Abstract

Analysis of 88 characters of external and internal body systems yielded a phylogenetic reconstruction of the Neotropical electric knifefish genus Sternarchorhynchus (Apteronotidae; Gymnotiformes). The results support a hypothesis of Sternarchorhynchus as the sister group to Platyurosternarchus. A series of synapomorphies, many involving major innovations of the neurocranium, jaws, suspensorium, and associated systems that permit an unusual mode of grasp-suction feeding, support the monophyly of both genera. Synapomorphies largely resolve relationships within Sternarchorhynchus with basal nodes strongly supported by characters pertinent to prey capture and initial processing of food items. These possible key innovations may provide Sternarchorhynchus with a competitive advantage over other clades of the Apteronotidae and account for the species diversity of the genus in Neotropical rivers. Adaptive radiation in Sternarchorhynchus was analysed. Habitat preference transitions repeatedly occurred in the genus between deep-river channel dwelling species and rheophilic species with preferences for higher energy setting including rapids and swift-flowing fluviatile settings. Twenty-two species of Sternarchorhynchus are described as new based on samples that originated in the smaller rivers draining into the Golfo de Paria, the Marowijne and Essequibo River basins, the Río Orinoco and in particular the Amazon River basin. The 32 species in Sternarchorhynchus make it the most speciose genus in the Apteronotidae.

INTRODUCTION

Electric knifefishes of the genus Sternarchorhynchus are tube-snouted, elongate, distinctly laterally compressed fishes with a broad range in many Atlantic drainages of South America. Sternarchorhynchus ranges from the Río Orinoco basin in Venezuela and Colombia (Mojica, 1999; Lasso et al., 2004a, b) south through the rivers of the Guianas (Planquette, Keith & Le Bail, 1996; Meunier, 2004) and the Amazon basin (Goulding, Leal Carvalho & Ferreira, 1988; Cox-Fernandes, 1999) to the upper Rio Paraná in the Río de La Plata basin (Britski, Silimon & Lopes, 1999; Campos-da-Paz, 2000). The genus is unknown from the trans-Andean and Caribbean versant portions of South America, the rivers of north-eastern and south-eastern Brazil that drain directly into the Atlantic Ocean, all portions of the Río de La Plata basin exclusive of the upper Rio Paraná system, and the entirety of the southern reaches of the continent.

Sternarchorhynchus species are moderate-sized predators (150 to 540 mm maximum total length) of benthic invertebrates (Goulding et al., 1988; Marrero & Taphorn, 1991). Their electrogenic abilities facilitate the location and capture of prey in soft mud and within the interstices between leaf litter and other detritus (Marrero & Winemiller, 1993; Crampton & Albert, 2006). Similarly they hunt for and capture aquatic insects, especially of the Chironomidae, Ephemeroptera, and Trichpotera, in small spaces in the river bottom and the tunnels and spaces in compacted clay nodules (Marrero, 1987; Marrero, Castillo & Machado-Allison, 1987; Marrero & Winemiller, 1993). Correlated modifications of the bones, musculature, and other soft tissues of the jaws, and suspensorium in the highly restructured, tube-like snouts, and anterior portions of the neurocranium allow extraction of prey items from these constricted refuges (Aguilera, 1986). These restructurings permit what has been termed ‘grasp-suction’ or ‘suction assisted by mechanical grasping’. Morphological adaptations permitting this highly specialized feeding mode are unique to Sternarchorhynchus and the African mormyriform genus Campylomormyrus amongst orders of fishes entirely restricted to freshwaters (Marrero & Winemiller, 1993; Winemiller & Adite, 1997).

A diversity of secondary sexually dimorphic features characterize genera in the Apteronotidae, with these modifications most prominent in the morphology of the snout and jaws (Cox-Fernandes, 1998). Amongst these are the hypermorphic growth of the snout in males of some species of Apteronotus(Cox-Fernandes, Lundberg & Riginos, 2002; Albert & Crampton, 2009) and the elaboration of the anterior portion of the jaws and dentition in Sternarchogiton(de Santana & Crampton, 2007). Many species of Sternarchorhynchus are characterized by secondary sexual dimorphism of multiple body systems. Noncephalic sexually dimorphic features of some species of Sternarchorhynchus involve pigmentation and more often the location of the anus and urogenital pore. More dramatic are the sexually dimorphic attributes involving the snout and jaws. Mature males of Sternarchorhynchus mormyrus have a wider snout than conspecific females of comparable body sizes, whereas the distinct fleshy dorsomedian ridge on the snout is limited to the males of Sternarchorhynchus kokraimoro. Some species in the genus demonstrate sexually dimorphic differences in the degree of curvature of the elongate snout. Most striking of the sexually dimorphic features in Sternarchorhynchus are the lengthened and transversely widened anterior portions of the dentaries in some larger males of ten species. This expanded region is rounded in the transverse plane and dorsally forms a bulbous structure crowned with a patch of variably enlarged teeth. Such enlarged dentition may be brought into play by males in confrontations during breeding contests (Marrero & Taphorn, 1991; Kirschbaum, 1995; Cox-Fernandes et al., 2002). Although these elaborations of the lower jaw and associated dentition occur only in mature males, they are absent in some conspecific males of comparable body sizes with well-developed testes. Presence versus absence of these modifications potentially reflects a social hierarchy wherein the jaw and dentition elaborations are limited to alpha males. Alternatively, variability in the expression of these features may reflect seasonality in the manifestation of the modifications of the lower jaw and associated dentition.

As in other gymnotiforms, the species of Sternarchorhynchus generate and receive weak electrical signals that function in electrocommunication and electrolocation. Electrocommunication serves to signal species identity and aspects of behaviour, whereas electrolocation permits the detection of organisms and objects in the surrounding aquatic system via distortions of the electric field generated by the individual [see Carr & Maler, 1986 and Albert & Crampton (2005) for discussions of the adaptations associated with these functions]. Members of Sternarchorhynchus generate at least four types of electric organ discharges (EODs). These periodic (wave-type) signals are interspecifically variable with discharges ranging between 944 and 1922 Hz and considerable overlap in distance ranges between congeners (Crampton & Albert, 2006: 688–689, figs 23.10–11). Some Sternarchorhynchus species have a unique EOD modulation pattern (active phase coupling) possibly associated with information exchange between potential mates (Langner & Scheich, 1978; Gottschalk & Scheich, 1979); however, the degree to which these unusual EOD patterns are universal across the genus is unknown.

Species of Sternarchorhynchus dwell in diverse habitats, with many members of the genus resident in the deep-water benthos of the larger rivers. In these habitats, the species of Sternarchorhynchus constitute a significant portion of the fish biomass (Lopez-Rojas, Lundberg & Marsh, 1984; Lundberg et al., 1987; Marrero & Taphorn, 1991). Jointly with other deep-water species of the Apteronotidae, they are preyed upon extensively by several catfish species of large body sizes (Marrero & Taphorn, 1991; Barbarino Duque & Winemiller, 2003). Some species of Sternarchorhynchus are adapted for life in smaller rivers and tributary streams where they often inhabit high-energy settings such as rapids and/or pools at the base of waterfalls (e.g. Sternarchorhynchus severii, Sternarchorhynchus caboclo; de Santana & Nogueira, 2006: 90).

Impoundments across the range of Sternarchorhynchus drowned many of the rapids and waterfalls that are preferred habitats for many species of the genus (Ferreira, 1995: 34–35; Campos-da-Paz, 2005: 399). Proposed development projects furthermore threaten other sites occupied or likely to be occupied by members of the genus. Habitat loss of this scale renders species of Sternarchorhynchus vulnerable to extinction at least at the local level (e.g. Sternarchorhynchus britskii in the upper Rio Paraná; Campos-da-Paz, 2005: 399; Triques, 2007: 125). Indeed, some species described as new herein are possibly either extinct or face imminent extinction.

Deep main river channels, rapids, pools at the bases of waterfalls and large rapids, and subterranean waters potentially inhabited by species of Sternarchorhynchus(Campos-da-Paz, 2000; Bichuette & Trajano, 2003) remain unsampled ichthyologically in many regions. When surveyed, the ichthyofauna in these difficult-to-collect habitats is nearly always incompletely sampled because of resource limitations and/or restrictions on the use of the collecting methods (e.g. rotenone) often prerequisite for exhaustive collections of such faunas. Consequently, nearly all species of Sternarchorhynchus described to date are based on one to relatively few specimens. One notable exception (Sternarchorhynchus roseni) was described from a locality that was the subject of long-term sampling efforts. Limited samples of many species of Sternarchorhynchus continue to bedevil systematic, ecological, and behavioural studies of the genus; problems typified by the restricted samples of some of the species described herein. Rectifying the problem of limited sample size is unlikely in the short term, if ever, because of resource limitations; a problem compounded by the accelerating anthropogenic modifications of many of the habitats occupied by species of Sternarchorhynchus.

Müller & Troschel (1848) described the first species now placed in Sternarchorhynchus as Sternarchus oxyrhynchus. The next three decades saw the proposal of Sternarchorhynchus by Castelnau (1855), the description of S. mormyrus by Steindachner (1868a), the proposal of the subgenus Rhamphosternarchus within Sternarchus by Günther (1870) for species now assigned to Sternarchorhynchus, and the description of Sternarchorhynchus curvirostris by Boulenger (1887). In a dramatic reversal of the practice of progressive description of additional species, Ellis (1913) placed S. mormyrus and S. curvirostris as junior synonyms of Sternarchorhynchus oxyrhynchus and recognized a monotypic Sternarchorhynchus. Ellis’ concept ofS. oxyrhynchus was of a geographically broadly distributed species characterized by pronounced variation in external head morphology and anal-fin ray counts.

This broadly inclusive S. oxyrhynchus held sway for over a half a century (e.g. Eigenmann & Allen, 1942: 318; Fowler, 1951: 431) until Fernández-Yépez (1967) resurrected S. curvirostris and S. mormyrus from the synonymy of S. oxyrhynchus based on differences between what he identified as samples of those nominal forms. Fernández-Yépez failed to specify the localities where his samples originated, but the illustrations in that publication make it clear that the material considered S. curvirostris was misidentified and that the purported specimens of S. mormyrus were probably another species. Notwithstanding these misidentifications, Fernández-Yépez correctly surmised that Sternarchorhynchus encompassed more than one species. Subsequent authors followed Fernández-Yépez and recognized S. oxyrhynchus, S. mormyrus, and S. curvirostris.

The century-plus hiatus in the description of nominal species of Sternarchorhynchus ended with Mago-Leccia's (1994) description of S. roseni from the Río Orinoco system. Soon thereafter, Campos-da-Paz (2000) summarized the state of knowledge of the taxonomy of Sternarchorhynchus, addressed the questions of the monophyly of, and intrarelationships within, the genus, and described two new species (S. britskii, Sternarchorhynchus mesensis). de Santana & Taphorn (2006; Sternarchorhynchus gnomus), de Santana & Crampton (2006; Sternarchorhynchus curumim), and de Santana & Nogueira (2006; S. severii, S. caboclo) described four additional species of Sternarchorhynchus.

Notwithstanding the recent flurry of species descriptions, Albert & Crampton (2005) and de Santana & Nogueira (2006) proposed that thorough analyses of Sternarchorhynchus across its range would reveal additional undescribed species. Results reported on herein confirm those predications; indeed those forecasts as to the number of undescribed species proved quite conservative. In addition to the ten previously recognized Sternarchorhynchus species, we describe 22 new species of the genus. Populations of Sternarchorhynchus that probably represent additional yet-to-be described species are identified, but formal descriptions of those forms deferred for various reasons. Nonetheless, the 32 species of Sternarchorhynchus account for 40% of the species in the Apteronotidae, the ghost electric knifefishes (de Santana, Castillo & Taphorn, 2006; this study).

Campos-da-Paz (2000) and Albert (2001) delved into the question of the relationships of and within Sternarchorhynchus and proposed Platyurosternarchus as a potential sister group to that genus. Those analyses encompassed solely the limited number of species then recognized within the genus. The numerous species described as new in Sternarchorhynchus since the phylogenetic analysis by Campos-da-Paz (2000), along with the continued description of phylogenetically informative characters within the Gymnotiformes, necessitate a re-evaluation of the evidence for the monophyly of the genus. More importantly it is appropriate to develop an intrageneric phylogenetic hypothesis incorporating the dramatically higher number of species now recognized within Sternarchorhynchus.

This study had four primary goals:

  1. To evaluate the hypothesis of the monophyly of Sternarchorhynchus and its sister-group relationship with Platyurosternarchus;

  2. To generate a hypothesis of the phylogenetic relationships within Sternarchorhynchus;

  3. To examine the correlations amongst major shifts in the jaws and associated structures, major transitions in body sizes, and shifts in habitat preferences in the species of Sternarchorhynchus within the context of the reconstructed phylogeny; and

  4. To provide a comprehensive revision of species of Sternarchorhynchus with associated keys, species accounts, and distribution maps.

MATERIAL AND METHODS

Hypotheses of phylogenetic relationships are based on cladistic methods first proposed by Hennig (1966) and subsequently elaborated by a series of authors (e.g. Nelson & Platnick, 1981; Wiley, 1981; Wiley et al., 1991; Forey et al., 1996). The method used in the search for the hypothesis best supported by the incorporated data was Wagner parsimony under which reversals of, and convergencies in, characters are permissible and quantified equally (Swofford & Maddison, 1987). The analysis was carried out using PAUP*4.0B10 (Swofford, 2003). A parallel analysis was undertaken in NONA (Goloboff, 1999) in its shell WINCLADA (Nixon, 1999–2002). The matrix of 88 characters and 34 terminal taxa was built in MacClade (Maddison & Maddison, 2005) with tree manipulations and character diagnoses generated through the same program. Characters were coded as unordered in the analysis of the data matrix. Outgroup comparisons served as the basis for polarity inferences of character. Rooting was a posteriori at the clade formed by Orthosternarchus and Sternarchorhamphus, which is hypothesized as a basal group in the Apteronotidae (Albert, 2001; Triques, 2005; Hilton et al., 2007). Ambiguous character distributions were resolved using the accelerated transformation optimization [ACCTRAN (Farris optimization)], which maximizes reversals over parallelism (de Pinna, 1991). Delayed transformation optimization (DELTRAN) optimizations are also discussed in Appendix 2.

Meristics and morphometrics

Many specimens of gymnotiforms suffer damage to or loss of their tails, most likely from predation, in some instances by other knifefishes (Lundberg et al., 1996: 666). Caudal region damage is often followed by a partial regeneration, with most so-damaged individuals recognizable by the clearly ‘truncated, or abruptly narrowed and/or abruptly depigmented tails.’ (Mago-Leccia, Lundberg & Baskin, 1985: 1); however, some damaged individuals lack these external indictors. We excluded specimens with apparently regenerated posterior regions of the body from counts of anal- and caudal-fin rays and from morphometrics involving total length, tail length, and tail depth.

Counts

Vertebral counts were obtained from radiographs and cleared and stained specimens. The Weberian apparatus in gymnotiforms incorporates the four anteriormost centra with the second and third elements very closely applied, or sometimes fused, to each other, but still apparent (Hopkins, 1991: 152; Campos-da-Paz, 2000: 524). Abdominal vertebrae are the anterior components of the series that bear a pair of ribs extending over and along the sides of the abdominal cavity and include the vertebrae of the Weberian complex. Transitional vertebrae lack both ribs and neural spines.

Counts of anal-, pectoral-, and caudal-fin rays were most often taken under a microscope using transmitted light with those data supplemented by counts taken from digital radiographs. In the count of the pectoral-fin rays, unbranched anterior rays are represented by lower case Roman numerals and branched rays by Arabic numbers. The count of branched rays includes all rays posterior to the anterior unbranched rays including the posteriormost ray in the series that is apparently sometimes unbranched. Anal-fin ray counts are presented in terms of anterior unbranched and total rays. Caudal-fin rays are the total number of those rays. Observations on the number and form of the branchiostegal rays are from cleared and stained specimens and radiographs.

Measurements

Morphological measurements are point-to-point distances utilizing digital callipers to 0.1 mm total length (TL) with data for each species presented in tabular form in each species account. Measurements were as follows: anal-fin base - the distance between the bases of the first and last rays of the anal fin; anus to anal-fin insertion - the distance from the posterior margin of the anus to the base of the first anal-fin ray; branchial opening - the height of the opening measured along the vertical; caudal length - the distance from the base of the posteriormost ray of the anal fin to the posteriormost point on the body; caudal-fin length - the distance from the hypural joint to the distalmost point along the posterior margin of the caudal fin; eye diameter - the horizontal width of the eye; greatest body depth - the greatest vertical extent of the body, usually situated slightly anterior to the dorsal-fin origin; head depth at eye - the head depth measured at the eye; head depth at nape - the head depth measured at the nape; head width - the head width measured at the opercular region; head length - the distance from the tip of the snout to the posterodorsal angle of the branchial opening; internarial distance - the distance between the posterior of the base of the tubular anterior naris and the anterior margin of the opening of the posterior naris; interocular width (interorbital width of some previous authors) – the minimum width between the dorsal margins of the orbits; length to end of anal-fin (the LEA of Mago-Leccia et al., 1985) – the distance from the tip of the snout to the end of the base of the anal fin; mouth length - the distance from the tip of the snout to the rictus of the mouth; pectoral-fin length - the distance between the base of the dorsalmost ray of the pectoral fin and the distalmost point on the margin of the fin; posterior naris to snout - the distance from the anterior border of the naris to the tip of snout; posterior naris to eye - the distance from the posterior border of the naris to the anterior margin of the eye; postocular distance - the distance from the posterior margin of the eye to the posterodorsal angle of the branchial opening; preanal-fin distance - the distance from the tip of the snout to the base of the first anal-fin ray; prepectoral-fin distance - the distance from the tip of the snout to the base of the dorsalmost pectoral-fin ray; snout length - the distance from the tip of the snout to the anterior margin of the eye; snout to anus - the distance from the tip of the snout to the anterior margin of the anus; tail depth - the depth of the tail measured immediately posterior of the base of the last anal-fin ray; and total length - distance from the tip of the snout to the end of the tip of the caudal fin.

Whenever possible, the sex of specimens was assessed via dissections in order to evaluate possible secondary sexual dimorphism. Ovaries of mature females are packed with yellow or cream-coloured eggs. Testes in mature males are, in contrast, white and smooth.

Abbreviations

Abbreviations used in text are: CS, cleared and stained; HL, head length; LEA, length from tip of snout to end of anal fin; and TL, total length.

Abbreviations for institutions and collections are: AMNH, American Museum of Natural History, New York; ANSP, Academy of Natural Sciences of Philadelphia; AUM, Auburn University Museum, Auburn; BMNH, The Natural History Museum, London [formerly British Museum (Natural History)]; CAS, California Academy of Sciences, San Francisco; CAS-SU, former Stanford University collections, now deposited in CAS; CM, Carnegie Museum, Pittsburg, fish collections now deposited at various depositories; CU, Cornell University, Ithaca; FMNH, Field Museum of Natural History, Chicago; IAVHP, Instituto Alexander von Humboldt, Colleción de Peces, Villa de Leyva, Colombia; INHS, Illinois Natural History Survey, Champaign; INPA, Instituto Nacional de Pesquisas da Amazônia, Manaus; IRScNB, Institute Royal de Sciences Naturelles de Belgique, Brussels; IDSM, Instituto de Desenvolvimento Sustenavel, Mamiraua, Tefé, Brazil; IU, Indiana University, Bloomington, fish collections now at various depositories; LACM, Los Angeles County Museum, Los Angeles; MBUCV, Museo de Biología de la Universidad Central de Venezuela, Caracas; MCNG, Museum de Ciencias Naturales, Guanare; MCP, Museu de Ciências e Tecnologia, Pontificia Universidade Católica do Rio Grande de Sul, Porto Alegre; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge; MNRJ, Museu National, Rio de Janeiro; MUSM, Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos, Lima; MZUSP, Museu de Zoologia da Universidade de São Paulo; NMW, Naturhistorisches Museum Wien, Vienna; ROM, Royal Ontario Museum, Toronto; UF, Florida Museum of Natural History, Gainesville; UMMZ, Museum of Zoology, University of Michigan, Ann Arbor; USNM, National Museum of Natural History, Smithsonian Institution, Washington; ZMA, Zoologisch Museum, Amsterdam; and ZMB, Museum für Naturkunde der Humboldt Universität, Zoologisches Museum, Berlin.

Locality data

Material examined for each species is geographically sorted by country and then, when available, major political divisions within countries, followed by specific localities within those divisions with the localities grouped to the degree possible by proximity. In the case of new species, the locality information is supplemented by data on collectors and date collected. Locality information (and collectors where appropriate) is followed by institutional abbreviation, catalogue number, and number of specimens in lot. Parenthetical information includes the range of total lengths of the specimens and when appropriate information about type status (for previously described species), type of preparation if not whole alcohol specimens, the sex of the specimens when determinable, and other pertinent information.

Nomenclature

Family level names for outgroups are those utilized in Checklist of the Freshwater Fishes of South and Central America [CLOFFSCA (Reis, Kullander & Ferraris, 2003)]. The narrow, posteriorly tapering, fleshy structure that arises from the mid-dorsal line of the posterior half of the body and runs along and is attached to the mid-dorsal groove on the body is termed the electroreceptive filament. This structure was equated with the adipose fin by Ellis (1913) and termed the dorsal thong by Mago-Leccia (1994). The homology of this structure remains uncertain but as indicated by the name it serves an electroreceptive function (see discussion by Franchina & Hopkins, 1996).

Osteological terminology is that of Weitzman (1962) with the following modifications. The use of epioccipital rather than epiotic corresponds to the practice proposed by Patterson (1975). Following Nelson (1969), the ossification traditionally termed the epihyal is referred to as the posterior ceratohyal, and the ceratohyal of many previous authors as the anterior ceratohyal. The ethmoid of Weitzman (1962) is termed the mesethmoid following Fink & Fink (1981, 1996). Vomer is used instead of prevomer, but with the vomer considered to be fused to the ventral ethmoid as is the case in various groups of fishes (Johnson & Patterson, 1996). The anterior portion of that combined bone is identified as the ventral ethmoid as is the practice in recent analyses involving gymnotiforms (Albert, 2001; Hilton et al., 2007). Intercalar is used instead of opisthotic as per the practice in recent studies (Albert, 2001). Following recent authors publishing on gymnotiforms we utilize endopterygoid rather than mesopterygoid (Fink & Fink, 1996). Recent literature on gymnotiform phylogeny includes alternative numbering systems for the gill arches (e.g., Albert, 2001). We follow Hilton et al.(2007) in using the number system for gill arches typical for studies of actinopterygian fishes. Nomenclature for muscles and associated tendons is that of Winterbottom (1974).

Osteological preparations

Samples were cleared and counterstained (CS) for cartilage and bone using the method outlined by Taylor & Van Dyke (1985). Removal of the pectoral girdle, suspensorium, and components of the head followed procedures outlined by Weitzman (1974). In order to identify anomalous features, two or more individuals of each species were prepared when specimens were of appropriate size and samples sufficiently large. The osteology of 23 of the 32 species of Sternarchorhynchus was examined via cleared and stained specimens. Cleared and counterstained specimens of these species are listed in Supporting Information Appendix S1. Specimens of Sternarchorhynchus axelrodi, Sternarchorhynchus freemani, Sternarchorhynchus galibi, Sternarchorhynchus kokraimoro, Sternarchorhynchus marreroi, Sternarchorhynchus taphorni, Sternarchorhynchus yepezi, and Sternarchorhynchus villasboasi could not be prepared in that fashion because of small sample sizes or because available specimens were of large body sizes. Radiographs served as supplementary sources of data for these species.

Available specimens of S. axelrodi are too large to clear and stain, but we were able to code the species for many osteological characters via radiographs of the type series (see Material examined under species account). That information and the external features allowed us to code the species for 73 of the 88 characters in the matrix. The question of missing data is problematic in phylogenetic analyses and has been discussed in various contexts (Novaceck, 1992; Wiens, 2003, 2005 and references therein). Wiens (2003, 2005) pointed out that even in instances of incomplete data, the available information will be phylogenetically informative when sufficient characters are coded. That has proved to be the case in this study, with the hypothesis of the phylogenetic relationships of S. axelrodi well supported.

Framework for outgroup comparisons

The phylogenetic analysis centres on relationships within Sternarchorhynchus rather than suprageneric relationships in the Apteronotidae, a problem that Hilton et al.(2007) concluded is ‘still the subject of debate’ and whose resolution extends beyond the scope of this project. For the purposes of the phylogenetic analysis herein, we focus on Platyurosternarchus as the sister group to Sternarchorhynchus according to results of the more encompassing phylogenetic hypothesis based on multiple character systems examined herein and in some previous analyses [Albert & Campos-da-Paz (1998: 429, fig. 2 – clade H), Campos-da-Paz (2000: 527 – clade C), Albert (2001: 72, fig. 43 – clade V)].

The question of the appropriate second outgroup to Sternarchorhynchus is less straightforward. Campos-da-Paz (1995) identified a series of similarities between Orthosternarchus tamandua and Sternarchorhamphus muelleri, a relationship reiterated by Alves-Gomes et al.(1995) based on genetic, morphologic and electrophysiologic data. Albert (2001), in turn, proposed that O. tamandua plus Sternarchorhamphus muelleri formed the sister group to the clade formed by Platyurosternarchus plus Sternarchorhynchus(Albert, 2001: 72, fig. 43 – clades U and V, respectively). Subsequent authors (Triques, 2005; Hilton et al., 2007) advanced additional evidence for the monophyly of the clade formed by O. tamandua and Sternarchorhamphus muelleri based on morphological data, but questioned the placement of the lineage formed by those two species as the sister group to the clade formed by Platyurosternarchus plus Sternarchorhynchus. In particular, Hilton et al.(2007) highlighted problematic homology assumptions underpinning that phylogenetic hypothesis (see comments under Convergence in elongation of snout in Discussion). That issue notwithstanding, Hilton et al. noted that all four genera (Orthosternarchus, Platyurosternarchus, Sternarchorhamphus, and Sternarchorhynchus) were relatively basal to ‘other apteronotid taxa.’ A lack of resolution in the phylogenetic scheme proposed by Triques (2005) renders it uninformative on this issue, but the pattern of relationships does not run counter to that proposal. As such the Orthosternarchus-Sternarchorhamphus clade serves as an appropriate secondary outgroup to Sternarchorhynchus for the purposes of our analysis.

Concepts of relationships within the rest of the Apteronotidae remain incompletely resolved (Crampton & Albert, 2006: fig. 23.11). Representative taxa from the remaining major clades across the rest of the Apteronotidae (Albert, 2001) were included as more distant outgroups for the purposes of the phylogenetic analysis (Apteronotus cuchillo, Apteronotus rostratus, Apteronotus albifrons, ‘Apteronotusapurensis, Sternarchella orthos, and Sternarchogiton labiatus). ‘Apteronotus’ apurensis is a member of a small clade that Albert & Campos-da-Paz (1998) hypothesized failed to form a monophyletic group with Apteronotus sensu stricto. These authors used the convention of ‘Apteronotus’ to highlight the uncertainty concerning the appropriate generic assignment for ‘A.’apurensis and the three other species in that clade.

Phylogenetic position of Ubidia

Divergent placements of the monotypic genus Ubidia necessitate comment. Triques (2005) concluded that Platyurosternarchus was the sister clade to Ubidia magdalenensis. Under the hypothesis advanced by that author, the PlatyurosternarchusUbidia clade formed the sister group to Sternarchorhynchus. A sister-group relationship between Ubidia and Platyurosternarchus was supported by one proposed synapomorphy –‘the marmorated pigmentation pattern of the body with large dark blotches formed of dense aggregations of chromatophores’ (Triques, 2005: 142; our translation). Marmoration was alternatively cited by Albert (2001) as synapomorphic for the members of the Apteronotus magdalenensis group [A. magdalenensis(= Ubidia magdalenensis) and A. cuchillo], which share several other hypothesized synapomorphies. In that analysis these species were deeply embedded within Apteronotus, with that genus separated from Platyurosternarchus and Sternarchorhynchus by several nodes supported by a series of characters (Albert, 2001: fig. 43). Both coding and homology questions pertain to the preferability of one of these alternative hypotheses as to the relationships of Ubidia with Apteronotus versus Platyurosternarchus.

Triques (2005) coded A. cuchillo as lacking marmoration. Examination of the types of A. cuchillo revealed marmoration formed of small scale, irregular, dark and light patterning over the entire head and body other than for the lightly coloured mid-dorsal region. Comparable patterning occurs in U. magdalenensis, albeit with the contrast between light and dark regions somewhat muted. Similar marmoration also occurs in the more recently described Apteronotus eschmeyeri(de Santana & Maldonado-Ocampo, 2005: fig. 5 a).

One could broadly characterize the coloration of Platyurosternarchus as marmorated; however, the pigmentation scheme of that genus differs notably from that of A. cuchillo, A. eschmeyeri, and U. magdalenensis and was characterized by Mago-Leccia (1994) as ‘highly diagnostic’ within the Gymnotiformes. The species of Platyurosternarchus have intense dark pigmentation on the ventral portions of the body with the darker patches in that region nearly contiguous and interrupted by some expansive more lightly pigmented regions (Ferreira et al., 2007: 164; de Santana & Vari, 2009: figs 1, 5). This pattern of expansive dark versus light pigmentation patches differs dramatically from the finer marmoration characteristic of A. cuchillo, A. eschmeyeri, and U. magdalenensis. Platyurosternarchus has more lightly coloured dorsolateral and dorsal regions of the body with some irregular large patches of somewhat darker pigmentation within the ground coloration; again a pattern absent in A. cuchillo, A. eschmeyeri, and U. magdalenensis. In light of these differences, we judge the marmoration in A. cuchillo, A. eschmeyeri, and U. magdalenensis as nonhomologous with the pigmentation pattern of Platyurosternarchus. This nonhomology eliminates support for the Ubidia–Platyurosternarchus clade proposed by Triques (2005) with those two genera plus Sternarchorhynchus thereby forming a trichotomy under that scheme of relationships. This, nonetheless, leaves Ubidia as potentially closely related to Sternarchorhynchus.

As noted, Albert (2001: fig. 43) proposed that a series of characters support the hypothesis that the relationships of Ubidia lie with a component of Apteronotus rather than Platyurosternarchus and Sternarchorhynchus. Apteronotus cuchillo and U. magdalenensis share distinctive juvenile coloration patterns, most notably lightly coloured bands proximate to the rear of the anal fin and the base of the caudal fin. These bands encircle the body within the dark background coloration (Schultz, 1949: 71, pl. 3, fig. A). Comparable ontogenetically vanishing contrasting coloration characterizes various Apteronotus species including A. eschmeyeri(de Santana et al., 2004). Neither species of Platyurosternarchus demonstrates a comparable pigmentation pattern. The common possession of this distinctive pigmentation pattern provides further support for the hypothesis that the relationships of U. magdalenensis lie with a component of the genus Apteronotus .

Coding of internal features of U. magdalenensis by Albert (2001) and Triques (2005) apparently was limited to characters shown in the partial illustrations of head osteology in the original description by Miles (1945: fig. 11). This impediment resulted in numerous uncoded characters and was encumbered by apparent errors in those illustrations. Of note is the lateral ethmoid which, although present in specimens of Ubidia examined in this study, was absent from the illustration with its absence versus presence coded as unknown by Triques (2005). A lateral ethmoid is always present in the species of Apteronotus contrary to the derived absence of that element in Platyurosternarchus and Sternarchorhynchus. Triques (2005) proposed other features uniting Ubidia to Platyurosternarchus plus Sternarchorhynchus. Two of the characters optimized at that clade rather than being universal for the three genera. Comments on the other proposed synapomorphies are provided under the discussion of the features in the analysis (characters 2, 23, 26, 68, 87) and ‘Convergence in the elongation of the snout’. The evidence from coloration and the lateral ethmoid, complemented by the information presented by Albert (2001: 76, 77, fig. 43, see synapomorphies for nodes Z, AA, AB, AC), support the hypothesis that the relationships of Ubidia lie with a subunit of Apteronotus rather than with Platyurosternarchus and Sternarchorhynchus. Ubidia is consequently not included as a proximate outgroup to Sternarchorhynchus in the phylogenetic analysis.

Phylogenetic results

Eighty-eight characters that vary within Platyurosternarchus and Sternarchorhynchus, diagnose one of those genera as monophyletic, or are informative for resolution of structure in the outgroup phylogeny were examined in the cleared and stained specimens (Supporting Information Appendix S1) and whole specimens listed under the taxonomic accounts. Characters were divided into two or more discrete character states, with multistate characters present for three characters. Characters were coded as unordered in the analysis of the data matrix (Appendix 1). A heuristic search with stepwise addition, simple additional sequence, branch-swapping tree bisection–reconnection, and ‘max’ trees saved in PAUP* 4.0b10 (Swofford, 2003) included all outgroups with branches of zero maximum length collapsed and yielded eight most parsimonious trees each with a length of 190, consistency index (CI) = 0.484, and retention index (RI) = 0.820. The analysis using minimum tree length of zero resulted in four trees of the same topology and scores. A parallel analysis in NONA (Goloboff, 1999) in its shell WINCLADA (Nixon, 1999–2002) using the same conditions as in PAUP* and a search strategy of Mult* yielded four trees with the same topology as in the PAUP* analysis with a length of 190, CI = 0.480, and RI = 0.810. The strict consensus tree generated in both of the two software packages was identical, with a length of 193, CI = 0.477, and RI = 0.814 and is presented in Figure 23. Only two of these trees were pertinent to alternative hypotheses of relationships within the lineage formed by Platyurosternarchus and Sternarchorhynchus and involved the highly imbedded clade 24 within the latter genus (Fig. 24). The other alternative trees pertain to differing patterns of relationships amongst more distant outgroup taxa and as such are not germane to the foci of this study.

Strict consensus of eight most parsimonious trees generated in PAUP*, length = 193, consistency index (CI) = 0.477, and retention index (RI) = 0.814, with all branches of zero maximum length collapsed and all characters unordered. Each of the most parsimonious trees has length = 190, CI = 0.484, and RI = 0.820. Synapomorphies only mapped for and within clade consisting of Platyurosternarchus and Sternarchorhynchus.

Strict consensus of eight most parsimonious trees generated in PAUP*, length = 193, consistency index (CI) = 0.477, and retention index (RI) = 0.814, with all branches of zero maximum length collapsed and all characters unordered. Each of the most parsimonious trees has length = 190, CI = 0.484, and RI = 0.820. Synapomorphies only mapped for and within clade consisting of Platyurosternarchus and Sternarchorhynchus.

Two alternative topologies of species within clade 24 with characters mapped under ACCTRAN optimization.

Two alternative topologies of species within clade 24 with characters mapped under ACCTRAN optimization.

CHARACTER DESCRIPTION AND ANALYSIS

The following discussion is arranged by discrete body systems ordered in an approximately anterior to posterior sequence. Characters involving multiple portions of an individual (e.g. coloration) are last. The description of each of the characters first includes a brief descriptor of the character followed by summaries of the alternative character states, the consistency and retention indices under the final phylogenetic hypothesis (Fig. 23), and finally a discussion of the distribution of character states amongst the examined species and members of the examined outgroups. Additional comments on the characters follow where appropriate.

Mouth

1. Extent of gape: (0) posterior terminus of gape not extending posteriorly to beyond vertical through posterior nares; (1) posterior terminus of gape extending posteriorly to beyond vertical through posterior nares (CI = 1.000; RI = 1.000).

Members of the proximate outgroups (Orthosternarchus, Sternarchorhamphus), along with Sternarchella orthos and all species of Sternarchorhynchus with the exception of S. goeldii and S. oxyrhynchus, have notably short mouths with the posterior limit of the gape falling short of the vertical through the posterior nares (state 0; Fig. 1B). Although the mouths in S. goeldii and S. oxyrhynchus are quite short along the snout axis compared to those in most fishes including many gymnotiforms (e.g. Magosternarchus, Lundberg et al., 1996: fig. 2) they are, nonetheless, distinctly longer than the form of the mouth present in all remaining species of Sternarchorhynchus. As a consequence, the gape in S. goeldii and S. oxyrhynchus extends posteriorly distinctly beyond the vertical running through the posterior nares (state 1; Fig. 1A) rather than terminating short of that landmark. This character could not be coded for the species of Platyurosternarchus or the outgroups other than Orthosternarchus, Sternarchorhamphus, and Sternarchella orthos. In those taxa the proportionally distinctly posterior location of the nares relative to the location of the apertures in Orthosternarchus tamandua, Sternarchorhamphus muelleri, and the species of Sternarchorhynchus means that that those openings cannot serve as homologous landmarks, thereby rendering coding impossible.

Lateral view of anterior portion of snout of A, Sternarchorhynchus oxyrhynchus, ANSP 162670; and B, Sternarchorhynchus caboclo, INPA 10594, showing differing degree of development of mouth and fold of skin at rear of rictus.

Lateral view of anterior portion of snout of A, Sternarchorhynchus oxyrhynchus, ANSP 162670; and B, Sternarchorhynchus caboclo, INPA 10594, showing differing degree of development of mouth and fold of skin at rear of rictus.

2. Presence or absence of small fold of skin on anterolateral surface of upper lip: (0) skin of upper lip flat and without fold; (1) skin of upper lip with distinct fold (CI = 0.500; RI = 0.900).

Variation in the morphology of the skin overlying the anterolateral surface of the upper lip was reported by Triques (2005: 26) for apteronotids. Triques (2005) reported the fold as present in S. oxyrhynchus. His material originated in the Amazon basin, a region outside the known range of the species and is likely to represent another species. Our analysis found that the upper lip lacks a skin fold in the species of Orthosternarchus, Platyurosternarchus, and Sternarchorhamphus. Within Sternarchorhynchus the fold is similarly absent in S. goeldii and S. oxyrhynchus(Fig. 1A). Both of those species of Sternarchorhynchus also share a mouth form unique in the genus (see character 1). All other species of Sternarchorhynchus have a distinct fold within the skin of the upper lip (Fig. 1B) with that feature also present in Sternarchella orthos amongst the more distant outgroups.

Upper jaw

3. Form of premaxilla: (0) somewhat to distinctly elongate from ventral view; (1) compact with longitudinal length approximately equal to transverse width (CI = 0.500; RI = 0.857).

As would be expected given the diversity of head forms across the Apteronotidae, the members of the family also demonstrate variation in the form of the premaxilla. In Platyurosternarchus crypticus and Platyurosternarchus macrostoma the premaxilla is anteroposteriorly variably elongate, a condition found in the other proximate outgroups to Sternarchorhynchus(Sternarchorhamphus muelleri, Campos-da-Paz, 1995: fig. 4; Orthosternarchus tamandua, Hilton et al., 2007: fig. 10) and various more distantly related gymnotiforms (Albert, 2001: fig. 6). An anteroposteriorly elongate premaxilla similarly characterizes a minority of the species of Sternarchorhynchus(S. axelrodi, S. goeldii, S. mormyrus, and S. oxyrhynchus; Fig. 2). The remaining species of Sternarchorhynchus have a compact form of the premaxilla in which the longitudinal length of the bone approximately equals its transverse width (Fig. 3). Looking beyond the immediate outgroups to Sternarchorhynchus, we find that comparable compact premaxillae are also present amongst examined apteronotids in the species of Apteronotus included in this study along with Sternarchella orthos(Lundberg et al., 1996: fig. 4c) and Sternarchogiton labiatus.

Premaxilla and anterior portion of neurocranium of Sternarchorhynchus goeldii, MCZ 46887; dorsal view, anterior to left.

Premaxilla and anterior portion of neurocranium of Sternarchorhynchus goeldii, MCZ 46887; dorsal view, anterior to left.

Premaxilla and anterior portion of neurocranium of Sternarchorhynchus roseni, MCNG 52591; dorsal view, anterior to left.

Premaxilla and anterior portion of neurocranium of Sternarchorhynchus roseni, MCNG 52591; dorsal view, anterior to left.

4. Presence or absence of teeth on premaxilla: (0) teeth absent; (1) teeth present (CI = 0.500; RI = 0.500).

Presence of premaxillary teeth is the common condition across the Gymnotiformes. The species of Orthosternarchus(Hilton et al., 2007: fig. 12), Sternarchorhynchus(Figs 2, 3), and Sternarchorhamphus(Campos-da-Paz, 1995: fig. 4) have premaxillary teeth present, albeit with the form of the dentition on the bone varying amongst those taxa. Premaxillary dentition is, in contrast, lacking in P. crypticus and P. macrostoma. Amongst the more distant outgroups included in the analysis, an edentulous premaxilla similarly occurs in Sternarchogiton labiatus. Other apteronotids not included in the outgroups that without premaxillary dentition are Apteronotus cuchillejo and the species of Adontosternarchus.

5. Degree of development of anterior portion of maxilla: (0) with distinct anteriorly pointed process; (1) without distinct anterior process (CI = 1.000; RI = 1.000).

Immediate outgroups to Sternarchorhynchus[Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), P. crypticus, P. macrostoma, Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 4)] have a distinct, somewhat to distinctly anteriorly pointed process of the maxilla from lateral view (state 0). That condition is also present in the more distant outgroup genera Apteronotus, Sternarchella, and Sternarchogiton. All species of Sternarchorhynchus alternatively have an anterior portion of the maxilla that lacks an anterior extension and have instead a rounded to square margin to that portion of the bone (state 1; Fig. 4).

Maxilla of A, Sternarchorhynchus roseni, MCNG 52591; B, Sternarchorhynchus mormyrus, USNM 306843, and C, Sternarchorhynchus goeldii, MCZ 46887; left side, lateral view; larger stippling represents cartilage.

Maxilla of A, Sternarchorhynchus roseni, MCNG 52591; B, Sternarchorhynchus mormyrus, USNM 306843, and C, Sternarchorhynchus goeldii, MCZ 46887; left side, lateral view; larger stippling represents cartilage.

6. Form of anterior portion of maxilla: (0) moderately expanded; (1) reduced and only slightly wider than main portion of bone; (2) distinctly expanded into somewhat square plate with irregular margins (CI = 1.000; RI = 1.000).

The anterior portion of the maxilla in most species of Sternarchorhynchus is moderately expanded into a plate-like region (state 0; Fig. 4A) with a similar condition also present amongst the examined outgroup species in the included species of Apteronotus, along with Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), Sternarchorhamphus muelleri and Sternarchogiton labiatus. Sternarchorhynchus axelrodi and S. mormyrus have that portion of the maxilla proportionally further expanded into an approximately square plate with irregular margins (state 2; Fig. 4B). Two other species of Sternarchorhynchus(S. goeldii, S. oxyrhynchus) alternatively have the anterior portion of the maxilla reduced in size relative to the conditions present in most congeners, with that region of the bone only slightly higher vertically than are the proximate posterior portions of the maxilla (state 1; Fig. 4C).

Platyurosternarchus has a highly expanded maxilla (Mago-Leccia, 1994: fig. 23d) of a form unique within the Apteronotidae (see discussion in de Santana & Vari, 2009). This renders it impossible to code Platyurosternarchus for the form of the anterior portion of the maxilla. The maxilla in Sternarchorhamphus muelleri is anteriorly distinctly expanded (Mago-Leccia, 1994: fig. 23h; Campos-da-Paz, 1995: fig. 4), similarly rendering it impossible to code that species for this character.

7. Form of ventral margin of maxilla: (0) relatively smooth; (1) with distinct concavity (CI = 0.500; RI = 0.900).

Although there occurs a pronounced diversity in the overall forms of the maxilla across the Apteronotidae (Mago-Leccia, 1994: fig. 23), most species in the family (e.g. Sternarchorhamphus muelleri, Campos-da-Paz, 1995: fig. 4), along with S. goeldii and S. oxyrhynchus, have a smooth, variably convex ventral margin of the main body of the bone (state 0; Fig. 4C). Other examined Sternarchorhynchus species alternatively, have distinct concavities along the main body of the maxilla (state 1; Fig. 4A, B). Such a concavity of the maxilla is also present in Orthosternarchus tamandua(Hilton et al., 2007: fig. 12) amongst the examined outgroups.

Lower jaw

8. Extent of fleshy pad at anterior of dentary: (0) absent or only slightly developed; when present not overlapping tip of snout in closed mouth; (1) well-developed, dorsally directed, and overlapping tip of snout in closed mouth (CI = 1.000; RI = 1.000).

The anterior portion of the fleshy covering of the dentary is slightly to moderately developed in outgroups to Sternarchorhynchus along with most members of the genus (state 0; Fig. 1B). Sternarchorhynchus goeldii and S. oxyrhynchus, in contrast, have a very well-developed, somewhat distally pointed, fleshy pad at the anterodorsal limit of the dentary (state 1; Fig. 1A). This dorsally directed pad overlaps the anterior portion of the snout when the mouth is closed.

9. Coronomeckelian bone: (0) present in juveniles and adults; (1) present as small rounded ossification in smaller specimens but absent in adults (CI = 1.000; RI = 1.000).

The coronomeckelian bone is typically present in most gymnotiforms and indeed most fishes. In P. crypticus and P. macrostoma the coronomeckelian is an anteroposteriorly elongate ossification running along the dorsal margin of the anguloarticular and the posterior portion of the well-developed Meckel's cartilage (Fig. 5). Comparable overall morphologies of the bone occur in the other outgroup taxa in this analysis [A. albifrons, A. cuchillo, A. rostratus, ‘A.’ apurensis, Orthosternarchus tamandua(Hilton et al., 2007: fig. 12b), Sternarchella orthos, and Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5)]. A small, round ossification positionally comparable to the coronomeckelian bone of other gymnotiforms is present in an approximately 40 mm TL specimen of an unidentified species of Sternarchorhynchus stewarti. That element is comparable to the form of that bone in adults of some genera of the Apteronotidae (e.g. Sternarchogiton labiatus).

Lower jaw of Platyurosternarchus macrostoma, FMNH 100730; left side, medial view, anterior to right; larger stippling represents cartilage.

Lower jaw of Platyurosternarchus macrostoma, FMNH 100730; left side, medial view, anterior to right; larger stippling represents cartilage.

Adults of all species of Sternarchorhynchus examined osteologically neither have any indication of the coronomeckelian bone (state 1; Figs 6, 7), nor do they retain a residual cartilage mass in the area primitively occupied by that ossification. The space occupied in outgroups by the coronomeckelian bone and Meckel's cartilage is filled inSternarchorhynchus by the dramatically anteriorly expanded anguloarticular characteristic of that genus (Figs 6, 7; see character 16).

Lower jaw of Sternarchorhynchus goeldii, MCZ 46887; left side, medial view, anterior to right.

Lower jaw of Sternarchorhynchus goeldii, MCZ 46887; left side, medial view, anterior to right.

Lower jaw of Sternarchorhynchus mareikeae, INPA 22901; left side, medial view, anterior to right.

Lower jaw of Sternarchorhynchus mareikeae, INPA 22901; left side, medial view, anterior to right.

Although the coronomeckelian bone was present in all non-Sternarchorhynchus gymnotiforms examined in this study, Mago-Leccia (1978: 36) reported that the element was of small size and ‘not ossified in some specimens’[our translation] of Eigenmannia virescens of the family Sternopygidae. It is uncertain whether that statement indicates that the coronomeckelian is present as a cartilaginous precursor to the bone or that the element is completely absent in the individuals in question. Regardless, the Sternopygidae is not phylogenetically proximate to Sternarchorhynchus and the absence of the ossification in a species of Eigenmannia is homoplastic relative to the condition in Sternarchorhynchus.

10. Meckel's cartilage: (0) present and well developed; (1) well developed in smaller individuals, but ontogenetically reduced and distinctly smaller in adults (CI = 1.000; RI = 1.000).

Meckel's cartilage is well developed across gymnotiforms and immediate outgroups toSternarchorhynchus[Orthosternarchus tamandua(Hilton et al., 2007: fig. 12b), Platyurosternarchus(Fig. 5), Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5)]; a condition comparable to that in most fishes. Within the Gymnotiformes, Meckel's cartilage most often has the form of an anteriorly attenuating body arising posteriorly from a distinct process on the medial wall of the anteromedial process of the anguloarticular and terminating anteriorly along the medial surfaces of that bone and the dentary (Fig. 5; Chardon & De La Hoz, 1974: fig. 6). Meckel's cartilage undergoes a progressive ontogenetic reduction in the species ofSternarchorhynchus. The cartilage is present and relatively well developed in small cleared and stained specimens of approximately 30 mm TL. At that body size, the cartilage arises from a distinct process that projects from the medial wall of the anguloarticular and then extends anteriorly to terminate a short distance posterior of the dentary symphysis. The notable difference in the form of Meckel's cartilage at that body size relative to the condition in outgroups is that inSternarchorhynchus the cartilage is distinctly narrower vertically even at its highest portion proximate to its contact with the process on the medial wall of the anguloarticular. At approximately 110 mm TL, the cartilage in cleared and stained preparations of Sternarchorhynchus is reduced vertically to a very narrow band that arises posteriorly at the bony process of the medial wall of the anguloarticular. However, larger examined cleared and stained specimens of the genus (approximately 115 mm TL) have the cartilage reduced to a small, very narrow band limited to the anterior quarter of the anguloarticular and falling far short posteriorly of the position of the process on the medial wall of the anguloarticular in smaller specimens (state 1). Individuals of 225 mm TL show no indication of that process on the anguloarticular. This ontogenetic reduction of Meckel's cartilage in the species ofSternarchorhynchus parallels the pronounced anterior development of the anguloarticular in the genus, which in adults fills the space occupied by Meckel's cartilage in outgroups (Figs 6, 7).

11. Relative lengths of posterodorsal and posteroventral processes of dentary: (0) posterodorsal process shorter than ventrolateral process; (1) posterodorsal and posteroventral processes of approximately same length (CI = 0.500; RI = 0.875).

Proximate outgroups to Sternarchorhynchus[Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), P. crypticus, P. macrostoma(Fig. 5), Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5)] along with other gymnotiforms have a posterodorsal process of the dentary extending dorsal of the anguloarticular that is, in turn, distinctly shorter than the posteroventral process of the dentary. The latter process runs ventral to the anguloarticular and sometimes along at least a portion of the ventral margin of the retroarticular (state 0; Fig. 5) (for conditions in some more distantly related gymnotiform taxa seeMago-Leccia, 1978: fig. 8; Mago-Leccia et al., 1985: fig. 3; Albert, 2001: fig. 10). The species of Sternarchorhynchus alternatively have posterodorsal and posteroventral processes of the dentary of approximately identical lengths or in some instances with the dorsal process slightly longer (Figs 6, 7). This condition is approximated in Sternarchella orthos(Lundberg et al., 1996: fig. 3c) amongst the outgroup taxa of this analysis. That species is, however, not closely related to Sternarchorhynchus(Albert, 2001) and the presence of this similar condition in these taxa is homoplastic.

12. Form of posteroventral process of dentary: (0) posteroventral process of dentary relatively robust and triangular overall; (1) posteroventral process of dentary very narrow and in form of elongate narrow strut of bone (CI = 1.000; RI = 1.000).

The posteroventral process of the dentary that extends ventral of the anterior portion of the anguloarticular is typically a relatively robust process in gymnotiforms including proximate outgroups such as Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), both species of Platyurosternarchus(Fig. 5), and Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 4). That morphology also occurs in the other outgroup apteronotids included in this study. Amongst all of those taxa, the process is either relatively broad vertically along most of its anteroposterior extent or gradually tapers to a point distally with a resultant overall triangular morphology (Mago-Leccia, 1978: fig. 8; Mago-Leccia et al., 1985: fig. 3; Albert, 2001: fig. 10). The species of Sternarchorhynchus have the posteroventral process of the dentary dramatically restructured into a very narrow strut of bone that borders a major portion of the ventral margin of the anteroposteriorly elongate anguloarticular (Figs 6, 7).

13. Posterior limit of posteroventral process of dentary: (0) posteroventral process of dentary reaching margin of posteroventral process of anguloarticular; (1) posteroventral process of dentary not reaching margin of posteroventral process of anguloarticular (CI = 0.200; RI = 0.200).

As mentioned under character 12, the posteroventral portion of the dentary of Sternarchorhynchus is a narrow, elongate process bordering the ventral margin of the anguloarticular (Fig. 6). The posterior extent of this process of the dentary varies within Sternarchorhynchus, with some species having the posterior terminus of the process falling short of the point where the posteroventral process of the anguloarticular arises from the main body of that ossification (state 1; Fig. 7). All other species in Sternarchorhynchus(Campos-da-Paz, 2000: fig. 6, for condition in S. mesensis) have the posteroventral process of the dentary extending further posteriorly (state 0; Fig. 6), with resultant contact of the posteroventral process of the dentary with the posteroventral process of the anguloarticular. A comparable contact between those elements occurs in Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 4), the species of Platyurosternarchus, and the more distant outgroups of this analysis (state 0).

14. Position of articulation between quadrate and dorsal surface of lower jaw: (0) located in region of articulation of anguloarticular and retroarticular, with anterodorsal portion of retroarticular contributing to joint of lower law with quadrate; (1) located distinctly anterior to region of articulation of anguloarticular and retroarticular, with anterodorsal portion of retroarticular well separated from joint of lower law with quadrate (CI = 1.000; RI = 1.000).

In Orthosternarchus tamandua(Hilton et al., 2007: fig. 12) and Sternarchorhamphus muelleri(Campos-da-Paz, 1995: figs 4, 5) the articulation of the articular condyle of the quadrate with the lower jaw is via an articular socket positioned along the posterodorsal margin of the jaw. In these species, the socket is situated along the dorsal portion of the joint between the anguloarticular and the retroarticular. Such a form of the articular socket occurs in other gymnotiforms (e.g. Rhabdolichops, Mago-Leccia, 1978: fig. 8) and is apparently the primitive condition for the Apteronotidae. In both Sternarchorhynchus and Platyurosternarchus, the area of quadrate–lower jaw articulation is shifted anteriorly relative to the joint between the anguloarticular and retroarticular. As a result, the articular socket on the lower jaw is formed solely by a portion of the posterodorsal region of the anguloarticular (Figs 5–7) with a distinct separation of the anterodorsal corner of the retroarticular and the quadrate–lower jaw articulation.

The degree of separation of the retroarticular from the joint between the lower jaw and quadrate increases ontogenetically in both Platyurosternarchus and Sternarchorhynchus. Smaller specimens of both Platyurosternarchus(P. crypticus, approximately 90 mm TL) and Sternarchorhynchus(Sternarchorhynchus cf. stewarti, approximately 40 mm TL) have the quadrate–lower jaw articulation proportionally closer to the anguloarticular–quadrate joint than in larger individuals of each species or adults of congeners.

An articulation of the quadrate solely with the anguloarticular resolves as a synapomorphy for Platyurosternarchus and Sternarchorhynchus under the results of this study (see also character 15). This condition is also present in A. albifrons, A. cuchillo, A. rostratus, ‘A.’ apurensis, Sternarchella orthos, and Sternarchogiton labiatus amongst the outgroups included in this analysis. The presence of this feature in these outgroup taxa is homoplastic to the condition in Platyurosternarchus and Sternarchorhynchus under present hypotheses of relationships within the Gymnotiformes (Albert, 2001).

15. Degree of separation along dorsal margin of anguloarticular between joint for articulation with quadrate and anguloarticular-retroarticular joint:(0) distance approximately equal to width of articular facet on anguloarticular that contacts quadrate; (1) distance approximately about 4–4.5 times width of articular facet on anguloarticular that contacts quadrate (CI = 1.000; RI = 1.000).

The site of articulation of the lower jaw with the quadrate is restricted to the anguloarticular in both Platyurosternarchus and Sternarchorhynchus(character 14), albeit differing in relative positions in the genera. Platyurosternarchus crypticus and P. macrostoma have a shorter, albeit, distinct separation of the condyle from the joint between the anguloarticular and retroarticular (state 0; Fig. 5). In contrast, the species of Sternarchorhynchus along with the outgroups A. albifrons, A. cuchillo, A. rostratus, ‘A.’ apurensis, Sternarchella orthos, and Sternarchogiton labiatus have a pronounced separation between these two landmarks (state 1; Figs 6, 7). This character could not be coded for Orthosternarchus tamandua and Sternarchorhamphus muelleri, in which the articular condyle on the lower jaw is formed jointly by the anguloarticular and retroarticular rather than being restricted to the dorsal margin of the anguloarticular.

16. Form of anterior portion and medial surface of anguloarticular: (0) anterior portion of anguloarticular broad with medial surface of bone having distinct process at posterior terminus of Meckel's cartilage; (1) anterior portion of anguloarticular distinctly pointed with medial surface of bone unelaborated (CI = 1.000; RI = 1.000).

The anterior portion of the anguloarticular that articulates with the posterior region of the dentary is relatively broad in most gymnotiforms (e.g. Albert, 2001: fig. 10) including the proximate outgroups to Sternarchorhynchus in this study [Orthosternarchus tamandua(Hilton et al., 2007: fig. 13), the species of Platyurosternarchus, and Sternarchorhamphus muelleri]. All of the outgroups also have a distinct process that arises from the medial surface of the anguloarticular and serves as the posterior limit of Meckel's cartilage (Fig. 5). Amongst the species of Sternarchorhynchus, the anterior portion of the anguloarticular has instead a distinctly anteriorly tapering overall triangular form (Figs 6, 7). Although a triangular anterior portion of the anguloarticular also occurs in some members of the Sternopygidae (e.g. Rhabdolichops; Mago-Leccia, 1978: fig. 8), the form of region is not distinctly anteriorly attenuate in those species. Those members of the Sternopygidae are, furthermore, distantly related to Sternarchorhynchus(Albert, 2001) and the similarities between these taxa and Sternarchorhynchus in this character are clearly homoplastic. As discussed under character 16, larger individuals of the species of Sternarchorhynchus have an unelaborated medial surface of the anguloarticular. The ontogenetic loss in Sternarchorhynchus of the process present in that region of the anguloarticular in other gymnotiforms correlates with the pronounced reduction of Meckel's cartilage in adults of all species of the genus (character 10).

17. Location of posterior terminus and form of posteroventral portion of the anguloarticular: (0) anguloarticular terminating posteriorly approximately at vertical through dorsal portion of joint between anguloarticular and retroarticular and without distinct posteriorly attenuating process extending under retroarticular; (1) anguloarticular terminating posteriorly at, to distinctly posterior of, vertical through dorsal portion of joint between anguloarticular and retroarticular and with distinct, posteriorly attenuating process extending along anterior margin of retroarticular (CI = 1.000; RI = 1.000).

Two highly divergent forms of the posteroventral portion of the anguloarticular are found in Sternarchorhynchus and the other gymnotiforms incorporated into this study. In Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5) and Orthosternarchus tamandua(Hilton et al., 2007: fig. 12b) the posteriormost portion of the anguloarticular is situated within the dorsal portion of the joint between the anguloarticular and retroarticular. This portion of the anguloarticular is incorporated into the socket receiving the articular condyle of the quadrate. The retroarticular in these two species has an anteriorly directed process extending under the posterior portion of the anguloarticular (state 0) with this condition also present in the other outgroups. The process is moderately developed in Orthosternarchus tamandua(Hilton et al., 2007: fig. 12) and extends further anteriorly in Sternarchorhamphus muelleri. The species of Platyurosternarchus and Sternarchorhynchus have the posteroventral portion of the anguloarticular extending further posteriorly, forming a distinct posteriorly attenuating process that extends under the anterior portion of the retroarticular (state 1; Figs 5–7). As such, the posterior terminus of the posteroventral portion of the anguloarticular in those genera is situated posterior of the vertical through the dorsal portion of the joint between the anguloarticular and the retroarticular.

18. Position of posterior limit of dentary: (0) posterior limit of dentary located distinctly anterior of vertical through vertical component of orbitosphenoid; (1) posterior limit of dentary located along vertical through vertical component of orbitosphenoid (CI = 0.500; RI = 0.800).

As discussed under characters 14 and 15, considerable variation occurs amongst the genera of interest in diverse aspects of the articulation of the lower jaw with the suspensorium. An additional difference reflective of the broader details of the arrangement of the bones of the head in these taxa involves the location of the position of the posteriormost point on the dentary with respect to the neurocranium. In S. goeldii and S. oxyrhynchus, along with the outgroups Orthosternarchus tamandua, P. crypticus, P. macrostoma, and Sternarchorhamphus muelleri, the posterior limit of the dentary lies considerably anterior of the vertical running through the vertically aligned section of the orbitosphenoid (state 0). In the remaining species of Sternarchorhynchus the posterior termination of the dentary is more posteriorly positioned relative to the orbitosphenoid, being situated along, or posterior of, the vertical through the vertically orientated anterior portion of the orbitosphenoid (state 1). State 1 is also present amongst the outgroups in Sternarchella orthos and Sternarchogiton labiatus.

Dentition

19. Form of teeth in upper and lower jaws: (0) teeth proportionally elongate with relatively little change in width other than distally and with slight posterior curvature distally; (1) teeth distinctly wider basally and tapering progressively to distinctly posteriorly recurved distal portion (CI = 1.000; RI = 1.000).

Species of Sternarchorhynchus and its immediate outgroup species, P. crypticus and P. macrostoma, along with the basal apteronotids Orthosternarchus tamandua(Hilton et al., 2007: fig. 12) and Sternarchorhamphus muelleri have teeth arranged in one or more rows along the dorsal portion of the dentary. These taxa also bear a patch of comparable dentition on the ventral surface of the premaxilla other than for the species of Platyurosternarchus, which lack upper jaw dentition. The tooth form differs amongst these taxa, with Orthosternarchus tamandua and Sternarchorhamphus muelleri having relatively elongate teeth on each jaw (state 0). The teeth in these genera become only slightly narrower along most of their length and terminate in a slightly posteriorly recurved point [tooth form termed villiform by Albert (2001: 16) and some other authors). Sternarchorhynchus alternatively has the teeth in both jaws tapering to a pronounced degree from proportionally wider bases with the distal curvature more developed (state 1). A similar form of distally tapering dentition occurs in the lower jaw of the species of Platyurosternarchus. However, this genus has an edentulous premaxilla. Such a form of dentition is also present in both jaws of the other outgroups included in the analysis exceptOrthosternarchus and Sternarchorhamphus.

20. Location of area of attachment of teeth on dentary: (0) teeth attaching to exterior surface of dentary; (1) teeth attaching to interior surface of dentary (CI = 1.000; RI = 1.000).

Attachment of the teeth on the dentary is consistently on the inner surface of the dentary in the species of Sternarchorhynchus, Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5), Orthosternarchus tamandua(Hilton et al., 2007: fig. 12), and the other apteronotid outgroups in this study other than P. crypticus and P. macrostoma. The dentition in Platyurosternarchus instead attaches to the outer surface of the dentary. Megadontognathus, an apteronotid genus not amongst the outgroup taxa of this study, has teeth arising from the outer surface of the dentary. Dentary dentition in this genus is limited to the posterior portion of the dorsal margin of the dentary (Mago-Leccia, 1994: fig. 58), whereas Platyurosternarchus has the dentition restricted to the anterior one-third of the dorsal margin of the bone. These positional differences indicate nonhomology of the dentition on the outer margin of the dentary in the two genera; a hypothesis supported by the pronounced phylogenetic separation of Megadontognathus and Platyurosternarchus(Albert, 2001).

Palatine arch

21. Degree of development of anterior portion of palatoquadrate cartilage: (0) moderately to well developed; (1) poorly developed and reduced to narrow, somewhat thread-like process (CI = 1.000; RI = 1.000).

The anterior portion of the palatoquadrate cartilage (the pars autopalatine of Hilton et al., 2007: fig. 11) ranges from moderately to well developed in most of the apteronotids incorporated into this study. Although the palatoquadrate cartilage is somewhat reduced in Platyurosternarchus relative to the condition in some other apteronotids, the cartilage remains an overall moderately well-developed, triangular structure (Fig. 8). Sternarchorhynchus species have the palatoquadrate cartilage much further reduced, resulting in a small structure (Figs 9, 10) sometimes having an almost thread-like form. Such a reduction is unique to Sternarchorhynchus amongst the taxa in this analysis.

Suspensorium and opercular series of Platyurosternarchus macrostoma, FMNH 100730; left side, lateral view, anterior to left; larger stippling represents cartilage.

Suspensorium and opercular series of Platyurosternarchus macrostoma, FMNH 100730; left side, lateral view, anterior to left; larger stippling represents cartilage.

Suspensorium and opercular series of Sternarchorhynchus chaoi, INPA 20851; left side, lateral view, anterior to left; larger stippling represents cartilage.

Suspensorium and opercular series of Sternarchorhynchus chaoi, INPA 20851; left side, lateral view, anterior to left; larger stippling represents cartilage.

Suspensorium and opercular series of Sternarchorhynchus goeldii, MCZ 46887; right side, lateral view, anterior to left; larger stippling represents cartilage.

Suspensorium and opercular series of Sternarchorhynchus goeldii, MCZ 46887; right side, lateral view, anterior to left; larger stippling represents cartilage.

22. Position of endopterygoid: (0) endopterygoid positioned along dorsomedial margin of lower jaw with no, or limited, overlap by ventral portion of endopterygoid of posterodorsal process of dentary and posterodorsal region of anguloarticular; endopterygoid largely positioned above dorsal margins of dentary and anguloarticular; (1) endopterygoid positioned along medial surface of dentary and anguloarticular with dorsal margin of endopterygoid located distinctly ventral of dorsal margins of dentary and anguloarticular (CI = 1.000; RI = 1.000).

Most members of the Gymnotiformes have the endopterygoid (the mesopterygoid or entopterygoid of some previous authors) positioned distinctly dorsal of the dorsal margin of the bones of the lower jaw (e.g. A. albifrons, Chardon & De La Hoz, 1974: fig. 20; state 0) or demonstrate limited overlap of the endopterygoid and the dorsal portions of the dentary and anguloarticular (e.g. species of Platyurosternarchus, Sternarchorhamphus muelleri, and more distant outgroups in the analysis; also state 0). The palatine arch is shifted ventrally relative to the lower jaw in the species of Sternarchorhynchus as part of the revamping necessitated by space restrictions along the vertical axis in the elongate tubular snout characteristic of the genus. One of the changes required to accommodate a dorsoventrally narrow snout is the shift dorsally of the lower jaw relative to the endopterygoid. This repositioning results in the endopterygoid being situated medial to the dentary and anguloarticular in Sternarchorhynchus(state 1). The dorsal margin of the endopterygoid is consequently positioned ventral of the dorsal border of the dorsal portions of the dentary and anguloarticular; a unique arrangement of these elements amongst examined gymnotiforms.

23. Form of anterior portion of endopterygoid: (0) anterior portion of bone attenuating, but with anterior region approximately as wide transversely as high; (1) anterior portion of bone very narrow and compressed transversely and distinctly higher than wide (CI = 1.000; RI = 1.000).

All examined outgroup gymnotiforms have a dorsoventrally higher anterior portion of the endopterygoid, which, nonetheless, extends medially to an extent such that it is approximately as wide transversely as it is high (state 0; Fig. 8). Sternarchorhynchus species have an unusual endopterygoid that is very narrow along the anteroposterior axis and reduced along the transverse plane. As a consequence, the endopterygoid is proportionally higher vertically than wide transversely state 1; (Figs 9, 10).

An endopterygoid with the anterior portion larger than the posterior section of the bone was proposed as a synapomorphy for Platyurosternarchus, Sternarchorhynchus, and Ubidia by Triques (2005; ossification termed the mesopterygoid therein). The landmark delimiting those portions of the bone was not identified. That issue aside, although the endopterygoid is distinctly elongate in Sternarchorhynchus(Fig. 9), the proportional length of the bone in Platyurosternarchus(Fig. 8; Albert, 2001: fig. 31a) is comparable to that not only in Ubidia(Miles, 1945: fig. 11) but also species of Apteronotus(e.g. A. albifrons, Chardon & De La Hoz, 1974: fig. 6; Apteronotus bonapartii, Hilton & Cox-Fernandes, 2006: fig. 5). Elongation of the element is thus of broader occurrence and a more encompassing analysis is necessary to determine the phylogenetic level at which it is applicable.

24. Form of posterior portion of endopterygoid: (0) relatively high vertically and broadly overlapping dorsal portion of quadrate and anterodorsal region of metapterygoid; not fused to quadrate or metapterygoid; (1) vertically reduced to very elongate, somewhat horizontally aligned process extending along and either tightly attached to lateral surface of quadrate and metapterygoid or fused to quadrate (CI = 1.000; RI = 1.000).

The typical form of the posterior portion of the endopterygoid in gymnotiforms including Orthosternarchus tamandua(Hilton et al., 2007: fig. 11), the species of Platyurosternarchus(Albert, 2001: fig. 31a), Sternarchorhamphus muelleri(Albert, 2001: fig. 31b), and examined outgroups is a vertically relatively expansive ossification closely applied and tightly attached to the lateral surface of portions of the quadrate and metapterygoid (state 0; Fig. 8). Sternarchorhynchus has the vertical extent of the posterior region of the endopterygoid dramatically reduced to an elongate process extending along the middle of the lateral portions of the quadrate and metapterygoid (state 1; Figs 9, 10). The reduction of the primitively broad contact between the overlapping endopterygoid, quadrate, and metapterygoid and the consequent decrease in the area of contact of the entopterygoid with the posterior elements of the palatine arch is offset by the very tight association of the reduced posterior portion of the endopterygoid with the lateral surface of the quadrate in the species of Sternarchorhynchus. Maximal connectivity of the endopterygoid and quadrate is achieved in S. goeldii and S. oxyrhynchus where these bones are fused into a single ossification without trace in adults of the original line of contact (Fig. 10; see character 25).

25. Association of quadrate and endopterygoid: (0) bones closely applied but separate; (1) bones fused (CI = 1.000; RI = 1.000).

The quadrate and endopterygoid are closely applied to each other but separate in examined outgroup gymnotiforms and in all species of Sternarchorhynchus other than S. goeldii and S. oxyrhynchus(state 0; Fig. 9). These two species have the quadrate and endopterygoid fused into a single anteroposteriorly elongate ossification without any indication of the original joint between those elements (state 1; Fig. 10).

26. Ascending process of endopterygoid: (0) absent; (1) present (CI = 0.250; RI = 0.500).

Diverse gymnotiforms have a variably elongate, dorsally directed ascending process of the endopterygoid arising from the dorsal surface of that bone (e.g. Sternarchella sima, Albert, 2001: fig. 31c). Such a process is present in S. mormyrus but absent across all of the other members of the genus examined osteologically (state 1). The feature was, nonetheless, included in the analysis because it may serve to define a small assemblage of species once the remaining recognized members of the genus are examined osteologically or further species of Sternarchorhynchus are described and evaluated as to their phylogenetic relationships. Within proximate outgroups such an endopterygoid process is encountered in P. crypticus but is absent in P. macrostoma. A comparable process is also present in smaller individuals of Sternarchorhamphus muelleri but is lost ontogenetically. We were unable to code S. axelrodi for this feature.

Possession of the process optimized as a synapomorphy for the clade consisting of Platyurosternarchus, Sternarchorhynchus, and Ubidia in the analysis by Triques (2005) that included a limited number of Sternarchorhynchus species. The process is only present in one species in the genus and one of the two species of Platyurosternarchus and as such does not serve to unit those three genera.

27. Attachment area of pterygocranial ligament: (0) limited to dorsal surface of endopterygoid; (1) involving both endopterygoid and quadrate (CI = 0.250; RI = 0.727).

The pterygocranial ligament extends posterodorsally from the suspensorium to an attachment on the orbitosphenoid. In examined outgroups along with S. caboclo, Sternarchorhynchus chaoi, S. curvirostris, S. curumim, S. goeldii, Sternarchorhynchus higuchii, Sternarchorhynchus mendesi, S. mesensis, S. mormyrus, S. oxyrhynchus, and S. roseni, the ligament attaches onto the suspensorium solely via a small dorsal process extending from the endopterygoid (state 0; Fig. 11A) that is best viewed from an oblique dorsoventral angle. Sternarchorhynchus britskii, Sternarchorhynchus cramptoni, S. gnomus, Sternarchorhynchus hagedornae, Sternarchorhynchus inpai, Sternarchorhynchus jaimei, Sternarchorhynchus mareikeae, Sternarchorhynchus montanus, Sternarchorhynchus retzeri, Sternarchorhynchus starksi, S. stewarti, and S. severii instead have the quadrate also contributing the attachment of the pterygocranial ligament on the suspensorium. This attachment site most often has the form of a dorsal process extending along the surface of the dorsal process of the entopterygoid and often completely overlapping the corresponding portion of the entopterygoid from medial view (state 1; Fig. 11B). We were unable to code S. axelrodi for this feature from radiographs of the type series.

Anterior portion of quadrate and associated structures of A, Sternarchorhynchus mormyrus, USNM 306843 (note: small bony process at base of pterygocranial ligament is part of endopterygoid), and B, Sternarchorhynchus retzeri, FMNH 11515; showing differing contributions of bones to attachment areas for pterygocranial ligament; process of endopterygoid shown in (A) is hidden by process of quadrate in (B); right side, medial view, anterior to left.

Anterior portion of quadrate and associated structures of A, Sternarchorhynchus mormyrus, USNM 306843 (note: small bony process at base of pterygocranial ligament is part of endopterygoid), and B, Sternarchorhynchus retzeri, FMNH 11515; showing differing contributions of bones to attachment areas for pterygocranial ligament; process of endopterygoid shown in (A) is hidden by process of quadrate in (B); right side, medial view, anterior to left.

28. Overall form of dorsal portion of quadrate: (0) with relatively broad, anterodorsal plate-like process; (1) without broad, anterodorsal plate-like process (CI = 1.000; RI = 1.000).

Outgroup gymnotiforms in this study including the immediate outgroups in this analysis (e.g. P. macrostoma, Sternarchorhamphus muelleri; Albert, 2001: fig. 31a, b) have a variably well-developed plate-like process of the quadrate that extends dorsal of the articular condyle that contacts the lower jaw (state 0; Fig. 8). In the species of Sternarchorhynchus, the quadrate is dorsoventrally shallower and lacks this dorsal process on the bone (state 1; Figs 9, 10).

29. Form of dorsal margin of quadrate: (0) straight to slightly convex; (1) concave (CI = 1.000; RI = 1.000).

A straight or convex dorsal margin along the plate-like dorsal process of the quadrate typifies most gymnotiforms including all proximate outgroups for Sternarchorhynchus in this analysis [e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 11); Sternarchorhamphus muelleri(Albert, 2001: fig. 31a, b)]. As described under character 28, Sternarchorhynchus species have the quadrate reduced vertically. Sternarchorhynchus axelrodi, S. goeldii, S. mormyrus, and S. oxyrhynchus, nonetheless, retain a straight to slightly convex dorsal margin of the reduced quadrate comparable to the form of that portion of the ossification in outgroups (state 0; Fig. 10). All other members of Sternarchorhynchus have a distinctly concave dorsal margin to the quadrate (state 1; Fig. 9).

30. Form of posterolateral portion of quadrate: (0) without lateral expansion; (1) with lateral expansion into rounded ridge terminating posteriorly in articular facet (CI = 1.000; RI = 1.000).

Outgroups have the lateral surface of the quadrate unelaborated (state 0), with the posterolateral region of this bone overlapped laterally by the endopterygoid [e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 11)]. The posterolateral portion of the quadrate is exposed laterally but unelaborated in all Sternarchorhynchus species other than S. goeldii and S. oxyrhynchus and is consequently coded as state 0 for these taxa (Fig. 9). Sternarchorhynchus goeldii and S. oxyrhynchus bear a lateral expansion of the lateral surface of the posterodorsal portion of the quadrate, with the expanded region having the form of a raised, laterally rounded, longitudinally aligned ridge terminating in a distinct posterior facet (state 1; Fig. 10) that articulates with a corresponding facet on the anterior surface of the metapterygoid (see character 39).

31. Form of posteroventral margin of main body of quadrate that receives anterior tip of symplectic: (0) with distinct notch; notch open laterally and most often also medially; (1) notch very small or absent and when present enclosed laterally and medially by vertical sheet of bone (CI = 0.333; RI = 0.500).

The narrow anterior portion of the symplectic fits into a distinct concavity in the posteroventral margin of the main body of the quadrate in proximate outgroups to Sternarchorhynchus together with all examined members of the genus other than S. mormyrus and perhaps S. axelrodi(state 0). In S. mormyrus the anteriormost portion of the symplectic fits into a dorsally open pocket bordered medially and laterally by vertical bony processes arising from the quadrate (state 1). Such a modification of the quadrate also appears to be present in S. axelrodi based on radiographs of several specimens of this species. Looking to more distant outgroups, we find that state 1 is present in A. albifrons, ‘A.’ apurensis, and Sternarchogiton labiatus. The character was included in the analysis as a potential synapomorphy for S. axelrodi and S. mormyrus that are herein proposed as sister taxa and perhaps for other species not examined osteologically in this study.

32. Position of area of articulation of symplectic with quadrate: (0) anterior portion of symplectic fitting into notch or bony pocket along posteroventral margin of quadrate located anterior of ventral limit of joint between quadrate and metapterygoid; (1) anterior portion of symplectic fitting into notch or pocket along posteroventral margin of quadrate located distinctly posterior of ventral limit of joint between quadrate and metapterygoid (CI = 0.500; RI = 0.500).

The notch or pocket in the posteroventral portion of the quadrate that accommodates the anterior tip of the symplectic is located anterior to, distinctly anterior of, the ventral terminus of the quadrate–metapterygoid joint in examined outgroups to Sternarchorhynchus and in all species in the genus other than S. curvirostris, S. goeldii, and S. oxyrhynchus(state 0; Fig. 9). These three species have the articulation of these ossifications shifted distinctly posteriorly relative to that landmark and situated ventral of the metapterygoid (state 1; Fig. 12).

Posterior portion of quadrate, anterior portion of metapterygoid and symplectic, and adjoining bones in Sternarchorhynchus curvirostris, USNM 163887; left side, lateral view, anterior to left; larger stippling represents cartilage.

Posterior portion of quadrate, anterior portion of metapterygoid and symplectic, and adjoining bones in Sternarchorhynchus curvirostris, USNM 163887; left side, lateral view, anterior to left; larger stippling represents cartilage.

33. Lateral closure of notch along posteroventral margin of quadrate that receives symplectic: (0) present; (1) absent (CI = 0.250; RI = 0.500).

Outgroup taxa in the analysis along with S. axelrodi, S. britskii, S. curvirostris, S. curumim, S. gnomus, S. goeldii, S. hagedornae, S. higuchii, S. inpai, S. mareikeae, S. mendesi, S. montanus, S. mormyrus, S. oxyrhynchus, S. severii, S. starksi, and S. stewarti have the lateral portion of the notch in the quadrate that receives the anterior tip of the symplectic covered, or largely covered, by a thin sheet of bone arising from the main body of the quadrate (state 0; Figs 10, 12). This thin bony sheet is absent in S. caboclo, S. chaoi, S. cramptoni, S. jaimei, S. mesensis, S. retzeri, and S. roseni(state 1; Fig. 9). A comparable bony sheet appears to be present in S. axelrodi based on radiographs.

34. Overlap laterally of anteroventral corner of metapterygoid by bony process of quadrate: (0) absent; (1) present (CI = 0.333; RI = 0.600).

The anteroventral corner of the metapterygoid proximate to the joint between this ossification and the quadrate is laterally exposed in most gymnotiforms including all examined outgroups to Sternarchorhynchus(state 0). Sternarchorhynchus curvirostris, S. gnomus, S. hagedornae, S. higuchii, S. mareikeae, and S. starksi have a further development dorsally of the bony sheet such that it overlaps the lateral surface of the notch in the posteroventral portion of the quadrate. This expansion results in the lateral overlap of the anteroventral corner of the metapterygoid by the dorsal process of the quadrate in these species (state 1; Fig. 12).

35. Location of posterior limit of posteroventral process of quadrate: (0) when present, process terminates approximately at, or slightly posterior of, vertical through ventral portion of joint between quadrate and metapterygoid; (1) process terminates approximately at vertical through two-thirds of length of metapterygoid or posterior of that point (CI = 1.000; RI = 1.000).

Most gymnotiforms either lack a posteroventral process of the quadrate that extends ventral of the symplectic and over the anteroventral process of the preopercle (e.g. Gymnotus carapo, Chardon & De La Hoz, 1974: fig. 4) or have a relatively short process that terminates approximately at, or slightly posterior of, the vertical through the ventral portion of the area of contact of the metapterygoid and quadrate (state 0). The latter condition is present in all examined outgroups to Sternarchorhynchus[e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 11); Platyurosternarchus(Albert, 2001: fig. 31a); Sternarchorhamphus muelleri(Albert, 2001: fig. 31b)]. Sternarchorhynchus species have a much more elongate posteroventral process of the quadrate. This process extends from the body of the ossification as a narrow flange of bone running ventral of the symplectic to a point at least as far posterior as the vertical located two-thirds of the distance along the length of the metapterygoid (state 1; Figs 9, 10, 12).

36. Overall form of symplectic: (0) symplectic relatively slender, with height of posteriormost portion of bone approximately one-fifth length of bone; (1) symplectic proportionally vertically higher, with height of posteriormost portion of bone approximately one-third length of bone (CI = 0.250; RI = 0.625).

Species of Sternarchorhynchus other than S. axelrodi, S. goeldii, S. mormyrus, and S. oxyrhynchus have a relatively slender symplectic that from a lateral view gradually increases posteriorly in its vertical extent. As a consequence, the height of the posteriormost portion of the symplectic is approximately one-fifth of its length (state 0). This symplectic form also occurs in A. cuchillo, A. rostratus, Orthosternarchus tamandua(Hilton et al., 2007: fig. 11), P. crypticus, and Sternarchorhamphus muelleri in the outgroups (state 0). Sternarchorhynchus axelrodi, S. chaoi, S. goeldii, S. mormyrus, and S. oxyrhynchus have instead a distinctly more posterodorsally angled dorsal margin of the symplectic with a resultant greater disparity in the height of the anterior versus posterior portions of the bone (state 1). State 1 is present amongst the examined outgroups in A. albifrons, ‘A.’ apurensis, P. macrostoma, Sternarchella orthos, and Sternarchogiton labiatus.

37. Anterodorsally directed process on dorsal margin of symplectic:(0) absent; (1) present (CI = 0.250; RI = 0.667).

The dorsal margin of the symplectic is straight to variably irregular in outgroups to Sternarchorhynchus(Hilton et al., 2007: fig. 11, for condition in Orthosternarchus tamandua) and nearly all species of the genus (state 0; Fig. 9). Sternarchorhynchus cramptoni, S. curumim, S. curvirostris, S. goeldii, S. hagedornae, S, mareikeae, S. oxyrhynchus, S. retzeri, S. starksi, and S. stewarti rather have the dorsal portion of the symplectic elaborated into an anteriorly and slightly medially directed process (state 1; Figs 10, 12).

38. Form of metapterygoid: (0) approximately triangular overall; (1) somewhat to distinctly horizontally elongate and approximately quadrilateral (CI = 1.000; RI = 1.000).

The metapterygoid in outgroups to Sternarchorhynchus is approximately triangular [e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 11); Platyurosternarchus(Albert, 2001: fig. 31a); Sternarchorhamphus(Albert, 2001: fig. 31b)] with the posterior portion tapering to a variably distinct point (state 0; Fig. 8). Species of Sternarchorhynchus, in contrast, have a more elongate metapterygoid with an overall quadrilateral form (state 1; Figs 9, 10).

39. Form of lateral surface of dorsal portion of metapterygoid: (0) overall flat or gently convex with minimal thickening; (1) with distinct rounded lateral ridge terminating anteriorly in well-developed articular surface (CI = 1.000; RI = 1.000).

Outgroups to Sternarchorhynchus have the posterior portion of the endopterygoid overlapping the anterodorsal region of the metapterygoid, with the latter ossification having a smooth lateral surface [e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 11 a)]. The dorsal portion of the metapterygoid in Sternarchorhynchus is not overlapped by the endopterygoid; however, all members of the genus other than S. goeldii and S. oxyrhynchus have the exposed portion of the metapterygoid unelaborated laterally or bearing at most some minor ridges and/or expansions on that surface (state 0; Fig. 9). Both S. goeldii and S. oxyrhynchus have a lateral elaboration of the dorsolateral portion of the metapterygoid expanded into a thickened, laterally rounded, anteroposteriorly aligned ridge. This metapterygoid expansion is particularly well developed anteriorly, where it forms a broad facet that articulates with a corresponding process of the quadrate (state 1, Fig. 10; see also character 30).

40. Association of posterior tip of posteroventral process of quadrate and dorsal margin of horizontal arm of preopercle: (0) tip of posteroventral process of quadrate positioned above dorsal margin of preopercle; (1) tip of posteroventral process of quadrate fitting into variably developed notch along dorsal margin of preopercle (CI = 1.000; RI = 1.000).

Outgroups to Sternarchorhynchus and S. goeldii, S. oxyrhynchus, and S. mormyrus have the elongate posteroventral process of the quadrate running along and closely applied to the dorsal margin of the horizontal arm of the preopercle. In that condition the tip of the quadrate process is neither enveloped posteriorly nor dorsally by the preopercle (state 0; Fig. 10). We were unable to code this feature for S. axelrodi. All remaining species of Sternarchorhynchus have the dorsal margin of the preopercle expanded dorsally in the region posterior to the tip of the posteroventral process of the quadrate. Consequently, the posteriormost portion of the quadrate lies within a depression along the dorsal margin of the preopercle (state 1; Figs 9, 12). Species with this expansion often, but not always, have the dorsally expanded portion of the preopercle elaborated into an anterodorsally directed process extending towards or over the tip of the quadrate process, sometimes encompassing the tip.

41. Anterior extent of preopercle: (0) terminating variably posterior of vertical through ventral portion of joint between quadrate and metapterygoid; (1) extending to location anterior of vertical through ventral portion of joint between quadrate and metapterygoid (CI = 0.143; RI = 0.625).

Examined apteronotid outgroups and S. axelrodi, S. chaoi, S. cramptoni, S. hagedornae, S. montanus, S. mormyrus, and S. starksi have the horizontal arm of the preopercle extending anteriorly to a point situated distinctly posterior of the ventral portion of the quadrate–metapterygoid joint (state 0; Figs 8, 9). Sternarchorhynchus britskii, S. caboclo, S. curumim, S. curvirostris, S. gnomus, S. goeldii, S. higuchii, S. inpai, S. jaimei, S. mareikeae, S. mendesi, S. mesensis, S. oxyrhynchus, S. retzeri, S. roseni, S. severii, and S. stewarti instead possess an anteroposteriorly longer horizontal arm of the preopercle terminating anteriorly somewhat, to distinctly, anterior of the vertical through the ventral portion of this joint (state 1; Fig. 10).

42. Form of laterosensory canal segment in vertical arm of preopercle: (0) in form of two shallowly anteriorly concave segments; (1) in form of two deeply anteriorly concave segments (CI = 0.167; RI = 0.545).

The portion of the laterosensory canal system in the vertical arm of the preopercle in Sternarchorhynchus has the form of two anteriorly concave segments that join, or nearly meet, approximately midway along the vertical length of the bone. Two major alternative morphologies of this system occur in the genus. In S. axelrodi, S. caboclo, S. chaoi, S. cramptoni, S. curumim, S. goeldii, S. mormyrus, S. oxyrhynchus, S. retzeri, and S. severii, the canals form deeply anteriorly convex arcs (state 1; Figs 9, 10, 13A). State 1 is also present in ‘A.’ apurensis and P. macrostoma in the outgroups. Sternarchorhynchus britskii, S. curvirostris, S. gnomus, S. higuchii, S. hagedornae, S. inpai, S. jaimei, S. mareikeae, S. mendesi, S. mesensis, S. montanus, S. roseni, S. starksi, and S. stewarti also have concave canal segments such as those in congeners, but with the convexity of the laterosensory canal segments much less pronounced and in the form of distinctly shallow arcs (state 0; Fig. 13B). State 0 is also present in the outgroups in the three examined species of Apteronotus, Orthosternarchus tamandua, P. crypticus, Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri.

Preopercle of A, Sternarchorhynchus severii, INPA 22898; and B, Sternarchorhynchus gnomus, INPA 25636; showing differing forms of laterosensory canals in bone; left side, lateral view, anterior to left.

Preopercle of A, Sternarchorhynchus severii, INPA 22898; and B, Sternarchorhynchus gnomus, INPA 25636; showing differing forms of laterosensory canals in bone; left side, lateral view, anterior to left.

43. Form of dorsal margin of opercle: (0) very slightly concave to distinctly convex; (1) distinctly concave (CI = 0.250; RI = 0.500).

Outgroups to Sternarchorhynchus have the dorsal margin of the opercle ranging from very slightly concave to distinctly convex, with the species of Platyurosternarchus plus Sternarchorhamphus muelleri having slightly concave margins to the bone (state 0; Fig. 8). That condition also occurs amongst more distant outgroups in A. albifrons, A. cuchillo, ‘A.’ apurensis, and Sternarchogiton labiatus. Sternarchorhynchus, in contrast, has the opercular margin distinctly concave (Figs 9, 10) with this condition (state 1) also present amongst outgroups in A. rostratus, Orthosternarchus tamandua(Hilton et al., 2007: fig. 11), and Sternarchella orthos(Lundberg et al., 1996: fig. 5a).

44. Form of posterodorsal corner of opercle: (0) rounded to pointed, but not forming distinct process; (1) extended in form of distinct ‘ear-like’ process (CI = 0.500; RI = 0.875).

The posterodorsal corner of the opercle varies in form across the Gymnotiformes, ranging most often from being gently rounded [e.g. Sternarchorhamphus(Albert, 2001: fig. 31b); Orthosternarchus tamandua(Hilton et al., 2007: fig. 11)] to somewhat concave [e.g. Electrophorus electricus(Albert et al., 2005: fig. 7)]. Species of Sternarchorhynchus, in contrast, have a distally rounded ‘ear-like’ posterior extension of this region of the opercle (state 1; Figs 9, 10). This process is present solely in Sternarchella orthos amongst outgroups in this study.

45. Association of posterodorsal corner of opercle and supracleithrum: (0) rounded posterior extension of opercle lying ventral of, or very slightly overlapping, ventralmost portion of supracleithrum; (1) rounded posterior extension of opercle extending over ventralmost portion of supracleithrum (CI = 0.250; RI = 0.625).

Sternarchorhynchus species other than S. axelrodi and S. mormyrus share state 0 in which the opercle demonstrates no or at most minimal overlap of the ventral portion of the supracleithrum. State 0 is present amongst outgroups in P. crypticus, Sternarchella orthos, and Sternarchorhamphus. Cleared and stained specimens of S. mormyrus have an extensive overlap of the ventralmost region of the supracleithrum by the posterodorsal portion of the opercle. Radiographs of specimens of its sister species, S. axelrodi, revealed a comparable degree of overlap of the two bones in that species (state 1). A similar degree of extensive overlap (state 1) occurs in outgroups in A. albifrons, A. cuchillo, A. rostratus, ‘A.’ apurensis, Orthosternarchus tamandua, P. macrostoma, and Sternarchogiton labiatus.

Hyoid arch

The number of branchiostegal rays within the Gymnotiformes ranges from two to five (Albert, 2001). Variation in the number of elements involves differences in the number of rays associated with the anterior and occasionally posterior ceratohyals. One to three ceratohyals can attach to the ventral margin of the anterior ceratohyal. The fourth element has a variable area of attachment ranging from the lateral surface of the posterior portion of the anterior ceratohyal to the lateral surface of the anterior region of the posterior ceratohyal. A fifth ray, when present, attaches to the area of the joint of the anterior and posterior ceratohyals (see Mago-Leccia, 1978: fig. 13 for illustration of this arrangement of five rays in Eigenmannia virescens) or to the lateral surface of the posterior ceratohyal (see de Santana & Nogueira, 2006: fig. 3 for illustration of this condition in S. caboclo). Various authors have used total ray counts to advance hypotheses of phylogenetic relationships (e.g. McAllister, 1968). Such data, albeit informative at a general level, fail to specify the presence versus absence of individual rays. Phylogenetically useful information can be better expressed in terms of individual elements; a procedure we use herein.

46. Presence or absence of anterior most branchiostegal ray attaching to ventral margin of anterior ceratohyal:(0) absent; (1) present (CI = 0.500; RI = 0.900).

One to three branchiostegal rays attach to the ventral margin of the anterior ceratohyal; however, in the absence of landmarks that allow us to homologize elements unambiguously we only coded the number of rays attaching to this bone when fewer than three rays are present. Nonetheless, based on positional information it appears that loss of branchiostegal rays proceeds in an anterior posterior sequence that leads us to cast the discussion within that framework. The first ray is absent in Orthosternarchus tamandua, which also lacks the apparent second ray, retaining only a single branchiostegal ray attaching to the anterior ceratohyal (Hilton et al., 2007: fig. 13). The first ray is missing in all species of Platyurosternarchus, the examined species of Apteronotus, and Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri amongst the outgroup taxa (state 0) and uniquely in S. caboclo within Sternarchorhynchus. Although this condition is autapomorphic for S. caboclo amongst osteologically examined species of Sternarchorhynchus, we retained it in the analysis because it may prove to be synapomorphic for a small clade once additional species of the genus are examined osteologically.

47. Presence or absence of posteriormost branchiostegal ray attaching either to lateral surface of area of articulation of anterior and posterior ceratohyals or to anterior region of lateral surface of posterior ceratohyal:(0) present; (1) absent (CI = 1.000; RI = 1.000).

The posteriormost, typically larger, branchiostegal ray in gymnotiforms attaches to the lateral surface of the hyoid arch within the area delimited by the area of joint between the anterior and posterior ceratohyals and the anterior region of the posterior ceratohyal. Variation in the area of attachment forms a continuum across the examined taxa and we coded only presence versus absence of the ray in this analysis. This posteriormost branchiostegal ray is absent (state 1) in both Sternarchorhamphus muelleri and Orthosternarchus tamandua(Hilton et al., 2007: fig. 13), but present in Platyurosternarchus and Sternarchorhynchus(state 0). This ray is similarly present in the examined species of Apteronotus and Sternarchella orthos and Sternarchogiton labiatus.

Sternarchorhamphus muelleri and S. caboclo each have four branchiostegal rays. That number is an equivalent character state under previous coding schemes focused on total numbers of rays rather than presence versus absence of individual elements. The reduced number of branchiostegal rays is achieved in different fashions in the two taxa, being a consequence of the loss of the anteriormost ray in S. caboclo versus the absence of the posteriormost ray in Sternarchorhamphus muelleri.

48. Relative size of first and second branchiostegal rays: (0) rays of approximately same length; (1) first ray approximately one-third length of second ray (CI = 1.000; RI = 0.000).

Variation occurs in both the relative overall size and length of the first and second branchiostegal rays, but we focus in this character on the length of the rays as this is a more readily quantifiable attribute. With the single exception of S. inpai all species of Sternarchorhynchus have the first and second rays of more or less comparable lengths (state 0). In S. inpai the first ray is shortened to approximately one-third of the length of the second ray (state 1). Although this condition is autapomorphic for S. inpai within Sternarchorhynchus in the present analysis, we included it herein because it may define a small clade within the genus in future studies that incorporate species not examined osteologically in this study. This character could not be coded in outgroups to Sternarchorhynchus or in the genus for S. caboclo, all of which lack the anteriormost ray.

49. Form of anterodorsal margin of third branchiostegal ray: (0) smoothly convex; (1) with distinct angular process (CI = 1.000; RI = 1.000).

The third branchiostegal ray has an overall smoothly convex margin along its anterodorsal region in both proximate outgroups to Sternarchorhynchus[e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 13); the br1 of those authors] and the more distantly related apteronotids in the analysis (state 0). Species of Sternarchorhynchus other than S. goeldii and S. oxyrhynchus similarly share state 0. Both S. goeldii and S. oxyrhynchus have the anterodorsal portion of the third branchiostegal ray expanded into a broadly angular pointed process (state 1).

50. Length of anterior branchiostegal ray attaching to lateral surface of the anterior ceratohyal: (0) 1.5 times or less of distance from anterior limit of ventral hypohyal to posterior limit of posterior ceratohyal; (1) approximately three times distance from anterior limit of ventral hypohyal to posterior limit of posterior ceratohyal (CI = 1.000; RI = 1.000).

Species of Sternarchorhynchus and Platyurosternarchus plus most examined outgroup apteronotids have the anterior branchiostegal ray attaching to the lateral surface of the anterior ceratohyal (the fourth branchiostegal ray under the scheme of five total rays detailed above) of a length equal to, or shorter than, 1.5 times the distance from anterior limit of the ventral hypohyal to posterior limit of posterior ceratohyal. Fourth branchiostegal rays of comparable relative lengths also occur in more distantly related outgroups [e.g. Eigenmannia virescens(Mago-Leccia, 1978: fig. 15)]. Sternarchorhamphus muelleri and Orthosternarchus tamandua have significantly longer fourth rays of lengths approximately three or more times the distance from the anterior limit of the ventral hypohyal to the posterior limit of the posterior ceratohyal (see Hilton et al., 2007: fig. 13 for Orthosternarchus tamandua). Although the derived elongation of the fourth branchiostegal ray is not pertinent to the question of relationships within Sternarchorhynchus, we included the character to provide structure to the outgroup phylogeny. A comparable elongation of the second branchiostegal ray attaching to the lateral surface of the posterior portion of the hyoid arch (the fifth ray under the numbering scheme herein) characterizes Sternarchorhamphus muelleri and is apparently autapomorphic for that species.

51. Form of dorsal margin of proximate portion of anterior branchiostegal ray attaching to lateral surface of the anterior ceratohyal:(0) dorsally pointed or rounded; (1) dorsally concave with angled margins (CI = 1.000; RI = 0.000).

The dorsal portion of the anterior ray attaching to the lateral surface of the anterior ceratohyal (the fourth ray under the system described above) is either dorsally rounded or pointed in outgroups to Sternarchorhynchus in the Apteronotidae. Sternarchorhynchus caboclo, in contrast, has a strongly concave dorsal margin to the ossification demarked by distinct angled areas both anteriorly and posteriorly. This character (state 1) is autapomorphic in the present study, but was retained in the analysis as a potential synapomorphy once additional species of Sternarchorhynchus are examined osteologically.

52. Relative size of anterior portion of urohyal: (0) relatively narrow and one-fifth or less of length of main body of bone; (1) relatively wide and one-third or more of length of main body of bone (CI = 0.333; RI = 0.500).

In Orthosternarchus tamandua, Sternarchorhamphus muelleri, S. goeldii, S. mormyrus, and S. oxyrhynchus, the anterior portion of the urohyal is narrow relative to the overall size of the bone with its transverse width approximately one-fifth or less of the length of the main body (state 0; Fig. 14A). Because of the reduction of the overall proportional length of the urohyal, the anterior portion of the urohyal is relatively wide relative to the overall size of the bone in species of Sternarchorhynchus other than S. goeldii, S. mormyrus, and S. oxyrhynchus. This is reflected in a transverse width of the urohyal that is approximately one-third or more of the length of the main body of the ossification (state 1; Fig. 14B). The urohyal is dramatically reduced in size and modified in form in the outgroup apteronotids other than P. crypticus, P. macrostoma, and Sternarchella orthos but, nonetheless, fits the definition of state 1 and was coded accordingly. This character was not coded for Sternarchogiton labiatus and the three examined species of Apteronotus, all of which have highly modified urohyals. It was impossible to code S. axelrodi for this character solely from radiographs of the types.

Urohyal of A, Sternarchorhynchus oxyrhynchus, USNM 228787, and B, Sternarchorhynchus hagedornae, USNM 391574; dorsal view, anterior at top.

Urohyal of A, Sternarchorhynchus oxyrhynchus, USNM 228787, and B, Sternarchorhynchus hagedornae, USNM 391574; dorsal view, anterior at top.

53. Form of anterior margin of urohyal: (0) straight or slightly irregular, but without distinct medial notch; (1) with distinct medial notch (CI = 0.200; RI = 0.556).

The anterior margin of the urohyal is transversely nearly straight or at most slightly irregular in Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchorhamphus muelleri, and most Sternarchorhynchus species (state 0; Fig. 14A). Sternarchorhynchus chaoi, S. cramptoni, S. curumim, S. hagedornae, S. higuchii, S. jaimei, S. mesensis, S. montanus, S. roseni, and S. stewarti alternatively have a distinct medial notch along the anterior margin of the urohyal (state 1; Fig. 14B). This character could neither be coded for the other outgroup apteronotids in which the urohyals are dramatically reduced in relative size and also modified in form, nor for S. starksi, which had a deformed urohyal in the single available cleared and stained specimen of the species and S. axelrodi in the absence of cleaned and stained specimens.

Gill arches

54. Form of basihyal: (0) without lateral processes; (1) with lateral processes (CI = 0.250; RI = 0.773).

The anteriormost component of the median ossifications of the lower half of the gill arches in all examined apteronotids is an elongate ossification identified by Hilton et al.(2007) as a fused basihyal plus first basibranchial. Underlying this assumption was the fact that the ossification occupies the area encompassed by those elements in some other gymnotiform families (e.g. Sternopygidae, see Mago-Leccia, 1978: fig. 7) or by the ossified basihyal plus cartilaginous first basibranchial in some taxa (e.g. Eigenmannia, Mago-Leccia, 1978: fig. 17). In Sternarchorhynchus the posterior limit of the anterior element terminates between the anterior portions of the first hypobranchials, the typical position for the end of the basihyal and as such it appears that basibranchial 1 is absent. We consequently identify the elongate anterior ossification as the basihyal. Apteronotus albifrons, A. cuchillo, A. rostratus, ‘A.’ apurensis, Orthosternarchus tamandua(Hilton et al., 2007: fig. 14), P. crypticus, P. macrostoma, Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri have this ossification ranging from rod-like to slightly expanded transversely anteriorly, but without distinct lateral processes (state 0; Fig. 15). Within Sternarchorhynchus an unelaborated form of the basihyal (state 0) is restricted to S. britskii, S. goeldii, S. higuchii, S. jaimei, S. oxyrhynchus, and S. roseni. Other species of Sternarchorhynchus have definite lateral processes along at least the posterior half of the bone (state 1; Figs 16, 17).

Ventral portion of gills arches of Sternarchorhynchus goeldii, MCZ 46887; dorsal view, anterior at top; larger stippling represents cartilage; small elements along anterior margins of ceratobranchials are highly reduced gill rakers.

Ventral portion of gills arches of Sternarchorhynchus goeldii, MCZ 46887; dorsal view, anterior at top; larger stippling represents cartilage; small elements along anterior margins of ceratobranchials are highly reduced gill rakers.

Ventral portion of gills arches of Sternarchorhynchus caboclo, INPA 10594; dorsal view, anterior at top; larger stippling represents cartilage; small elements along anterior margins of ceratobranchials are highly reduced gill rakers.

Ventral portion of gills arches of Sternarchorhynchus caboclo, INPA 10594; dorsal view, anterior at top; larger stippling represents cartilage; small elements along anterior margins of ceratobranchials are highly reduced gill rakers.

Ventral portion of gills arches of Sternarchorhynchus hagedornae, USNM 391574; dorsal view, anterior at top; larger stippling represents cartilage; highly reduced gill rakers along anterior margins of ceratobranchials not illustrated.

Ventral portion of gills arches of Sternarchorhynchus hagedornae, USNM 391574; dorsal view, anterior at top; larger stippling represents cartilage; highly reduced gill rakers along anterior margins of ceratobranchials not illustrated.

55. Presence or absence of medial ridge on posterior portion of dorsal surface of basihyal:(0) present; (1) absent (CI = 1.000; RI = 1.000).

Various gymnotiforms have a medial ridge on the dorsal surface of the posterior portion of the basihyal (Triques, 1993: figs 12, 13). Such a process (state 0; Figs 15–17) is present in all outgroups in this study other than A. rostratus, Orthosternarchus tamandua, and Sternarchella orthos and all species of Sternarchorhynchus other than S. inpai and S. montanus(state 0). This ridge is absent in those two species (state 1). This character could not be coded for in S. axelrodi from radiographs.

56. Degree of separation between basihyal and second basibranchial: (0) ossifications in close proximity with posterior limit of basihyal extending posteriorly nearly to transverse to posterior limit of ossified portions of first hypobranchial; (1) ossifications distinctly separated with posterior limit of basihyal extending posteriorly only to, or proximate to, anterior region of ossified portion of first hypobranchial (CI = 1.000; RI = 1.000).

One notable variable amongst examined taxa involves the degree of the extension posteriorly of the basihyal. The condition typical for most apteronotids including the outgroups to Sternarchorhynchus in this study has a relatively small gap between the posterior limit of the basihyal and the anterior margin of the second basibranchial (state 0). This posterior position of the terminus of the basihyal is reflected in the location of the end of the ossification approximately at the transverse to the posterior limit of the ossified portion of the first hypobranchial (Hilton et al., 2007: fig. 14) for an illustration of this condition in Orthosternarchus tamandua). The examined species of Sternarchorhynchus, in contrast, have the posterior limit of the basihyal situated between the anterior portions of the contralateral first hypobranchials or anterior of that position (state 1; Figs 15–17). This anterior position of the terminus of the basihyal results in a major gap between that ossification and the anterior limit of the first hypobranchial.

57. Form of first hypobranchial: (0) triangular overall with transverse dimension of ossified portion slightly less than one-half length of that region; (1) elongate with transverse dimension of ossified portion approximately one-quarter to one-fifth length of that region (CI = 1.000; RI = 1.000).

An overall triangular first hypobranchial (state 0; Figs 15–17) is broadly distributed across all examined taxa in the Apteronotidae and indeed elsewhere in the Gymnotiformes (Mago-Leccia, 1978: fig. 7 for condition in a representative of the Sternopygidae). Within the Apteronotidae, a distinct elongation of the first hypobranchial occurs in both Orthosternarchus tamandua and Sternarchorhamphus muelleri(state 1) amongst the taxa examined in this study. Although not directly informative as to intrageneric relationships within Sternarchorhynchus, we retained the character in the analysis to provide structure within the outgroup phylogeny.

58. Form of second basibranchial: (0) with midlength constriction; (1) with lateral margins running approximately in parallel (CI = 0.250; RI = 0.700).

The majority of species of Sternarchorhynchus along with all of the examined outgroups have the second basibranchial constricted to a variable, but nonetheless pronounced, degree along its midlength (state 0). As a consequence, the element has a variable hour-glass shape from dorsal view (Figs 15, 16). In some outgroup taxa [e.g. Orthosternarchus tamandua(Hilton et al., 2007: fig. 14)], the second basibranchial is proportionally more elongate with a relatively less pronounced but still obvious midlength constriction. A subset of the species of Sternarchorhynchus(S. chaoi, S. cramptoni, S. curvirostris, S. hagedornae, S. jaimei, S. mesensis, S. mormyrus, S. retzeri, S. severii, S. starksi, and S. stewarti) have a bony plate extending laterally from the lateral margins of the constricted portion of the main body of the second basibranchial (state 1; Fig. 17). These lateral extensions result in parallel lateral margins to the second basibranchial from a dorsal view, albeit with the constricted main body of the bone still obvious; more so, in transmitted light (state 1). This character could not be coded for S. axelrodi from radiographs.

59. Form of anterior portion of second hypobranchial: (0) without distinct anterior extension from main body; (1) with distinct anterior extension from main body (CI = 1.000; RI = 1.000).

The second hypobranchial is a variably anteroposteriorly elongate bone amongst gymnotiforms, typically with an progressively tapering anterior region that extends along the lateral margin of the second basibranchial. Although outgroups to Sternarchorhynchus along with S. goeldii and S. oxyrhynchus have the anterior portion of this ossification somewhat pointed overall, the region is not developed into a distinct narrow, often somewhat slightly medially curving process (state 0; Fig. 15). All Sternarchorhynchus species other than S. goeldii and S. oxyrhynchus have the anterior portion of the second hypobranchial more attenuate, with this anterior extension often slightly medially curving anteriorly (state 1; Figs 16, 17).

60. Degree of development of medial portion of second hypobranchial in adults: (0) with distinct medial process extending ventral of second basibranchial and approaching contralateral second hypobranchial; (1) without distinct medial process and with contralateral second hypobranchials separated by second basibranchial (CI = 0.500; RI = 0.000).

The second hypobranchial in adults of nearly all outgroups and all species of Sternarchorhynchus has a medial extension directed towards its contralateral partner in the area ventral of the second basibranchial. Amongst species for which we had a size range of osteological preparations, the extension is less developed in smaller individuals but ontogenetically expands into a distinctly pointed process (state 0; Figs 15, 17). Uniquely amongst the species examined herein, S. caboclo and S. higuchii lack that medial process of the second hypobranchial. They instead have the contralateral ossifications completely separated by the second basibranchial and the cartilage that lies between that ossification and the third basibranchial (state 1; Fig. 16). This character could not be coded for in S. axelrodi from radiographs.

61. Degree of constriction of medial portion of fourth ceratobranchial: (0) constriction limited when present; (1) constriction very distinct (CI = 1.000; RI = 1.000).

When viewed from the dorsal view, the fourth ceratobranchial has nearly parallel lateral and medial margins, or tapers slightly anteriorly in Orthosternarchus tamandua(Hilton et al., 2007: fig. 14) and Sternarchorhamphus muelleri(state 0). Most examined apteronotids and more distantly related gymnotiform taxa either retain that morphology of the fourth ceratobranchial or have on occasion a lateral expansion of the anterior portion of that ossification (e.g. Rhabdolichops troschelii, Mago-Leccia, 1978: fig. 7). Species of Platyurosternarchus and Sternarchorhynchus, however, have the anterior portion of the fourth ceratobranchial transversely constricted along the portion of the bone somewhat distal of the region where the ossified portion of the ceratobranchial merges into the cartilage that caps the bone (state 1; Figs 15–17). Such constriction of the fourth ceratobranchial is particularly pronounced in S. goeldii amongst species of the genus represented by cleared and stained specimens in this analysis. That condition may serve to define a subunit of that genus once additional congeners are examined osteologically.

62. Condition of fourth infrapharyngobranchial: (0) totally cartilaginous; (1) ossified (CI = 1.000; RI = 1.000).

The fourth infrapharyngobranchial is an anteroposteriorly elongate, posteriorly wider cartilaginous body without ossification centres in both Orthosternarchus tamandua(Hilton et al., 2007: fig. 14) and Sternarchorhamphus muelleri. An entirely cartilaginous fourth infrapharyngobranchial is, furthermore, general amongst other outgroups in the Apteronotidae (e.g. Adontosternarchus, Mago-Leccia et al., 1985: fig. 5) and across the Gymnotiformes (e.g. the sternopygids Eigenmannia and Rhabdolichops, Mago-Leccia, 1978: figs 9, 17). It thus represents the primitive condition across the Gymnotiformes (state 0). The species of Sternarchorhynchus and Platyurosternarchus have a well-ossified fourth infrapharyngobranchial with an overall triangular, posteriorly widening form (state 1; Fig. 18). The ossified portion of the fourth infrapharyngobranchial is bordered anteriorly by a small cartilage cap that contacts the cartilages of the third epibranchial and third infrapharyngobranchial and terminates posteriorly along a transversely elongate cartilage that broadly articulates with the cartilage along the anterior margin of the fourth epibranchial.

Dorsal portion of gills arches of Sternarchorhynchus hagedornae, USNM 391574; right side, ventral view, anterior at top; larger stippling represents cartilage.

Dorsal portion of gills arches of Sternarchorhynchus hagedornae, USNM 391574; right side, ventral view, anterior at top; larger stippling represents cartilage.

63. Degree of medial development of cartilage along anterior margin of first epibranchial: (0) cartilage extending along lateral, but not anterior margin of cartilage at anterior of second infrapharyngobranchial; (1) cartilage expanded medially and extending along all, or nearly all, of anterior margin of cartilage along anterior margin of second infrapharyngobranchial (CI = 1.000; RI = 1.000).

Two primary conditions of the cartilages along the anterior limits of the first epibranchial and the second infrapharyngobranchial occur in Sternarchorhynchus and its proximate outgroups. Orthosternarchus tamandua(Hilton et al., 2007: fig. 14), Sternarchorhamphus muelleri, and other examined outgroup apteronotids have a moderately sized cartilage along the anterior margin of the first epibranchial. This cartilage extends solely along the lateral margin of the cartilage that caps the anterior limit of the ossified portion of the second infrapharyngobranchial. In this configuration, the cartilage that anteriorly caps the second infrapharyngobranchial retains a distinct transversely exposed margin anteriorly (state 0). Platyurosternarchus and Sternarchorhynchus have the cartilage along the anterior portion of the first epibranchial further expanded medially to a notable degree. Therefore that cartilage overlaps not only the lateral margin of the cartilage that caps the second infrapharyngobranchial anteriorly, but extends to varying degrees along the anterior margin of that cartilage. As a consequence of this expansion of the anterior cartilage of the first epibranchial, the cartilage at the anterior margin of the second infrapharyngobranchial in Platyurosternarchus and Sternarchorhynchus lacks a broadly exposed margin anteriorly; a derived condition (state 1; Fig. 1).

Myorhadoi

64. Form of ventral myorhadoi in central portions of body: (0) central portion of myorhadoi without ventral or dorsal elaborations; (1) central portion of myorhadoi with ventral elaboration; (2) central portion of myorhadoi with ventral and dorsal elaborations (CI = 0.667; RI = 0.900).

Myorhadoi are the superficial series of the intermuscular bones that extend along the length of the body in gymnotiforms and are variably present in other fishes. The most common form of the large series of these ossifications present across the Gymnotiformes is a bony rod that progressively subdivides distally at both ends with these terminal processes extending further into the musculature. Such an unelaborated bony rod along the central region of the myorhadoi occurs in Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchorhamphus muelleri, Sternarchella orthos, and Sternarchorhynchus goeldii amongst the taxa in this study (state 0). All other examined taxa have elaborations of the central rod-like section of at least some of the myorhadoi; with such extensions taking either of two forms (states 1 or 2). Apteronotus albifrons, Sternarchogiton labiatus, and all species of Sternarchorhynchus with the exception of S. goeldii have an elaboration of the portion of the central section of the portion of the myorhadoi series located in the region of the body approximately one-quarter the distance between the rear of the abdominal cavity and the end of the anal fin. The myorhadoi in that region have a relatively long, curving, anteromedially directed extension arising from the medial surface of the rod-like central portion of the bone (states 1 and 2; Fig. 19A, B). Sternarchorhynchus chaoi uniquely also has a shorter anterodorsally orientated process arising from the dorsal surface of the myorhadoi at a point approximately directly opposite to the base of the larger anteroventral extension of the ossification (state 2; Fig. 19B). This character could not be coded for S. axelrodi solely based on radiographs of the types.

Myorhadoi from central portion of body of A, Sternarchorhynchus higuchii, INPA 20855; and B, Sternarchorhynchus chaoi, INPA 20851; lateral view, anterior to left.

Myorhadoi from central portion of body of A, Sternarchorhynchus higuchii, INPA 20855; and B, Sternarchorhynchus chaoi, INPA 20851; lateral view, anterior to left.

Position of anus and urogenital papilla

65. Ontogenetic shift in position of anus: (0) position of anus relatively invariant in juveniles and adults with anus situated posterior to vertical through eye in adults; (1) position of anus distinctly ontogenetically variable and shifting anteriorly with growth to location anterior to vertical through eye in adults (CI = 0.125; RI = 0.500).

Orthosternarchus tamandua, P. crypticus, P. macrostoma, and Sternarchorhamphus muelleri lack an ontogenetic anterior migration of the anus and urogenital papilla. Positional transitions were absent in A. cuchillo, A. rostratus, Sternarchella orthos, and Sternarchogiton labiatus amongst the more distant outgroups in this analysis (state 0). Campos-da-Paz (2000) remarked that the position of the anus (and presumably the associated urogenital papilla) was ontogenetically variable in a subset of the few species of Sternarchorhynchus recognized at the time of his analysis. Our analysis, involving as it does a significantly greater number of now-recognized species, documents that many Sternarchorhynchus species demonstrate an ontogenetic shift anteriorly of the anus and urogenital papilla (state 1; see listing of species in Appendix 1). An anterior shift of the position of the anus and urogenital papilla occurs in A. albifrons and ‘A.’ apurensis' amongst the more distantly related apteronotids incorporated into the analysis. This character was not coded for species of Sternarchorhynchus for which available ontogenetic series are too restricted to demonstrate potential shifts in the position of the anus and urogenital papilla.

Neurocranium

66. Presence or absence of scythe-shaped process along lateral margin of ventral ethmoid: (0) absent; (1) present (CI = 1.000; RI = 1.000).

The ventral ethmoid is fused to the anterior portion of the vomer to form a compound ossification (see discussion in Albert, 2001: 6–7). The lateral margin of the ventral ethmoid lacks any pronounced scythe-shaped elaborations in outgroups to Sternarchorhynchus(state 0). Species of Sternarchorhynchus with the exception of S. goeldii and S. oxyrhynchus(Fig. 2) have contralateral, well-developed, posteriorly directed, scythe-shaped processes arising from the lateral margins of what is considered positionally to be the ventral ethmoid (state 1; Fig. 3). Each scythe-shaped process has a very small distal cartilage in specimens of approximately 40 mm TL, but is completely ossified in adults and separate from any cartilage masses associated with the main body of the vomer and/or parasphenoid.

The positionally somewhat similar expansions on the lateral margin of the ventral ethmoid in examined species of Apteronotus, along with Orthosternarchus tamandua(Hilton et al., 2007: fig. 10c), Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri lack the scythe-shaped process present in the vast majority of Sternarchorhynchus species. These outgroups have, in contrast, posterolaterally directed articular surfaces on the ventral ethmoid that form the anterior limit of anteroposteriorly elongate cartilage bodies that extend along the length of the vomer and sometimes parasphenoid. In light of their significantly different overall morphology and association with the vomerine cartilages, those processes of the ventral ethmoid in these outgroups are coded as state 0.

67. Relative length of the mesethmoid in adults: (0) distance from tip of mesethmoid to vertical through posteriormost limit of that bone less than, to distinctly less than, distance from anteriormost point on frontal to anterior margin of parietal; (1) distance from tip of mesethmoid to vertical through posteriormost limit of that bone equal to, or longer than, distance from anteriormost point on frontal to anterior margin of parietal (CI = 1.000; RI = 1.000).

One of the striking features of Sternarchorhynchus is the pronounced development of the snout into an elongate overall tubular structure. Elongation of the snout is achieved via diverse alterations within the Apteronotidae (see Convergence in elongation of snout under Discussion), but in Sternarchorhynchus the pronounced lengthening of the mesethmoid is one of the major contributing factors to the elongation of the dorsal portion of the neurocranium. The mesethmoid in adults of that genus spans approximately one-half the distance between the tip of the snout and the anterior margin of the opercle (state 1; Campos-da-Paz, 2000: fig. 6), with the extension reflected in the relative length of the mesethmoid versus the frontal. Juveniles of Sternarchorhynchus have a proportionally much shorter mesethmoid (compare Albert, 2001: fig. 17A, B) with that ossification progressively increasing in length ontogenetically. Although Orthosternarchus tamandua amongst the proximate outgroups also has an elongate overall head, the proportional elongation of the mesethmoid is much less pronounced in that species (Hilton et al., 2007: fig. 10) than in Sternarchorhynchus. We also did not encounter dramatically elongated mesethmoids amongst outgroup apteronotids in this study (state 0).

68. Presence or absence of lateral ethmoid: (0) present; (1) absent (CI = 1.000; RI = 1.000).

A lack of the lateral ethmoid was ambiguously polarized as a possible synapomorphy for Platyurosternarchus plus Sternarchorhynchus by Albert & Campos-da-Paz (1998) and Albert (2001; cited therein as lateral ethmoid not ossified). Triques (2005) alternatively proposed that the lateral ethmoid was present in both genera as is the case in the vast majority of gymnotiforms, albeit sometimes represented as a cartilaginous element. The ambiguity in the earlier studies as to the applicability of the absence of a lateral ethmoid as a synapomorphy for Platyurosternarchus plus Sternarchorhynchus was a consequence of optimization resulting from the lack of an ossified lateral ethmoid in the examined specimens of Orthosternarchus tamandua. An ossified lateral ethmoid was, however, reported as present in Orthosternarchus tamandua by Hilton et al.(2007: 6, fig. 10), albeit reduced in size and shifted anteriorly relative to the typical condition in apteronotids. A lateral ethmoid is similarly present in the cleared and stained material of Orthosternarchus tamandua examined herein. In light of the consequent recoding of the condition in Orthosternarchus from lateral ethmoid absent to present, the absence of the lateral ethmoid in Platyurosternarchus and Sternarchorhynchus(state 1) resolves in this study as an unambiguous synapomorphy for those genera.

69. Degree of development of the ventral portion of the orbitosphenoid: (0) well developed and extensive along anteroposterior axis and extending anteriorly along dorsal margin of parasphenoid; (1) relatively narrow along anteroposterior axis and not extending anteriorly along dorsal margin of parasphenoid (CI = 1.000; RI = 1.000).

The orbitosphenoid extends ventrally from the frontal to contact the dorsal margin of the parasphenoid in gymnotiforms and many other ostariophysans. In the case of Orthosternarchus tamandua(Hilton et al., 2007: fig. 10b), the species of Platyurosternarchus crypticus, and Sternarchorhamphus muelleri the ventral portion of the orbitosphenoid proximate to the parasphenoid is anteroposteriorly elongate with a definite anterior extension along the dorsal surface of the latter bone. This condition (state 0) occurs in the examined outgroup species of Apteronotus and ‘Apteronotus’ along with Sternarchella orthos and Sternarchogiton labiatus. Sternarchorhynchus, in contrast, has a more pillar-like form of the ventral portion of the orbitosphenoid with either a limited anterior extension where it contacts the parasphenoid or a lack of such an elaboration. The orbitosphenoid in Sternarchorhynchus similarly has at most a limited posterior extension along the dorsal margin of the parasphenoid (state 1; Fig. 20).

Anterior portion of braincase and proximate bones of A, Sternarchorhynchus hagedornae, USNM 391574; and B, Sternarchorhynchus mormyrus, USNM 306843; left side, lateral view, anterior to left; larger stippling represents cartilage.

Anterior portion of braincase and proximate bones of A, Sternarchorhynchus hagedornae, USNM 391574; and B, Sternarchorhynchus mormyrus, USNM 306843; left side, lateral view, anterior to left; larger stippling represents cartilage.

70. Form of posterior margin of orbitosphenoid: (0) straight and abutting anterior margin of pterosphenoid along its entire length; (1) concave and contacting pterosphenoid only at dorsal and ventral limits of areas of contact of orbitosphenoid and pterosphenoid (CI = 1.000; RI = 1.000).

Examined outgroups to Sternarchorhynchus and all species of that genus other than S. axelrodi and S. mormyrus have a continuous vertical cartilaginous joint between the orbitosphenoid and pterosphenoid [state 0; Fig. 20A; see also Hilton et al.(2007: fig. 10b), for illustration of ventral portion of joint]. Cleared and stained and radiographed individuals of S. mormyrus instead have a distinctly anteriorly concave posterior margin of that portion of the orbitosphenoid. Contact between the orbitosphenoid and the pterosphenoid is thus limited to separate areas of articulation situated ventral and dorsal to concavity (state 1; Fig. 20B). Although we lacked cleared and stained specimens of S. axelrodi, the anterior concavity of the posterior margin of the orbitosphenoid is obvious in radiographs of the large specimens and the species was coded as having state 1.

71. Form of anterior margin of pterosphenoid: (0) straight and abutting anterior margin of orbitosphenoid along its entire length; (1) concave and contacting orbitosphenoid only at dorsal and ventral limits of areas of contact of pterosphenoid and orbitosphenoid (CI = 1.000; RI = 1.000).

This character is associated with character 70. Outgroups to Sternarchorhynchus and all examined species of this genus other than S. axelrodi and S. mormyrus have a straight, vertical anterior margin of the orbitosphenoid (state 0; Fig. 20A). The cleared and stained specimens of S. mormyrus in contrast have a distinctly posteriorly concave anterior margin of the pterosphenoid. That concavity in conjunction with the adjoining concavity along the posterior margin of the orbitosphenoid results in a large foramen within the lateral region of the braincase. Contact of the pterosphenoid with the orbitosphenoid along the primitively continuous joint is as a consequence reduced to two separate areas of articulation; one situated dorsal to and the second ventral of the foramen (state 1; Fig. 20B). An identical posterior concavity of the anterior margin of the pterosphenoid (state 1) is obvious in radiographs of large individuals of A. axelrodi.

Apteronotus albifrons, A. cuchillo, and ‘Apteronotus’ apurensis in the outgroups have a posteroventral extension of the pterosphenoid that contacts the posterior portion of the orbitosphenoid slightly above the point where that bone articulates with the parasphenoid. This process and a small anterior concavity on the orbitosphenoid jointly delimit a small transverse foramen that at first consideration might be considered homologous with the foramen discussed in the previous paragraph and under character 70 (see Hilton & Cox-Fernandes, 2006: fig. 4, for a comparable condition in A. bonapartii). The aperture in A. albifrons, A. cuchillo, and ‘A.’ apurensis is positioned distinctly ventral of the opening in S. axelrodi and S. mormyrus and is formed by ventral extensions of the orbitosphenoid and pterosphenoid rather than by the main body of these bones such as is the case in S. axelrodi and S. mormyrus. These apertures are clearly nonhomologous with the foramen in the two species of Sternarchorhynchus.

72. Anterior limit of anterior fontanel: (0) extending anterior of anterior limit of orbitosphenoid; (1) falling short of anterior limit of orbitosphenoid (CI = 0.500; RI = 0.875).

Many ostariophysans including gymnotiforms have a median frontoparietal fontanel that is typically separated into anterior and posterior components by the transversely aligned epiphyseal bar. Variation in the extent of the anterior component of the fontanel delimited by the frontals is pertinent to the groups of interest. In both Platyurosternarchus and Sternarchorhamphus the frontal component of the fontanel system is proportionally elongate with the anterior limit of the aperture being situated anterior of the orbitosphenoid. This condition (state 0) is also present in the outgroups Apteronotus and ‘Apteronotus’ along with Sternarchella orthos and Sternarchogiton labiatus. Orthosternarchus tamandua(Hilton et al., 2007: fig. 10a) and Sternarchorhynchus species have less extensive frontal fontanels that fall distinctly short of the anterior limit of the orbitosphenoid (state 1).

Pectoral girdle

73. Presence or absence of mesocoracoid: (0) present; (1) absent (CI = 1.000; RI = 1.000).

The mesocoracoid is a relatively small, vertically elongate ossification running between the cleithrum dorsally and the region of the joint between the coracoid and scapula ventrally. The possession of a mesocoracoid is typical for gymnotiforms [e.g. species of Gymnotus(Albert & Miller, 1995: fig. 5; Albert et al., 2005: fig. 16)] and this ossification is present in Orthosternarchus tamandua(Hilton et al., 2007: fig. 18b) and Sternarchorhamphus muelleri. A mesocoracoid is absent in Platyurosternarchus and Sternarchorhynchus and amongst the outgroups of immediate interest also in the species of Apteronotus and ‘Apteronotus’ along with Sternarchella orthos and Sternarchogiton labiatus(state 1).

74. Anterior extent of slender anteroventral process of coracoid: (0) reaching to medial surface of cleithrum and terminating in small cartilage; (1) terminating anteriorly distinctly short of medial surface of cleithrum and with resultant gap filled by distinct cartilage mass (CI = 0.500; RI = 0.000).

The coracoid extends anteriorly as an elongate, typically anteriorly attenuating splint of bone amongst gymnotiforms. This condition similarly occurs in outgroups to Sternarchorhynchus examined in this study (Hilton et al., 2007: fig. 18 for condition in Orthosternarchus tamandua). Anteriorly, this portion of the bone lies proximate to the medial surface of the cleithrum and is capped by at most a small cartilage body. This morphology of the coracoid process (state 0) also characterizes all Sternarchorhynchus species other than S. britskii and S. caboclo. In these two species there is a distinct gap between the anterior terminus of the anteroventral process and the proximate portion of the cleithrum (state 1). Filling that gap is an anteroposteriorly elongate cartilage much larger than the cartilage mass in that position in congeners. This character could not be coded for S. axelrodi based on radiographs of the available specimens.

Postcleithra and proximate independent ossifications

Postcleithra are variably shaped, often flattened ossifications embedded in the superficial tissues of the body and positioned in part medial and more so posterior to the pectoral girdle, in particular the cleithrum. These elements occur broadly across within the Characiformes, albeit with zero to three postcleithra present in different families (comments by Vari, 1995: 26). Postcleithra are typically reduced to one element in the Cypriniformes and lost in the Siluriformes (Fink & Fink, 1981). Species of Sternarchorhynchus along with some other gymnotiforms have been reported to possess one or more postcleithra, but with these elements absent in some taxa within the order [e.g. Sternopygus macrurus(Lundberg & Mago-Leccia, 1986: fig. 8); Electrophorus electricus(Brousseau, 1976: 100)]. Nonetheless, positioning of ossifications termed postcleithra by diverse authors differs considerably across the Gymnotiformes; variation that raises questions as to the homology of some of those bones.

Amongst immediate outgroups to Sternarchorhynchus, we find that Orthosternarchus tamandua has a single large, plate-like postcleithrum partially overlapped by the pectoral girdle (Hilton et al., 2007: fig. 18). This element and the ossifications in the species of Platyurosternarchus along with Sternarchorhamphus muelleri are positionally equivalent to the first postcleithrum in the Characiformes [e.g. Brycon meeki(Weitzman, 1962: fig. 18)] including the basal families Distichodontidae and Citharinidae (Vari, 1979: fig. 35). Ossifications located in this position in Orthosternarchus and Sternarchorhamphus are homologous with the first postcleithrum of other orders of the Ostariophysi. A comparable first postcleithrum is also present in Sternarchorhynchus, with the bone followed by one or more additional postcleithra in some species.

Diverse gymnotiforms have additional ossifications of uncertain homology located in the region anterodorsal to the first postcleithrum. The apteronotid genus Adontosternarchus bears three smaller, overlapping, longitudinally elongate ossifications in that area (Mago-Leccia et al., 1985: fig. 6). A positionally homologous anteroposteriorly elongate ossification occurs in the basal gymnotiform genus Gymnotus. More pertinently, a subset of the species of Sternarchorhynchus have an anteroposteriorly elongate bone situated distinctly anterodorsal to the element that is positionally homologous with the first postcleithrum or the series of postcleithra in most groups in the Characiformes. The simultaneous presence in many gymnotiforms of one to three first postcleithra positionally homologous to those in many other ostariophysans plus these additional ossifications makes it clear that the more dorsally positioned bones in various species of Sternarchorhynchus are not postcleithra in terms of homology, notwithstanding their locations posterior to the cleithrum. This is likely to be the case with the bone or series of dorsally positioned bones present in some outgroup gymnotiforms.

For the purposes of the following discussion, we utilize a positional concept for postcleithra. The element partially overlapped by the posterior portion of the cleithrum is herein coded as the first postcleithrum in keeping with its positional equivalence to the ossification of that name in other ostariophysans [e.g. Brycon meeki(Weitzman, 1962: fig. 18)]. Sequentially posterior elements in that series are termed the second and third postcleithra. In light of this convention, the ossification(s) situated anterodorsal to the first postcleithrum in Adontosternarchus, Gymnotus, and some species of Sternarchorhynchus coded under character 75 are elements of uncertain homology.

75. Presence or absence of ossification located in superficial tissues of body anterodorsal to first postcleithrum:(0) absent; (1) present (CI = 0.167; RI = 0.615).

An independent ossification situated posterior of the cleithrum and anterodorsal to the first postcleithrum occurs in S. axelrodi, S. britskii, S. chaoi, S. curvirostris, S. gnomus, S. goeldii, S. hagedornae, S. inpai, S. montanus, S. mormyrus, S. oxyrhynchus, S. severii, and S. starksi(state 1; Fig. 21). Amongst the outgroup apteronotids examined in this study, this ossification is present in ‘A.’ apurensis. No ossification occurs in this region (state 0) in the remaining Sternarchorhynchus species or in A. albifrons, A. rostratus, Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri in the outgroups.

Posterior portion of cleithrum, postcleithra, and independent ossification in A, Sternarchorhynchus hagedornae, USNM 391574; and B, Sternarchorhynchus mormyrus, USNM 306843; left side, lateral view, anterior to left.

Posterior portion of cleithrum, postcleithra, and independent ossification in A, Sternarchorhynchus hagedornae, USNM 391574; and B, Sternarchorhynchus mormyrus, USNM 306843; left side, lateral view, anterior to left.

Three ossifications identified as postcleithra were illustrated in the same general region by Mago-Leccia et al.(1985: fig. 8) in the apteronotid Adontosternarchus devenanzii. These bones differ in orientation and form from the ossification present in that area in some Sternarchorhynchus species. This makes it difficult to evaluate the possible homology between the elements in Adontosternarchus and the single bone present in some species of Sternarchorhynchus. As discussed above, in light of positional differences, we do not equate these ossifications with the postcleithra found in various gymnotiforms and characiforms. Furthermore, Adontosternarchus and Sternarchorhynchus are not close relatives (Albert, 2001) and the ossifications situated in the superficial tissues anterodorsal to the first postcleithrum even if homologous in those genera, nonetheless, demonstrate a homoplastic distribution.

76. Form of ossification located anterodorsal to first postcleithrum: (0) rounded or ovoid; (1) elongate (CI = 0.250; RI = 0.400).

When present, the independent ossification reported under character 75 has either of two forms in Sternarchorhynchus and other examined apteronotids. A rounded, ovoid, or slightly irregularly rectangular form of the bone (state 0; Fig. 21A) occurs in S. chaoi, S. gnomus, S. hagedornae, S. inpai, S. montanus, S. severii, and S. starksi and in the outgroup in ‘A.’ apurensis. In contrast, an anterodorsally distinctly elongate form of the ossification (state 1; Fig. 21B) is present in S. axelrodi, S. britskii, S. curvirostris, S. goeldii, S. mormyrus, and S. oxyrhynchus. This ossification is absent and could not be coded in the remaining Sternarchorhynchus species examined osteologically along with A. albifrons, A. rostratus, Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri in the outgroups.

77. Number of postcleithra: (0) one postcleithrum; (1) two postcleithra; (2) three postcleithra (CI = 0.400; RI = 0.400).

A single postcleithrum (state 0), the first postcleithrum under the numbering system discussed above, is the only element of that series present in Orthosternarchus tamandua(Hilton et al., 2007: fig. 18) and Sternarchorhamphus muelleri. A second postcleithrum located posterior to the first postcleithrum is present in Platyurosternarchus and Sternarchorhynchus for a total of at least two postcleithra in these genera [states 1 (Fig. 20A, B) and 2]. Outgroups with a second postcleithra are A. cuchillo, ‘A.’ apurensis, and Sternarchogiton orthos. Sternarchorhynchus curvirostris, S. inpai, S. mendesi, S. mesensis, and S. roseni have a third postcleithrum (state 2).

78. Form of second postcleithrum: (0) rounded or slightly longitudinally ovoid; (1) horizontally elongate (CI = 0.250; RI = 0.500).

The second postcleithrum is located immediately posterior of the first element in that series that is overlapped, in turn, by the posterior margin of the cleithrum. Two morphologies of the second postcleithrum occur within Platyurosternarchus, Sternarchorhynchus, and the other examined apteronotids bearing that ossification. In P. crypticus, P. macrostoma, S. goeldii, S. mormyrus, and S. oxyrhynchus the bone is horizontally elongate (state 1; Fig. 21B), whereas in all other species of Sternarchorhynchus the ossification is either round or only slightly longitudinally ovoid (state 0; Fig. 21A). An elongate morphology of the second postcleithrum (state 1) also occurs in A. cuchillo and ‘A.’ apurensis. This character could not be coded for outgroups that lack a second postcleithrum or for S. axelrodi based on radiographs of the types.

Weberian complex

79. Form of intercalarium: (0) transversely elongate; (1) rhomboidal or rectangular; (2) medially tapering and triangular overall (CI = 0.667; RI = 0.500).

The intercalarium is one of the set of ossicles within the pars auditum of the Weberian apparatus. These ossicles, along with associated soft tissues, the swimbladder, and components of the neurocranium are directly involved in the detection of sound in the water column. The intercalarium in gymnotiforms is a relatively small ossification located within the interosseus ligament that extends anterodorsally from the anterior tip of the tripus and then continues anteriorly to the scaphium (De La Hoz & Chardon, 1984: fig. 24, for illustration of arrangement in Sternopygus macrurus). Three morphologies of this ossification occur in Platyurosternarchus, Sternarchorhynchus, and the examined outgroup apteronotids. State 0, a transversely elongate form of the intercalarium, is limited to P. crypticus and P. macrostoma amongst examined taxa. In Sternarchorhynchus species with the exception of S. mesensis and S. starksi the intercalarium is overall rectangular or posteriorly narrower medially, but, nonetheless, extends a distinct distance medially along the axis of the interosseus ligament (state 1; Fig. 22A). A rhomboidal form of the ossification occurs in A. cuchillo, ‘A.’ apurensis, Orthosternarchus tamandua, and Sternarchorhamphus muelleri. A rectangular morphology of the intercalarium is present in A. albifrons, A. rostratus, Sternarchella orthos, and Sternarchogiton labiatus(state 1). In S. mesensis and S. starksi the bone distinctly tapers medially to a point and has an overall triangular form when examined from a dorsal view (state 2; Fig. 22B). It was impossible to code this character for S. axelrodi from radiographs of the types.

Tripus, intercalarium, and associated portions of interosseus ligament of A, Sternarchorhynchus oxyrhynchus, USNM 228787; and B, Sternarchorhynchus mesensis, MNRJ 13216; left side, dorsal view, anterior at top; dashed bands indicate ligament.

Tripus, intercalarium, and associated portions of interosseus ligament of A, Sternarchorhynchus oxyrhynchus, USNM 228787; and B, Sternarchorhynchus mesensis, MNRJ 13216; left side, dorsal view, anterior at top; dashed bands indicate ligament.

80. Form of tripus: (0) progressively narrowing away from area of attachment to the centrum of the third vertebra and with relatively narrow distal tip; (1) not distinctly narrowing distally and relatively wide for distal half of bone (CI = 1.000; RI = 0.000).

The tripus, another of the set of ossicles within the pars auditum of the Weberian apparatus, progressively attenuates as it extends distally from its area of attachment on the centrum of the third vertebra both in outgroups to Sternarchorhynchus and most members of that genus (De La Hoz & Chardon, 1984: fig. 24, for illustration of comparable arrangement in Sternopygus macrurus). The distal portion of the tripus has an abrupt change in angle towards the midline in Platyurosternarchus and is more gradually angled in that direction in most species of Sternarchorhynchus(state 0; Fig. 22A). Regardless of the details of the morphology of that region of the tripus, the distal half of the ossification becomes progressively narrower and is quite narrow distally relative to the more anterior portions of the bone. This condition is also the case in the examined outgroups to the genus. In S. mesensis the distal half of the tripus is of approximately constant width and forms a wide overall process with a more obtuse distal tip. The width of the posterior portion of the tripus is approximately one-half that of the anterior region of the bone (state 1; Fig. 22B). Although the character is autapomorphic for S. mesensis amongst the species of Sternarchorhynchus included in this study, it may serve as a synapomorphy for a small subunit of the genus once additional species of the genus are included in the analysis. It was impossible to code this character for S. axelrodi based on radiographs of the types.

Caudal fin

81. Ossification of hypural plate: (0) plate ossified; (1) plate cartilaginous (CI = 1.000; RI = 1.000).

An ossified hypural plate is common to both Sternarchorhynchus and Platyurosternarchus, whereas the plate is cartilaginous in Orthosternarchus tamandua(Hilton et al., 2007: fig. 17a) and Sternarchorhamphus muelleri[state 1; note: the specimen illustrated by Fink & Fink (1981) as Sternarchorhamphus with an ossified hypural plate is rather P. crypticus]. The plate is also ossified in other examined outgroup apteronotids, although with some specimens of A. rostratus having only a slightly developed ossification node in the central portion of what appears to be an otherwise cartilaginous plate. Other specimens of this species, in contrast, have a well-ossified hypural plate, with A. rostratus coded as having that condition (state 0).

Scalation

82. Scales along mid-dorsal region of body anterior to origin of electroreceptive filament: (0) present; (1) reduced to some extent and sometimes largely absent (CI = 0.250; RI = 0.750).

The mid-dorsal region of the body in most gymnotiforms is scaled, albeit with scales sometimes sparse and hidden to varying degrees by the overlying skin (state 0). This condition is present amongst the outgroups in A. albifrons, A. cuchillo, and A. rostratus. Most examined species of Sternarchorhynchus similarly have the mid-dorsal region of the body anterior of the origin of the electroreceptive organ scaled (state 0). Scales are lacking in variably extensive portions of the mid-dorsal areas anterior to the origin of the electroreceptive that region of the body in ‘A.’ apurensis, Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchella orthos, Sternarchogiton labiatus, Sternarchorhamphus muelleri, S. axelrodi, S. chaoi, S. goeldii, S. jaimei, S. mormyrus, and S. oxyrhynchus(state 1).

83. Degree of reduction of scaling along mid-dorsal portion of body anterior to origin of electroreceptive filament: (0) scales absent along entire mid-dorsal region to origin of electroreceptive filament; (1) scales absent along mid-dorsal region only to region approximately at vertical through posterior margin of pectoral fin (CI = 1.000; RI = 1.000).

Sternarchorhynchus species and members of the outgroup with reductions in the scales along the mid-dorsal region of the body demonstrate two different degrees of that reduction. In S. axelrodi, S. goeldii, S. mormyrus, and S. oxyrhynchus along with the outgroups‘A.’ apurensis, Orthosternarchus tamandua, P. crypticus, P. macrostoma, Sternarchella orthos, Sternarchogiton labiatus, and Sternarchorhamphus muelleri the scales are absent along the entire mid-dorsal region to the origin of the electroreceptive filament (state 0). Sternarchorhynchus chaoi and S. jaimei have less extensive reduction of the scale covering in the mid-dorsal region of the body, with the scales missing from the rear of the head approximately to the vertical through the posterior margin of the pectoral fin (state 1). This character was not coded for species that retain scales anterior by some degree to the electroreceptive filament origin, whether in the form of complete or sparse scaling.

84. Degree of development of mid-dorsal scales when present along mid-dorsal portion of body anterior to origin of electroreceptive filament:(0) scales well developed and obvious; (1) scales sparse and covered to different degrees by skin (CI = 0.500; RI = 0.667).

Species of Sternarchorhynchus that retain scales along the mid-dorsal region anterior of the origin of the electroreceptive filament vary in the degree to which the scales are present and readily obvious externally. Most species of the genus with a scaled mid-dorsal region (all species other than S. hagedornae, S. higuchii, S. mendesi, and S. roseni) have very obvious scales in that region (state 0). State 0 is also present in the outgroups in A. albifrons, A. cuchillo, and A. rostratus. In S. hagedornae, S. higuchii, S. mendesi, and S. roseni the mid-dorsal scales in the region anterior to the electroreceptive region, although present, are somewhat sparse and covered to varying degrees by skin (state 1). Given the presence of scales along at least part of the mid-dorsal area, this condition differs from the lack of scales in that region in characters 82 and 83. This character could not be coded for species lacking scales along the mid-dorsal region anterior to the electroreceptive filament (characters 82, 83).

Coloration

85. Presence or absence of narrow, mid-dorsal, lightly coloured stripe on head: (0) mid-dorsal portion of head with pigmentation comparable to that of laterally adjoining regions or with pigmentation darker; (1) mid-dorsal region of head with narrow, mid-dorsal, lightly coloured stripe (CI = 0.333; RI = 0.833).

Immediate outgroups to Sternarchorhynchus(Orthosternarchus, Platyurosternarchus, Sternarchorhamphus) along with Sternarchella orthos and Sternarchogiton labiatus amongst more distant outgroups all have the coloration of the mid-dorsal portions of the head comparable to, or darker than, that of the laterally adjoining regions of the head (state 0). The majority of species of Sternarchorhynchus(state 1; Appendix 1), in contrast, have a narrow to very narrow, mid-dorsal, distinctly more lightly coloured stripe extending from, or proximate to, the tip of the snout posteriorly to at least the interorbital region and sometimes further posteriorly on the head (state 1). Some species with such a stripe have that pigmentation pattern continuous with a comparable mid-dorsal stripe along the body that extends to varying degrees posteriorly towards, or sometimes beyond, the origin of the electroreceptive filament. Amongst the outgroups, such a mid-dorsal lightly coloured stripe (state 1) is also present in A. albifrons, A. cuchillo, and A. rostratus. Two species of Sternarchorhynchus, S. britskii and S. mesensis, were reported to have only an indistinct mid-dorsal stripe on the snout in certain individuals (Campos-da-Paz, 2000), but these species are coded as state 1 in light of the presence of the stripe to some degree in the material of these species examined in this study.

86. Presence or absence of narrow, lightly coloured stripe on lateral surface of snout: (0) lateral surface of snout with pigmentation comparable to that of adjoining regions or with irregular marbling; (1) lateral surface of snout with distinct, more lightly coloured band (CI = 0.333; RI = 0.857).

All examined apteronotid outgroups to Sternarchorhynchus have uniform coloration of the lateral surface of the snout or when the pigmentation in this region is non-uniform it is marbled or otherwise irregular (state 0). Such uniform pigmentation along the snout is common to S. curvirostris, S. goeldii, S. oxyrhynchus, S. roseni, and S. starksi. In contrast, S. axelrodi, S. britskii, S. caboclo, S. chaoi, S. cramptoni, S. curumim, S. gnomus, S. hagedornae, S. higuchii, S. inpai, S. jaimei, S. mareikeae, S. mormyrus, S. mendesi, S. mesensis, S. montanus, S. retzeri, S. severii, and S. stewarti have a distinct, more lightly coloured band extending along the lateral surface of the snout in the region anterior to the eye and sometimes continuing nearly to the tip of the snout (state 1).

87. Presence or absence of dark coloration along basal portion of anal fin: (0) fin hyaline to slightly dusky overall, but lacking distinct band of dark pigmentation basally; (1) fin hyaline to slightly dusky overall, with distinct band of dark pigmentation basally (CI = 1.000; RI = 0.000).

Anal fin coloration in Sternarchorhynchus ranges from hyaline to somewhat dusky overall, but most species of the genus along with all outgroups in this analysis lack a band of distinctly darker pigmentation extending along the basal portion of the fin (state 0). Such a band of dark, basal pigmentation on the anal fin is limited to S. curvirostris amongst species of the genus (state 1). Although autapomorphic for this species in this analysis, we included this feature because future analyses may find it to be more broadly distributed with the inclusion of additional species.

Triques (2005) proposed that dark coloration of the caudal peduncle and adjoining regions was synapomorphic for a clade formed by Platyurosternarchus, Sternarchorhynchus, and Ubidia. This pigmentation is rare amongst the species of Sternarchorhynchus. As discussed under the framework for outgroup comparisons, the caudal pigmentation of Platyurosternarchus and Ubidia differs in various details with the coloration of the latter genus more similar to that of a subunit of Apteronotus.

88. Presence or absence of dark coloration along distal portions of anal fin: (0) fin hyaline to slightly dusky overall, but lacking distinct band of dark pigmentation along distal portion of fin; (1) fin hyaline to slightly dusky overall, and with distinct band of dark pigmentation distally (CI = 0.167; RI = 0.375).

Anal fin coloration in preservative in Sternarchorhynchus ranges from hyaline to somewhat dusky overall, but most species in the genus along with A. albifrons, A. cuchillo, A. rostratus, Orthosternarchus tamandua, P. crypticus, and P. macrostoma lack a band of distinctly darker pigmentation along the distal margin of the anal fin (state 0). ‘Apteronotus’ apurensis and Sternarchorhamphus muelleri in the outgroups together with S. axelrodi, S. cramptoni, S. goeldii, S. montanus, S. mormyrus, S. oxyrhynchus, and S. retzeri have instead a distinct band of dark pigmentation along the distal portions of the anal fin (state 1). This darkly pigmented band covers one-fifth to one-half of the distal portions of much of the anterior and middle anal-fin rays and in some instances all, or nearly all, of the posterior rays of the fin. The dark coloration on the anal fin in the species of Platyurosternarchus is limited to the posteriormost portion of the fin and extends to the base of the involved fin rays (de Santana & Vari, 2009); a very different and thus nonhomologous pattern.

Sternarchorhynchus roseni demonstrates variation in the presence or absence of dark pigmentation on the anal fin, with the presence of such coloration correlated with the possession in mature males of an anteriorly expanded dentary with enlarged dentition. This correlation raises the possibility that the presence of dark pigmentation is a seasonal, sexually dimorphic feature for males and we consequently code S. roseni as unknown for this character.

DISCUSSION

Phylogenetic reconstruction and key innovations

Twenty-four nodes were resolved in the phylogenetic analysis for, and within, the clade formed by Platyurosternarchus plus Sternarchorhynchus(clade numbering in Fig. 23). Reconstructed relationships amongst the outgroup taxa in this study beyond the hypothesis of a sister-group relationship between Platyurosternarchus and Sternarchorhynchus should not be viewed as well-supported phylogenetic hypotheses and support for those portions of the phylogeny is not detailed on the cladogram. The potential uncertainty in outgroup relationships is derivative of several limitations with the underlying data. Taxon sampling amongst apteronotids outside of Platyurosternarchus and Sternarchorhynchus was of necessity sparser than within those genera that were the subject of in-depth analyses by, respectively, de Santana & Vari (2009) and herein. Notwithstanding the fact that some characters were incorporated into the analysis in order to provide structure to the outgroup phylogeny, we did not search for new characters informative about relationships amongst and within the outgroup taxa incorporated into this study. Characters previously reported in the literature as informative about relationships within the Apteronotidae were typically excluded from this analysis unless germane to the core questions phylogenetic addressed in the study. The evidence for the sister-group relationship between Orthosternarchus tamandua and Sternarchorhamphus muelleri arrived at herein does, however, conform with the hypotheses advanced by Campos-da-Paz (1995) and various subsequent authors.

The results strongly support the hypothesis of a sister-group relationship between Platyurosternarchus and Sternarchorhynchus advanced by authors starting with Albert & Campos-da-Paz (1998), albeit with some differences in the underlying evidence (see Synapomorphy list in Appendix 2 for details). Several of the synapomorphies common to those two genera are of particular note in being unambiguous modifications unusual not only within the Apteronotidae but in some instances across the Gymnotiformes. Striking from that perspective are the restructuring of the posterior portion of the anguloarticular (character 17), the very distinct constriction of the medial portion of the fourth ceratobranchial (character 61), the well-ossified fourth infrapharyngobranchial (character 62), and the absence of the lateral ethmoid (character 68). The results of this study and those of de Santana & Vari (2009) also support the hypothesis of the monophyly of Platyurosternarchus. A number of the derived modifications at the level of Platyurosternarchus similarly involve the jaws and dentition. These include the absence of teeth on premaxilla (character 4), and the attachment of teeth to the exterior surface of the dentary (character 20).

Of particular note within the clade formed by Platyurosternarchus plus Sternarchorhynchus are the extensive series of synapomorphies for Sternarchorhynchus(Synapomorphy list in Appendix 2 for details), with a high percentage (77%) of the 27 characters informative as to the monophyly of Sternarchorhynchus being unambiguous characters uniquely derived at the level of the genus. These numerous synapomorphies are by far the greatest number of such features defining any genus within the Gymnotiformes (Albert, 2001, for other genera). Seven of the 27 synapomorphies for Sternarchorhynchus involve restructurings of components of the jaws (character 5, the form of the maxilla; character 9, the reduction of the coronomeckelian bone; character 10, the reduction of Meckel's cartilage; character 11, the relative development of the posterodorsal and posteroventral processes of the dentary; character 12, the form of the posteroventral process of the dentary; character 15, the position on the lower jaw of the articular facet contacting the quadrate; and character 16, the form of the anterior portion of the anguloarticular). Seven additional synapomorphies forSternarchorhynchus involve alterations of the suspensorium so that it can be accommodated in the vertically constricted tubular snout or are correlated with the remodelling of components of the jaws (character 21, the reduction of the anterior portion of the palatoquadrate cartilage; character 22, the shift ventrally in the relative position of the endopterygoid; character 23, the reduction of the anterior portion of the endopterygoid; character 24, the reduction of the posterior portion of the endopterygoid and the closer association or fusion of that ossification with the quadrate; character 28, the loss of the dorsal portion of the quadrate; character 35, the shift of the posterior limit of the posteroventral portion of the quadrate; and character 38, the horizontal elongation of the metapterygoid). Except for character 11, none of these modifications was manifested within the examined outgroups. Of the restructurings of the neurocranium at this level of the phylogeny, one synapomorphy (character 67, the increase in the proportional distance from the tip of the mesethmoid to the vertical through the posteriormost limit of that bone) is clearly correlated with the dramatic proportional elongation of the snout characteristic of Sternarchorhynchus. These 15 modifications of the jaws, suspensorium, and anterior portion of the neurocranium all correlate with the distinctive form of the jaws, snout, and associated soft tissue systems that permit grasp-suction or suction assisted by mechanical grasping in the species of Sternarchorhynchus(comments under Adaptive radiation in the next section).

The synapomorphies for Sternarchorhynchus that are heavily weighed towards modifications of the jaws, suspensorium, anterior portion of the neurocranium, and associated systems are recognized herein as key innovations [i.e. special set of characteristics –sensuLiem (1973), Lauder (1981)] within a comparative phylogenetic framework as proposed by Stiassny & Jensen (1987). The concept of key innovation has been used in diverse fashions, usually focusing on attributes purported to account for the success for a lineage in any of several ways, including species richness and morphological diversity. Herein we focus on species richness resultant from morphological modifications, specifically asymmetrical divergence in species numbers between Platyurosternarchus and Sternarchorhynchus following the criterion of Guyer & Slowinski (1993).

That being said, there remains controversy on the issue of whether key innovations serve as triggers of diversification (e.g. Cracraft, 1990) and it is difficult to test for such features (Hodges, 1997). One difficulty with the discussion of key innovations is the lack of a standard to assess diversity and species richness (Jensen, 1990). Jensen (1990) reviewed the process for historical testing for key innovations. Historical testing uses convergence to search for independent corroboration of hypotheses relating evolutionary novelties with phylogenetic patterns (Jensen, 1990). That author proposed that historical testing must be a two-step process. The first step involves a comparison of the species diversity of a taxon exhibiting a proposed key innovation or innovations with that of its sister group. Following that is the comparison of sister groups in independent lineages in which an identical novelty has evolved. This ascertains whether comparable disparate patterns of species richness occur in each of the independent lineages.

In the present study, the first step involves a comparison of species diversity of Sternarchorhynchus versus Platyurosternarchus. The second step is a comparison of the species richness in the two genera of freshwater fishes in which grasp-suction feeding has been reported (Sternarchorhynchus and the African electric fish genus Campylomormyrus), versus their sister taxa (Platyurosternarchus and Gnathonemus, respectively) neither of which demonstrates that feeding system (for further information on the species diversity and relationships amongst Mormyriformes genera see Moller, 1995; Sullivan, Lavoué & Hopkins, 2000; Feulner et al., 2007).

In both the South American and African lineages, the groups in which grasp-suction feed mode has evolved (Sternarchorhynchus with 32 species and Campylomormyrus with 14-plus species) are more diverse than are sister taxa that do not demonstrate that mode of feeding (Platyurosternarchus with two species and Gnathonemus with five species, respectively; for further details see Adaptive radiation and Convergence in elongation of snout sections). Campylomormyrus is forecast to be even more diverse than presently recognized (Feulner et al., 2007). This consistent pattern of relative species richness between Sternarchorhynchus and Campylomormyrus versus their sister groups satisfies the second criterion advanced by Jensen (1990). We thus propose that the various synapomorphies for Sternarchorhynchus noted above, in particular the restructuring of components of the jaws, suspensorium, and anterior portion of the neurocranium that permit the grasp-suction feeding as likely key innovations linked to the adaptive radiation within that genus.

Looking at the distribution of synapomorphies of and within Sternarchorhynchus, particularly of uniquely derived features, we find a remarkable asymmetry in the occurrence of these attributes across the phylogeny of the genus (Fig. 23). Clades 1 to 7 in Sternarchorhynchus involve 39 such synapomorphies contrary to the presence of only one uniquely derived synapomorphy across clades 8 to 24. The pronounced loading of these characters at the basal levels within the clade formed by Platyurosternarchus and Sternarchorhynchus correlates with a number of novel modifications of various body systems at these levels. These potential key innovations particularly load at the level of clade 3 where they involve the synapomorphies for the species of Sternarchorhynchus associated with capture and initial processing of prey items. Such loading of major modifications applies, albeit to a lesser extent, at the level of other basal clades within that genus (clades 4 to 7), all of which are defined by multiple synapomorphies. With few exceptions, clades 8 to 24 are instead defined by single homoplastically distributed synapomorphies that involve aspects of the feeding system. The underlying cause of this pattern of synapomorphy distribution is impossible to discern from the morphological data, nor is relative timing of cladogenesis determinable in the absence of any benchmark vicariance events that provide a temporal context to the phylogeny. One explanation involves rapid speciation at less inclusive levels within the phylogeny of the genus. Such a clustering of modifications at the basal clades is congruent with expectations under the hypothesis of adaptive radiation (Foote, 1997; Schluter et al., 1997; Schluter, 2000; also comments under Adaptive radiation below).

Having arrived at a series of innovations that permit the very unusual, albeit clearly highly functional, grasp-suction feeding mode at the level of Sternarchorhynchus(clade 3), there followed less dramatic modifications of the jaws, suspensorium, and gill arches at basal levels within the genus. Clade 4, consisting of S. goeldii and S. oxyrhynchus, is characterized by several unique distinctive modifications of these body systems (character 1, the mouth length; character 6, the reduction of the anterior portion of the maxilla; character 8, the presence of a fleshy, dorsally directed pad at the anterior margin of the dentary; character 25, the fusion of the quadrate and endopterygoid; character 30, the lateral expansion of the posterolateral portion of the quadrate; and character 39, the lateral expansion of the anterolateral portion of the metapterygoid). In sum, these result in a mouth form unique to S. goeldii and S. oxyrhynchus within Sternarchorhynchus. Presumably, these modifications correlate with different diets or modes of prey capture in these species versus their congeners. Unfortunately, the limited detailed available information on feeding habits in, and prey items of, the species of Sternarchorhynchus precludes a test of this hypothesis at this time.

Clade 5, the sister lineage to clade 4, includes all species of Sternarchorhynchus with the exception of S. goeldii and S. oxyrhynchus. Clade 5 is characterized by fewer unique features that clearly involve the feeding system and the elongate snout than is clade 4 (character 7, the position of the posterior limit of the dentary; character 59, the anterior extension from the main body of the second hypobranchial; character 66, the presence of a scythe-shaped process on the lateral margin of the ventral ethmoid). Clade 7, in turn, has two unique synapomorphies associated with the jaws and suspensorium (character 3, the compact form of the premaxilla; character 29, the concave dorsal margin of the quadrate). The remaining clades (8–24) either lack or have only homoplastically distributed synapomorphies associated with the food acquisition and initial processing systems and the associated elongate preorbit. The lack of further substantive modifications of the jaws, suspensorium, and anterior portion of the neurocranium at, or within, clade 8 is likely to reflect functional constraints of those portions of the head by the key innovations present at the levels of the more inclusive clades (3, 5, 7).

As would be expected, the Bremer support values [Treerot V.2C (Sorenson, 1999); 20 replicates and 10 000 trees] parallel the same pattern of asymmetry in the distribution of nonhomoplastic synapomorphies discussed above. Clade 1 (Platyurosternarchus plus Sternarchorhynchus) has a Bremer value of 4. Within that clade, Platyurosternarchus(clade 2) has a value of 2, whereas Sternarchorhynchus(clade 3) with many more synapomorphies has a value of 20. Less inclusive basal clades and their values are: clade 4, Bremer support 8; clade 5, Bremer support 4; clade 6, Bremer support 5; and clade 7, Bremer support 5. The remaining clades in the phylogeny (8–24) all have low support values of 1 other than for clade 20, which has a value of 2.

The overall pinnate form of the phylogeny at the level of Platyurosternarchus and Sternarchorhynchus and the asymmetrical distribution of the number of species across the phylogeny (Fig. 23) evokes the phenomenon discussed by Stiassny & de Pinna (1994), who documented instances in which basal taxa within lineages are proportionally depauperate in numbers of species relative to their sister groups. As discussed previously, key innovations are a potential basis for the species richness within Sternarchorhynchus. The underlying cause for the relatively depauperate situation in Platyurosternarchus is unknown, but some authors (Brooks and McLennan, 1991; Hodges, 1997) have proposed that such clades may possess attributes that constrain diversification. Stiassny & de Pinna (1994) also commented that basal taxa in pinnate phylogenetic schemes often have geographically extremely constricted distributions. The broad distribution of P. macrostoma in the immediate outgroup along with the extensive ranges of S. goeldii in the Amazon basin and of S. oxyrhynchus of the Río Orinoco system of the basal clade within Sternarchorhynchus run counter to the pattern highlighted by those authors. Although the distribution of S. goeldii and S. oxyrhynchus(Figs 42, 63) is more restrictive than the broader Amazon basin, the river systems of the Guyanas, and the upper Rio Paraná inhabited by their congeners, the distributional ranges of these two species that constitute the sister clade to all other species of Sternarchorhynchus, nonetheless, cover significant portions of the overall distribution of the genus.

Map of central and northern South America showing geographical distribution of Sternarchorhynchus goeldii sp. nov.(1 = holotype locality; species also collected in region of type locality of Sternarchorhynchus higuchii sp. nov., star), Sternarchorhynchus hagedornae sp. nov.(2 = holotype locality), S. higuchii sp. nov.(3 = holotype locality), and Sternarchorhynchus inpai sp. nov.(4 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus goeldii sp. nov.(1 = holotype locality; species also collected in region of type locality of Sternarchorhynchus higuchii sp. nov., star), Sternarchorhynchus hagedornae sp. nov.(2 = holotype locality), S. higuchii sp. nov.(3 = holotype locality), and Sternarchorhynchus inpai sp. nov.(4 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus oxyrhynchus(1 – Essequibo River, reported type locality of species, but see Remarks in species account concerning possible erroneous locality information), and Sternarchorhynchus retzeri sp. nov.(2 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus oxyrhynchus(1 – Essequibo River, reported type locality of species, but see Remarks in species account concerning possible erroneous locality information), and Sternarchorhynchus retzeri sp. nov.(2 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Adaptive radiation

The various forms of the snout and jaws in diverse groups of gymnotiforms are associated with differences in the diet and accessibility of certain food items. Restructurings of the bones, musculature, and other soft tissues of the anterior portions of the neurocranium and jaws (Aguilera, 1986) in Sternarchorhynchus permit the extraction of prey items from constricted refuges in compacted clay masses, decaying wood, debris, and mud on river bottoms. These dramatic restructurings are proposed above as potential key innovations that facilitate ‘grasp-suction’ or ‘suction assisted by mechanical grasping’ in the members of the genus. In that system, food items are first mechanically removed from the constricted refuges favoured by potential prey by pincer-like manipulations of the small toothed jaws and then drawn into the oral cavity via suction. Marrero & Winemiller (1993) and Winemiller & Adite (1997) pointed out that this very unusual, highly specialized feeding mode is unique to Sternarchorhynchus and the African mormyriform genus Campylomormyrus amongst those orders of fishes entirely restricted to freshwaters. Paralleling this functional similarly is the fact that Campylomormyrus is a strikingly speciose member of the Mormyridae (Feulner et al., 2007); a situation comparable to that of Sternarchorhynchus within the Apteronotidae. Feulner et al.(2007) proposed that the phenomenon of high numbers of species represents an adaptive radiation within Campylomormyrus. Does that phenomenon also apply to Sternarchorhynchus?

In his discussion on key innovations in cichlids, Liem (1973) noted that Mayr (1960) and Bock (1965) suggested that adaptive radiations of organisms would not occur until the evolutionary novelty reached a certain degree of stability or development. The features recognized as key innovations of Sternarchorhynchus were primarily acquired at the level of clade 3 and achieved a degree of stability at clade 8. More recently, Schluter (2000) defined adaptive radiation as the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Schluter proposed four criteria that characterized that phenomenon: origin from a common ancestor, rapid speciation, a correlation between environment and phenotype, and trait utility.

Common ancestry

The hypothesis of the monophyly of Sternarchorhynchus is supported by a series of synapomorphies thereby filling the first of the criteria proposed by Schluter (2000).

Rapid speciation

Rates of speciation are difficult to determine in the absence of any evidence (i.e. detailed fossil record or appropriate molecular data) that permits calibration of the age of the different lineages; however, the pronounced disparity between the two species within Platyurosternarchus(de Santana & Vari, 2009) and the minimum of 32 species within Sternarchorhynchus(see also comments under Possible additional undescribed species) confirm significantly different rates of speciation and/or extinction between these genera. No evidence of extinction exists for Platyurosternarchus, the two members of which are morphologically quite similar. That fact plus the broad range of P. macrostoma across the Amazon, Orinoco, and Essequibo river basins is congruent with the hypothesis of a minimal degree of speciation within Platyurosternarchus. Regardless of this issue, Sternarchorhynchus demonstrates a significantly accelerated rate of speciation over its sister clade as evidenced by the disparity in the number of species in each genus [two species of Platyurosternarchus(de Santana & Vari, 2009) versus 32-plus species of Sternarchorhynchus(results herein); also comments in Stiassny & Jensen (1987) concerning the need to evaluate concepts such as species richness in a phylogenetic context]. At least some of the studies focused on patterns of morphological divergence within clades in the fossil record have demonstrated that morphological disparity typically is maximized towards the base of the clade with that phenomenon followed by an increase in the diversity of species (Foote, 1997). The pattern highlighted by Foote (1997) is well manifested in the Platyurosternarchus–Sternarchorhynchus lineage, wherein the basal lineages are characterized by the possession of a disproportionate percentage of the derived morphological modifications known across that lineage. Within Sternarchorhynchus this is followed by a dramatic increase in species richness in clades that demonstrate relatively little pronounced morphological restructurings in the examined body systems.

Phenotype–environment correlation and trait utility

A correlation between environment and phenotype as typified by phylogeny is clear in some of the basal clades within the genus (comments at beginning of Discussion above). Different forms of the mouth in Platyurosternarchus versus Sternarchorhynchus and within basal lineages of the latter genus can be assumed to reflect different feeding regimes permitting exploitation of specific food resources; a difference fitting the criterion of trait utility. The most conspicuous phenotypic differences amongst species of Sternarchorhynchus involve snout size and shape; more specifically differences in the proportional length, vertical height, transverse width and degree of central curvature of the snout. To date we lack the necessary information on feeding behaviours and dietary preferences to engage in a rigorous test of the hypothesis of phenotype–environment correlation and trait utility. Nonetheless, the overall evidence indicates that adaptive radiation is a driving mechanism that accounts to some unspecified degree for the speciose nature of Sternarchorhynchus relative to other genera of the Apteronotidae. Further analysis incorporating data from life history studies of all species in Sternarchorhynchus evaluated within a phylogenetic analysis is necessary to test this hypothesis.

Convergence in elongation of snout

Elongate preorbital regions of the head (= snout) are manifested in two units of the Gymnotiformes; the Apteronotidae (Compsaraia, Orthosternarchus, Platyurosternarchus, Sternarchorhamphus, Sternarchorhynchus, and some species of Apteronotus) and the Rhamphichthyidae (Gymnorhamphichthys and Rhamphichthys). In addition, both the Apteronotidae and Rhamphichthyidae include numerous species lacking comparable elongations of the preorbital portion of the head. The extension of the snout in subunits of the two families is clearly convergent under present concepts of phylogenetic relationships within the Gymnotiformes (Albert & Campos-da-Paz, 1998; Albert, 2001).

Focusing on the Apteronotidae, the very different forms of the elongation of the anterior region of the head in Compsaraia and Apteronotus relative to the snout morphology in Sternarchorhynchus brings to the fore questions as to the equivalence of the preorbital expansion in these taxa (Cox-Fernandes et al., 2002: fig. 1; and Hilton & Cox Fernandes, 2006: fig. 1, relative to the species of Apteronotus; and Mago-Leccia, 1994: fig. 90, for Compsaraia compsus identified as Porotergus compsus therein). More significantly, neither Compsaraia nor the species of Apteronotus with elongate preorbital regions are closely related to Sternarchorhynchus(Albert, 2001; Crampton & Albert, 2006; results of this study). Thus, the elongate snouts in these other apteronotid taxa are convergent with the lengthened preorbital portion of the head characteristic of Sternarchorhynchus.

Elongation of the preorbital region was proposed by Albert & Campos-da-Paz (1998) and Albert (2001) as a feature supporting a clade within the Apteronotidae consisting of Orthosternarchus, Platyurosternarchus, Sternarchorhamphus, and Sternarchorhynchus. More recently, Hilton et al.(2007) revisited this issue and highlighted the fact that although these four genera share an elongate preorbital region, the mode of lengthening derives from different underlying morphological modifications in Orthosternarchus and Sternarchorhamphus on the one hand versus Platyurosternarchus and Sternarchorhynchus on the other. These differences manifest themselves most clearly in the degree of proportional development of the jaws that is best reflected in the relative position of the articulation of the lower jaw with the suspensorium.

Hilton et al.(2007) noted that the joint of the lower jaw with the articular condyle of the quadrate is positioned distinctly further posteriorly in both Platyurosternarchus and Sternarchorhynchus(Fig. 25B for position of joint in latter genus) than is the case in Orthosternarchus tamandua and Sternarchorhamphus muelleri. The combination of a posterior position of the joint and an elongate snout is achieved by pronounced proportional longitudinal elongation of various elements of the lower jaw (dentary, anguloarticular, Meckel's cartilage at least in juveniles) inPlatyurosternarchus(Fig. 5) and Sternarchorhynchus(Figs 6, 7) relative to corresponding elements in Orthosternarchus tamandua(Hilton et al., 2007: fig. 12) and Sternarchorhamphus muelleri(Campos-da-Paz, 1995: fig. 5). Snout elongation in Orthosternarchus tamandua(Hilton et al., 2007: fig. 11) and Sternarchorhamphus muelleri in which the lower jaw–quadrate joint is anteriorly positioned is in large part a function of anteroposterior elongation of elements posterior of the lower jaw–quadrate joint; most notably the preopercle, interopercle, and subopercle. Expansions of these ossifications in Orthosternarchus and Sternarchorhamphus contrasts with the more typical apteronotid morphology of these bones in Platyurosternarchus and Sternarchorhynchus.

Radiograph of head and snout of Sternarchorhynchus yepezi, holotype, MCNG 44286; showing A, expanded portion of dentary and elaboration of associated dentition in sexually dimorphic males; B, point of articulation between quadrate and lower jaw; and C, elongate mesethmoid.

Radiograph of head and snout of Sternarchorhynchus yepezi, holotype, MCNG 44286; showing A, expanded portion of dentary and elaboration of associated dentition in sexually dimorphic males; B, point of articulation between quadrate and lower jaw; and C, elongate mesethmoid.

Preorbital elongation results from restructuring of two different systems in the species of Platyurosternarchus and Sternarchorhynchus(the anteroposterior elongation of the components of the lower jaw and anterior portions of the suspensorium) versus Orthosternarchus tamandua and Sternarchorhamphus muelleri(the anteroposterior elongation of the posterior portion of the suspensorium and some elements of the opercular series). The lengthening of the snout in these two lineages is thus patently nonhomologous, notwithstanding the external similarities in that region in the head.

Differences also occur in the mode of preorbital elongation and in particular in the morphology of the jaws in Sternarchorhynchus versus its sister group, Platyurosternarchus. Although members of each genus have what could be considered to be elongate preorbital regions, the mouth in Platyurosternarchus is relatively elongate, forming 30.0–37.9% of the HL (de Santana & Vari, 2009), whereas it is reduced in the species of Sternarchorhynchus to a small opening achieving a maximum of 12.4% of the HL in those species with longer mouths.

Looking at the underlying bones, we find that the modifications associated with the elongation of the snout in Sternarchorhynchus are unique to that genus, not being approximated in Platyurosternarchus let alone other examined apteronotids. Of particular note is the pronounced elongation of the mesethmoid in Sternarchorhynchus(Fig. 25C), which is a major factor in the lengthened preorbit, whereas the frontal is only moderately extended in this genus. In contrast, the mesethmoids in Orthosternarchus tamandua, P. crypticus, P. macrostoma, and Sternarchorhamphus muelleri are of proportional lengths comparable to those in outgroup apteronotids, whereas these species demonstrate a pronounced elongation of the frontals. Orthosternarchus, Platyurosternarchus, and Sternarchorhamphus have anteroposteriorly lengthened orbitosphenoids, whereas Sternarchorhynchus has a form of this bone proportionally comparable to the smaller ossification present in apteronotid taxa with shorter snouts.

A ventral curvature of the snout that terminates below the horizontal to opercular region was proposed as a synapomorphy for a clade formed by Platyurosternarchus, Sternarchorhynchus, and Ubidia in an analysis by Triques (2005). Elongation of the snout that is prerequisite for the extension of the snout to ventral of the horizontal to opercular region was achieved via different modifications in these genera and as such the curvature is not homologous across these genera.

As discussed above, grasp-suction or suction assisted by mechanical grasping is unique to two genera amongst freshwater fishes; Sternarchorhynchus in the Neotropics and Campylomormyrus, a member of the order Mormyriformes that inhabits rivers in the western and central portions of sub-Saharan Africa (Feulner et al., 2007: fig. 1). Notwithstanding the striking convergence between these two genera in their feeding systems, numerous differences occur in the underlying osteological systems in Sternarchorhynchus versus Campylomormyrus. Given the pronounced phylogenetic distance between these genera, this result is expected and in the interests of efficiency, we discuss only a few of the larger scale differences.

The mesethmoid of Sternarchorhynchus is distinctly elongate anteroposteriorly (character 67), and the primary contributor amongst the bones of the dorsal portion of the neurocranium to the elongation of the snout. In Campylomormyrus the elongation of the preorbital region of the head is, in contrast, largely a function of the proportional lengthening of the frontal, with the mesethmoid being relatively short along its anteroposterior axis (Taverne, 1968: figs 2, 3, 6). Campylomormyrus has a distinctly anteroposteriorly lengthened orbitosphenoid with that elongation being one of the distinguishing characteristics of the genus (Taverne, 1968: fig. 9). Sternarchorhynchus, by contrast, has a relatively anteroposteriorly compact orbitosphenoid. Finally, the coronomeckelian ossification is lost ontogenetically in Sternarchorhynchus(character 9), but this bone is distinctly lengthened along the anteroposterior axis in Campylomormyrus.

These and correlated differences in other elements of the preorbital region in Sternarchorhynchus versus Campylomormyrus amply demonstrate that the two genera arrived at functionally convergent forms of jaws and associated systems for prey capture and ingestion via different evolutionary paths in South America versus sub-Saharan Africa. Interesting, the convergences in the overall external form of the preorbital region of the head in Campylomormyrus and Sternarchorhynchus correlate with the independent acquisition in these two genera of two other features; body elongation of varying degrees and electrogenic abilities. A correlation between body elongation and grasp-suction feeding may not derive directly from this feeding mode, but rather may be functionally associated with the requirements of the generative and receptive systems associated with electrolocation and electrocommunication. Both electrolocation and electrocommunication, however, provide distinct advantages in the habitats occupied by Campylomormyrus and Sternarchorhynchus. Electrolocation facilitates the targeting of potential prey items in the mud, leaf litter, and interstices of clay nodules under the limited light conditions characteristic of most tropical rivers inhabited by Campylomormyrus and Sternarchorhynchus. Detection and fine-scale location of prey is a prerequisite so that the species of these genera can then bring into action their unique ‘grasp-suction’ feeding system. Electrocommunication is also undoubtedly critical in such habitats ‘for signaling of identity or behavior states and intentions’ (Crampton & Albert, 2006: 649) to at least congeners. As is the case with electrolocation, this ability provides a major advantage in the dark waters inhabited by Campylomormyrus and Sternarchorhynchus on the two continents.

Sexual dimorphism

The comments in the introduction summarized the several types of sexual dimorphism that occur in the Apteronotidae. Sexual dimorphism of diverse forms, including snout form and modifications of the lower jaw and dentary teeth, arose several times in the family under present hypotheses of relationships (Rapp Py-Daniel & Cox-Fernandes, 2005). Rapp Py-Daniel & Cox-Fernandes (2005) proposed that the sexual dimorphism in some Sternarchorhynchus species is most parsimoniously interpreted as having arisen independently relative to other instances of that phenomenon in the Apteronotidae. Our conclusions, in particular the evidence supporting the hypothesis of a sister-group relationship of Sternarchorhynchus with Platyurosternarchus, a genus in which sexual dimorphism is unknown (de Santana & Vari, 2009), are consistent with this hypothesis. The distribution of sexual dimorphism within Sternarchorhynchus reveals interesting intrageneric patterns. The following comments should be considered with the caveat that various species of Sternarchorhynchus are known from limited samples, in some instances consisting solely of probable juveniles. It is probable that the totality of sexual dimorphism across the genus is more extensive than presently documented.

Survey of extensive samples of Platyurosternarchus, the sister group to Sternarchorhynchus, failed to reveal sexually dimorphic features in that genus (de Santana & Vari, 2009). A lack of such modifications similarly characterized the species of the basal clade 4 (S. goeldii and S. oxyrhynchus) within Sternarchorhynchus. Lack of sexual dimorphism in the species of clade 4 is noteworthy in that we examined large series of specimens of all sizes of those two species and the absence of the trait is unlikely to be an artefact of limited samples. Evidence as to the presence versus absence of sexual dimorphism is unavailable for some species in clade 5 (all species of Sternarchorhynchus with the exception of S. goeldii and S. oxyrhynchus), but notwithstanding that limitation, the phenomena manifests itself in 12 of the 22 species in clade 5 (S. axelrodi, S. caboclo, S. cramptoni, S. curumim, S. gnomus, S. hagedornae, S. higuchii, S. inpai, S. mareikeae, S. mormyris, S. retzeri, S. roseni) with those species distributed across the clade. Sexual dimorphism within this lineage typically involves the positions of the anus and urogenital papillae, with the variation in these features sometimes correlated with sexually dimorphic coloration differences.

One particularly striking form of sexual dimorphism is unique amongst gymnotiforms to some members of clade 17. Some mature males in five of the ten species in clade 17 (S. cramptoni, S. hagedornae, S. higuchii, S. retzeri, S. roseni) have the anterior portion of the dentary expanded both laterally and dorsally. The resultant dorsally bulbous structure is transversely rounded along the longitudinal axis of the snout and surmounted by a series of enlarged dentary teeth (Fig. 25A; also Rapp Py-Daniel & Cox-Fernandes, 2005: fig. 5b). Males of these species possibly use this modified lower jaw during antagonistic encounters (Marrero & Taphorn, 1991; Kirschbaum, 1995; Cox-Fernandes et al., 2002). Several of the species within clade 17 not known to demonstrate sexual dimorphism in the form of the lower jaw and dentary dentition are represented by limited samples in this study. An examination of larger samples of those forms would be likely to reveal the presence of these derived modifications in other species in this lineage. These elaborations of the dentary and dentary dentition potentially represent a complex of synapomorphies for the species of clade 17. In the absence of definitive information on the presence or absence of these sexually dimorphic elaborations in all species of clade 17, we deferred from incorporating those features into the phylogenetic analysis. Two other species of Sternarchorhynchus not incorporated into the phylogenetic analysis because of limited samples (S. taphorni, S. yepezi) also demonstrate these pronounced elaborations of the lower jaw and associated dentition in some mature males. Thus, they are likely to be components of clade 17 or a slightly more inclusive lineage.

Body size and habitat

The innovations described previously potentially provided a competitive advantage to Sternarchorhynchus relative to other clades of Apteronotidae during the colonization of Neotropical rivers and may account for the speciose nature of this genus within the context of the Apteronotidae. Two other aspects of morphology and behaviour may have contributed to the success of the genus. Examination of body size within the context of the arrived-at phylogeny reveals repetitive shifts in maximum body sizes within Sternarchorhynchus that probably permit the exploitation of a more diverse suite of resources. Similarly, preferred habitats for the species of the genus demonstrate definite patterns when evaluated within the context of phylogeny, with such shifts further expanding the habitats available to members of Sternarchorhynchus.

Platyurosternarchus, the sister group to Sternarchorhynchus, grows to a relatively large body size (415 mm TL; de Santana & Vari, 2009). Basal clades within Sternarchorhynchus(clades 4, 6) include the largest species in the family, with examined specimens of clade 4 exceeding 460 mm TL and those of clade 6 reaching approximately 550 mm TL. Species of clade 7 are with few exceptions of shorter to distinctly shorter maximum known body sizes, with 16 of the 20 species in that clade (S. britskii, S. caboclo, S. chaoi, S. curumim, S. curvirostris, S. gnomus, S. hagedornae, S. higuchii, S. inpai, S. jaimei, S. mareikeae, S. mendesi, S. mesensis, S. montanus, S. severii, S. stewarti) not known to achieve TLs greater than 275 mm with some of these species sexually mature by 150 mm TL. Body sizes of those standard lengths apparently represent the primitive condition for the species of clade 7. Reversals to distinctly larger body sizes occur three times within the clade, with all of the involved species achieving TLs of at least 375 mm. These are S. starksi(clade 16), S. cramptoni and S. retzeri that jointly form clade 30, and S. roseni(clade 24).

The second transition of note within the phylogenetic scheme for Sternarchorhynchus involves habitat preferences. In the discussion, we follow the definitions used by Crampton & Albert (2006: 659) for streams (terra firme streams and small rivers not exposed to seasonal floodplain flooding) and rivers (deep, swiftly flowing channels). Platyurosternarchus species are somewhat generalists in terms of habitat, being collected both in streams and along the margins of larger rivers (de Santana & Vari, 2009). The large body size Sternarchorhynchus species of basal clades 4 and 6 (S. axelrodi, S. goeldii, S. mormyrus, S. oxyrhynchus) are inhabitants of large rivers. This is also the case for the large body-sized S. roseni of clade 24. Such a correlation between large body sizes and larger water bodies would seem logical given the reported dietary preferences for deep-water invertebrates that characterize these species. Alternative preferences for rapid water conditions were documented for an unidentified Sternarchorhynchus species by Schwassmann & Carvalho (1985: 237) and this attribute has been found to be broadly distributed across the genus. Twelve of the 19 intermediate-sized species of clade 7 have an overall preference for fluviatile habitats, with the other seven species in that lineage showing repeated independent transitions to life in rapids. Rheophilic habits characterize S. caboclo(clade 8), S. severii(clade 9), S. inpai(clade 11), S. mareikeae(clade 13), S. jaimei(clade 22), and S. higuchii(clade 24). Sternarchorhynchus hagedornae(clade 17) also lives in high-energy stream settings albeit without evidence that it occurs in rapids. Transitions from the ancestral riverine habitat preferences thus appear to have occurred multiple times independently within Sternarchorhynchus. Furthermore, S. jaimei and S. kokraimoro, species not included in the phylogeny because of limited samples, also were captured in the high energy habitat of the rapids of the lower Rio Xingu. An absence of information as to the phylogenetic position of these two species renders it impossible to determine whether these represent one or more additional independent transitions to rheophilic conditions. Nonetheless, the evidence indicates that rheophilic stenotopy with rapids and other high-energy situations were acquired independently on multiple occasions within Sternarchorhynchus as was the case with at least some groups of Neotropical cichlids (comments in Kullander & Ferreira, 2005). More extensive collecting efforts in diverse habitats throughout the river systems inhabited by the apparently rheophilic species of Sternarchorhynchus are prerequisite for an evaluation of the degree to which these species are actually exclusively stenotopic for rapids and other areas of very rapid water flow.

Rheophily, the tendency to thrive in or have a preference for running water, occurs in numerous groups of South American fishes [summary in Lima & Zuanon (2004: 120)] including other apteronotids [e.g. Megadontognathus(Campos-da-Paz, 1999); some species of Sternarchogiton]. Preference for rapidly flowing water and rapids is correlated with diverse morphological modifications in some groups of fish, whereas in other taxa the adaptations to high energy habitats are behavioural. We were unable to identify morphological modifications unique to all of the rheophilic species of Sternarchorhynchus to the exclusion of their congeners. Although a long tail such as is present in some species of the genus living in rapidly flowing waters might on first consideration be thought to correlate with rheophily, comparable elongations of the tail occur amongst congeners inhabiting lentic water bodies and slower flowing streams and rivers. Behavioural observations of the rheophilic species of Sternarchorhynchus in nature are lacking to date. Nonetheless, in the absence of any manifest external morphological shifts, it appears that the rheophily typical of some species of Sternarchorhynchus is likely to be a consequence of as-of-yet undetermined behavioural adaptations.

TAXONOMIC ACCOUNTS

The dramatic increase in the number of species of Sternarchorhynchus makes it impossible to associate confidently many of the literature records for the long-described species S. curvirostris, S. mormyrus, and S. oxyrhynchus with one of the species recognized in this revision. Particularly problematic in this respect are citations of one or more of these species of Sternarchorhynchus within compendia of species purportedly inhabiting various rivers, regions, or countries. Myriad drainage basins and regions originally cited as inhabited by a single species of Sternarchorhynchus rather include two or more often more members of the genus in their ichthyofaunas. We thus restrict the synonymies for S. curvirostris, S. oxyrhynchus, and S. mormyrus to the original descriptions of the species and major subsequent publications (e.g. Ellis, 1913; Mago-Leccia, 1994). Exceptions involve instances where evidence in a publication or the examination of samples that served as the basis for the record in question allowed us to associate unequivocally a published citation with a particular species.

SternarchorhynchusCastelnau, 1855

SternarchorhynchusCastelnau, 1855: 91 [Campos-da-Paz, 2000: 525, concerning page numbering; type species: Sternarchorhynchus mulleri, Castelnau, 1855(= Sternarchus oxyrhynchus, Müller & Troschel, 1848); type by monotypy; masculine].

RhamphosternarchusGünther, 1870: 4 (type species Sternarchus oxyrhynchus, Müller & Troschel, 1848; type by subsequent designation of Jordan, 1919; proposed as subgenus of Sternarchus, Bloch & Schneider, 1801; masculine).

SternarchorchynchusMago-Leccia, 1978: 14 (misspelling).

Diagnosis:

Synapomorphies for Sternarchorhynchus were discussed under the preceding phylogenetic reconstruction and are summarized in the synapomorphy list for the genus in Appendix 2(clade 3). Although these characters delimit the genus as monophyletic, they overwhelmingly involve internal characters and are therefore inappropriate to determine whether whole specimens at hand are members of Sternarchorhynchus. The following combination of derived and plesiomorphic characters serve to delimit members of the genus.

Sternarchorhynchus shares with other species of the Apteronotidae the presence of a caudal fin and a fleshy dorsal electroreceptive filament, features that readily distinguish the family from other groups in the Gymnotiformes. Species of Sternarchorhynchus differ from other genera in the Apteronotidae by the combination of the possession of an elongate, tubular snout that is compressed laterally and slightly to strongly curved ventrally anteriorly; the length of the snout (usually more than 50% of HL), the small mouth (usually less than 12% of HL), the pronounced distance from the posterior naris to the eye (usually more than 37% of HL), and the posterior position of the origin of the midsaggital electroreceptive filament (approximately at, or posterior to, the middle of TL and situated approximately at 55 to 70% of TL).

Remarks:

Campos-da-Paz (2000: 527) detailed the history of the recognition, or lack thereof, of Sternarchorhynchus by authors following Castlenau's (1855) proposal of the genus. As discussed by Campos-da-Paz, various authors failed to recognize Sternarchorhynchus for nonspecified reasons, perhaps because they were not aware of Castlenau's description of the genus.Günther (1870: 4), for example, was apparently unaware of Sternarchorhynchus because he proposed Rhamphosternarchus as a subgenus of Sternarchus, with that genus including three species amongst which was S. oxyrhynchus, the type species of Sternarchorhynchus. The Rhamphosternarchus of Günther was based on the presence in member species of a ‘snout produced into a long tube’, one of the distinguishing attributes for Sternarchorhynchus. Eigenmann & Ward (1905: 166) rectified the problem by explicitly noting that Rhamphosternarchus was a synonym of Sternarchorhynchus.

Common features of species of Sternarchorhynchus:

The species of Sternarchorhynchus share a distinctive bauplan with many of the features delimiting species involving meristics and specific morphometric features and/or coloration rather than significant changes in overall external head and/or body morphology. In the interest of space efficiency, we describe external features common to all species of Sternarchorhynchus in this section rather than reiterating them in each species description. Conditions of the intragenerically variable external features for each species are detailed in each species account.

Body elongate and distinctly laterally compressed (cultiform), more so in region posterior to abdominal region. Greatest body depth located in area of abdominal cavity or slightly posterior to that region. Dorsal profile of body ranges between straight to very slightly convex. Anteriormost perforated lateral line scale located along vertical through pectoral-fin origin.

Head laterally compressed, widest in opercular region and deepest at nape. Eye very small, laterally positioned on dorsal half of head, and completely covered by thin membrane. Cephalic pores small. Snout distinctly elongate, overall tubular but somewhat compressed laterally. Premaxilla of small size. Anterior naris located at end of small tube and proximate to tip of snout. Posterior naris ellipsoid, without tubular extension. Posterior naris located distinctly closer to tip of snout than to anterior margin of eye. Branchial opening constricted to short vertical aperture situated along posterior margin of opercle and slightly anterior to vertical through pectoral-fin origin. Branchial membranes joined at isthmus. Anus and urogenital papilla adjacent and ventrally positioned. Location of both structures intragenerically and ontogenetically variable in many species and sexually dimorphic in some species.

Scales cycloid, small and present over all, or nearly all, of body from rear of head to anterior portion of base of caudal fin. Scales absent or sparse in some species along mid-dorsal line of body anterior to origin of electroreceptive filament. Lateral line scales often larger than those of scale rows immediately dorsal and ventral of lateral line series.

Pectoral fin long, broad, and distally pointed. Anal fin elongate and extending from under head posteriorly for most of length of body. Dorsal electroreceptive filament arising on posterior portion of body and progressively narrowing posteriorly and inserting into and attached to narrow mid-dorsal grove. Caudal and pectoral fins present, but dorsal and pelvic fins absent as in all gymnotiforms.

Distribution:

A distinct majority of the species of Sternarchorhynchus inhabit portions of the Amazon basin (22 of 32 species, Table 1), with the genus occurring across major portions of that drainage system. Many of the apparent major gaps in the distribution of the genus across the Amazon basin are most likely to be a function of the lack of collecting in habitats appropriate for the members of the genus. This issue is general for the Neotropical freshwater fish fauna (Vari & Malabarba, 1998). Seven species of Sternarchorhynchus occur in the Río Orinoco system and the independent river systems of north-eastern Venezuela that drain into the Golfo de Paria (Table 1). One of these species, S. mormyrus, is the only member of the genus shared with the Amazon basin (comments under that species description).

Occurrence of the species of Sternarchorhynchus in river systems and regions of cis-Andean South America

Species River basins and regions 
S. axelrodi    x  x       
S. britskii          x   
S. caboclo            x 
S. chaoi       x      
S. cramptoni   x   x x      
S. curumim       x      
S. curvirostris         x    
S. freemani  x           
S. galibi  x           
S. gnomus x            
S. goeldii   x   x x x     
S. hagedornae         x    
S. higuchii   x          
S. inpai     x        
S. jaimei   x          
S. kokraimoro           x  
S. mareikeae     x        
S. marreroi x            
S. mendesi x            
S. mesensis    x         
S. montanus         x    
S. mormyrus x  x    x  x    
S. oxyrhynchus x            
S. retzeri   x   x x      
S. roseni x            
S. schwassmanni    x         
S. severii            x 
S. starksi      x       
S. stewarti         x    
S. taphorni         x    
S. yepezi x            
S. villasboasi           x  
Species River basins and regions 
S. axelrodi    x  x       
S. britskii          x   
S. caboclo            x 
S. chaoi       x      
S. cramptoni   x   x x      
S. curumim       x      
S. curvirostris         x    
S. freemani  x           
S. galibi  x           
S. gnomus x            
S. goeldii   x   x x x     
S. hagedornae         x    
S. higuchii   x          
S. inpai     x        
S. jaimei   x          
S. kokraimoro           x  
S. mareikeae     x        
S. marreroi x            
S. mendesi x            
S. mesensis    x         
S. montanus         x    
S. mormyrus x  x    x  x    
S. oxyrhynchus x            
S. retzeri   x   x x      
S. roseni x            
S. schwassmanni    x         
S. severii            x 
S. starksi      x       
S. stewarti         x    
S. taphorni         x    
S. yepezi x            
S. villasboasi           x  

River systems and regions are as follows: A, Rio Orinoco basin; B, Guianas; C, Rio Negro; D, Rio Tocantins/Araguaia; E, Rio Trombetas; F, mainstream Amazon below Manaus; G, mainstream Amazon above Manaus; H, Rio Madeira; I, western Amazonian tributaries; J, upper Rio Paraná; K, Rio Xingu; L, Rio Branco.

The sparse records of Sternarchorhynchus in the rivers of the Guianas (two species) may be in part a function of the limited collecting efforts in appropriate habitats, but the genus is unknown even in some relatively well sampled basins in this region (Mol et al., 2007). Similar gaps in distributions at the generic level occur in other components of the ichthyofauana in the Guianas. A single species of Sternarchorhynchus occurs in the Essequibo River system of Guyana, one species in the Lawa River of the Marowijne River (= Fleuve Maroni) system along the border between Suriname and French Guiana. Sternarchorhynchus has also been reported from the Fleuve Oyapock (= Rio Oiapoque) along the boundary between French Guiana and Brazil (Planquette, Keith & Le Bail, 1996: 406). We were unable to examine the specimens that served as the basis for that report. A single species of Sternarchorhynchus, S. britskii, is known from the upper portions of the Rio Paraná basin in Brazil; that being the only member of the genus occurring in a drainage system south of the Amazon basin.

Diversity:

The 32 recognized species of Sternarchorhynchus represent approximately 40% of the present known diversity at the species level within the Apteronotidae, making it by far the most speciose genus in the family, with the approximately 15 recognized species of Apteronotus(sensu stricto; Albert & Crampton, 2005: 363) making it the next most diverse genus in the family. As we note under the Possible additional undescribed species section, the 32 recognized species of Sternarchorhynchus undoubtedly represent an underestimate of the actual diversity in the genus. Under present concepts of generic limits and species richness, Sternarchorhynchus is one of the most speciose genera within the Gymnotiformes. Only Gymnotus in the Gymnotidae, that is estimated to include at least 35 species (Albert & Crampton, 2005; Albert et al., 2005) is as speciose as Sternarchorhynchus.

An evaluation of the diversity of Sternarchorhynchus relative to that of Gymnotus shows some notable differences in the proportional density of species in the two genera across their range. Most obviously, Sternarchorhynchus is restricted to the cis-Andean portions of South America with only one species, S. britskii, known from a river basin south of the Amazon, whereas Gymnotus has a massive distribution from Mexico to Argentina to both sides of the Andean Cordilleras. Gymnotus is, furthermore, able to tolerate a much broader range of habitats, being able to absorb atmospheric oxygen via a portion of the gas-bladder (Campos-da-Paz, 2003: 483); an adaptation that opens up low oxygen-level backwaters and swamps as potential habitats. Sternarchorhynchus lacks such modifications and is apparently limited to well-oxygenated habitats. The more restricted geographical range and habitat tolerances of the species of Sternarchorhynchus render its speciose nature all the more impressive in terms of proportional density of species. In part this density is made possible by the ability by many species of Sternarchorhynchus to exploit deep river habitats, which contrasts with the absence of all genera of the Gymnotidae from such habitats (Crampton, 2007: 302).

Remarks on secondary sexual dimorphism:

Various species within one clade within Sternarchorhynchus together with various other Gymnotiformes (e.g. Sternarchogiton; de Santana & Crampton, 2007: 390: fig. 1a; Compsaraia, Albert & Crampton, 2009: fig. 1) demonstrate particularly striking sexual dimorphic remodelling of the lower jaw of various forms. Some males with well-developed testes amongst the Sternarchorhynchus species in clade 17 have the dentary anteriorly lengthened and transversely expanded into a dorsally bulbous structure that is rounded from a dorsal view (Fig. 25A; comments on Sexual dimorphism under Discussion above). This enlarged portion of the dentary bears a patch of distinctly enlarged, typically slightly posteriorly recurved dentition. This patch of enlarged teeth in males of this clade may be utilized in confrontations during breeding contests (Marrero & Taphorn, 1991: 129).

Such dramatic elaborations of the lower jaw and dentary dentition are unknown in some species of the clade represented by limited numbers of specimens and larger samples are necessary to determine whether the condition is indeed absent in these species. We have examined conspecific males of the species characterized by such dentary modifications with well-developed testes that nonetheless lack elaborations of the lower jaw and associated dentition. This is despite those individuals being of body sizes comparable to conspecific males with those attributes. Thus, one cannot assume that large specimens of a species within clade 17 that lack an expansion of the dentary and elaborations of the dentary dentition are females. The significance of these differences amongst conspecific males of comparable sizes cannot be elucidated from museum specimens, but potentially reflects social structure amongst groups of males. Alternatively, it may be a function of seasonality in the manifestation of this secondary sexually dimorphic feature.

Keys to the species of Sternarchorhynchus

Keys are presented for the species of Sternarchorhynchus by major drainage basins (Amazon, Orinoco, Paraná) or region (Guianas) because each of those units have largely endemic faunas of Sternarchorhynchus species. As an aid to the identification of species, their distributions are detailed in Table 1. The alternative of a single, all-inclusive key for all members of the genus would be quite complex and involve working through multiple couplets in order to identify materials of many species only one of which (S. mormyrus) occurs in more than one river basin (Amazon plus Orinoco). Sternarchorhynchus mormyrus is included in the keys for the species in both of those basins. The diagnoses under each species account serve to separate differentially each species from all congeners. Sternarchorhynchus mesensis demonstrates variation in the degree to which the narrow, lightly coloured stripe on the head and body is obvious. Some specimens of the species in isolation might be considered to lack the stripe and the species consequently appears in two places in the key for the members of the genus in the Amazon basin.

Differences in anal-fin ray numbers, both anterior unbranched and total rays, proved to be the most informative meristic features for distinguishing Sternarchorhynchus species. Frequency distributions of these values for the species are provided in Tables 2 and 3 to assist readers with the identification of specimens at hand. Supplementing the information in the keys, diagnoses, and tables are the phylogenetically informative characters discussed under Character description and analysis. That evidence underpins our recognition of multiple species within a well-resolved hypothesis of relationships. Autapomorphies characterizing a number of species are listed in Appendix 2.

Frequency of distribution of anterior anal-fin rays in species of Sternarchorhynchus

 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 
S. axelrodi                      
S. britskii                      
S. caboclo                     
S. chaoi                   
S. cramptoni                  
S. curumim                         
S. curvirostris                          
S. freemani                        
S. galibi                      
S. gnomus                        
S. goeldii                
S. hagedornae                     
S. higuchii                    
S. inpai                        
S. jaimei                       
S. kokraimoro                         
S. mareikeae                      
S. marreroi                        
S. mendesi                     
S. mesensis                       
S. montanus                         
S. mormyrus                     
S. oxyrhynchus                       
S. retzeri                    
S. roseni                   
S. schwassmanni                         
S. severii                        
S. starksi                      
S. stewarti                     
S. taphorni                          
S. yepezi                         
S. villasboasi                        
 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 
S. axelrodi                      
S. britskii                      
S. caboclo                     
S. chaoi                   
S. cramptoni                  
S. curumim                         
S. curvirostris                          
S. freemani                        
S. galibi                      
S. gnomus                        
S. goeldii                
S. hagedornae                     
S. higuchii                    
S. inpai                        
S. jaimei                       
S. kokraimoro                         
S. mareikeae                      
S. marreroi                        
S. mendesi                     
S. mesensis                       
S. montanus                         
S. mormyrus                     
S. oxyrhynchus                       
S. retzeri                    
S. roseni                   
S. schwassmanni                         
S. severii                        
S. starksi                      
S. stewarti                     
S. taphorni                          
S. yepezi                         
S. villasboasi                        

Frequency of distribution of total anal-fin rays in species of Sternarchorhynchus

 153 157 161 165 169 173 177 181 185 189 193 197 201 205 209 213 217 221 225 229 233 237 241 245 
S. axelrodi                     
S. britskii                     
S. caboclo                   
S. chaoi                     
S. cramptoni                  
S. curumim                       
S. curvirostris                     
S. freemani                      
S. galibi                      
S. gnomus                    
S. goeldii                 
S. hagedornae                   
S. higuchii                    
S. inpai                      
S. jaimei                  
S. kokraimoro                       
S. mareikeae                    
S. marreroi                       
S. mendesi                      
S. mesensis                     
S. montanus                       
S. mormyrus                    
S. oxyrhynchus                     
S. retzeri                   
S. roseni                    
S. schwassmanni                       
S. severii                      
S. starksi                   
S. stewarti                     
S. taphorni                         
S. yepezi                       
S. villasboasi                     
 153 157 161 165 169 173 177 181 185 189 193 197 201 205 209 213 217 221 225 229 233 237 241 245 
S. axelrodi                     
S. britskii                     
S. caboclo                   
S. chaoi                     
S. cramptoni                  
S. curumim                       
S. curvirostris                     
S. freemani                      
S. galibi                      
S. gnomus                    
S. goeldii                 
S. hagedornae                   
S. higuchii                    
S. inpai                      
S. jaimei                  
S. kokraimoro                       
S. mareikeae                    
S. marreroi                       
S. mendesi                      
S. mesensis                     
S. montanus                       
S. mormyrus                    
S. oxyrhynchus                     
S. retzeri                   
S. roseni                    
S. schwassmanni                       
S. severii                      
S. starksi                   
S. stewarti                     
S. taphorni                         
S. yepezi                       
S. villasboasi                     

Anal-fin ray counts arranged in groups of four starting with listed number.

Key to the species ofSternarchorhynchusoccurring in the Amazon basin

  • Gape of mouth aligned with main axis of snout, with gape elongate and extending posteriorly distinctly beyond vertical through posterior naris S. goeldii sp. nov.

    Gape short, terminates posteriorly at, or slightly anterior of, vertical through anterior naris 2

  • Scales either lacking or sparse and covered by skin middorsally at least in part of region from rear of head to insertion of electroreceptive filament 3

    Scales present and obvious along mid-dorsal region of body from rear of head to insertion of electroreceptive filament 9

  • Mid-dorsal scales missing along entire mid-dorsal region to insertion of electroreceptive filament 4

    Mid-dorsal scales either sparse and covered by skin, or missing along mid-dorsal region solely from rear of head to point approximately along vertical through rear of pectoral fin 5

  • Total anal-fin rays 200–217 S. axelrodi sp. nov.

    Total anal-fin rays 222–245 S. mormyrus

  • Dorsal portion of head and body with at most obscure mid-dorsal, lightly coloured stripe on head S. higuchii sp. nov.

    Dorsal portion of head and body with distinct narrow mid-dorsal, lightly coloured stripe on head and often on body 6

  • Mid-dorsal scales completely absent along body approximately to vertical through rear of pectoral fin 7

    Mid-dorsal region of body with scales present, but sparse and covered by skin 8

  • Lateral line extending posteriorly to point three scales short of base of caudal fin; total anal-fin rays 165–178, premaxilla with eight to 13 teeth, postocular distance 39.4–41.3% of HL S. chaoi sp. nov.

    Lateral line extending posteriorly to base of caudal fin; total anal-fin rays 180–195, premaxilla with four to seven teeth, postocular distance 41.5–49.1% of HL S. jaimei sp. nov.

  • Anterior unbranched anal-fin rays 32–35, total anal-fin rays 173–193, length of anal-fin base 86.6–89.6% of LEA, head depth at nape 53.1–59.2% of HL S. hagedornae sp. nov.

    Anterior unbranched anal-fin rays 25–26, total anal-fin rays 164–166, length of anal-fin base 80.2–83.9% of LEA, head depth at nape 49.5–49.6% of HL S. schwassmanni sp. nov.

  • Mid-dorsal region of head and body without narrow, lightly coloured stripe 10

    Mid-dorsal region of head and body with distinct, narrow, lightly coloured stripe 13

  • Lateral line extending posteriorly to point three to five scales short of base of caudal fin, eight to nine series of scales present above lateral line in mid-dorsal region of body S. kokraimoro sp. nov.

    Lateral line extending posteriorly to base of caudal fin, ten to 13 series of scales present above lateral line in mid-dorsal region of body 11

  • Total anal-fin rays 153–174, branchiostegal rays four S. caboclo

    Total anal-fin rays 171–189, branchiostegal rays five 12

  • Premaxillary teeth seven to ten, teeth in outer tooth row of dentary seven to eight, prepectoral-fin distance 18.3–20.6% of LEA, internarial distance 2.4–2.8% of HL S. curumim

    Premaxillary teeth 11–14, teeth in outer row of dentary ten to 11, prepectoral-fin distance 21.4–23.8% of LEA, internarial distance 3.2–4.7% of HL S. mesensis

  • Mid-dorsal lightly coloured stripe limited to head 14

    Mid-dorsal lightly coloured stripe commencing on snout and extending posteriorly to insertion of electroreceptive filament 17

  • Lateral line extending posteriorly to point four to eight scales anterior of base of caudal fin 15

    Lateral line extending posteriorly to base of caudal fin 16

  • Total anal-fin rays 156–167, caudal length 12.6–15.6% of LEA, postocular distance 33.1–41.1% of HL, anal fin somewhat dusky, but not dark S. inpai sp. nov.

    Total anal-fin rays 186, caudal length 6.8% of LEA, postocular distance 43.5–46.4% of HL, anal fin dark S. taphorni sp. nov.

  • Premaxillary teeth 12–13, teeth in outer row of dentary 12–13, tip of electroreceptive filament located four to seven scales posterior of vertical through termination of base of anal fin, pectoral-fin length 35.0–40.9% of HL, postocular distance 36.0–41.3% of HL S. mareikeae sp. nov.

    Premaxillary teeth eight to nine, teeth in outer row of dentary eight to nine, tip of electroreceptive filament located two scales posterior of vertical through termination of base of anal fin, pectoral-fin length 43.6–47.1% of HL, postocular distance 44.4–46.0% of HL S. villasboasisp. nov.

  • Distal portions of anal fin with dark band covering one-third to one-half of most of fin rays and sometimes all of posterior rays 18

    Distal potions of anal fin without distinct dark band 20

  • Postocular distance 31.6–34.9% of HL, caudal length 7.1–10.3% of LEA S. retzeri sp. nov.

    Postocular distance 37.4–43.7% of HL, caudal length 5.3–8.4% of LEA 19

  • Premaxillary teeth eight to 12, distance from anus to snout 8.9–12.0% of LEA, pectoral-fin length 38.0–46.5% of HL, tail depth 23.6–29.6% of caudal length S. cramptoni sp. nov.

    Premaxillary teeth six to seven, distance from anus to snout 6.9–7.3% of LEA, pectoral-fin length 47.8–49.1% of HL, tail depth 21.2–21.7% of caudal length S. montanus sp. nov.

  • Anal fin distinctly dark basally S. curvirostris sp. nov.

    Anal fin hyaline or slightly dusky, but not distinctly dark basally 21

  • Caudal length 6.1–7.1% of LEA S. stewarti sp. nov.

    Caudal length 9.0–16.5% of LEA 22

  • Preanal distance 14.2–17.0% of LEA, head length 20.5–23.6% of LEA, pectoral-fin length 27.0–35.3% of HL S. mesensis

    Preanal distance 9.4–12.7% of LEA, head length 14.8–18.1% of LEA, pectoral-fin length 40.6–53.2% of HL S. starksi sp. nov.

Key to the species ofSternarchorhynchusoccurring in the Río Orinoco basin

  1. Gape of mouth aligned with main axis of snout, with gape elongate and extending posteriorly distinctly beyond vertical to posterior naris S. oxyrhynchus

    Gape short, terminates posteriorly at, or slightly anterior of, vertical through anterior naris 2

  2. Mid-dorsal region of head and body without narrow, lightly coloured stripe or stripe obscure S. gnomus

    Mid-dorsal region of head and body with distinct, narrow, lightly coloured stripe 3

  3. Scales along mid-dorsal region of body either lacking along at least part of region or scales sparse and covered by scales 4

    Scales present and obvious along mid-dorsal region of body from rear of head to insertion of electroreceptive organ S. marreroi sp. nov.

  4. Total anal-fin rays 174–188 5

    Total anal-fin rays 193–245 6

  5. Total anal-fin rays 174–182; anterior unbranched anal-fin rays 29–46; postocular distance 38.8–40.8% of LEA; caudal-fin length 18.1–33.0% of caudal length; scales above lateral line at midbody six to nine S. mendesisp. nov.

    Total anal-fin rays 182–188; anterior unbranched anal-fin rays 23–29; postocular distance 43.8–46.2% of LEA; caudal-fin length 38.8% of caudal length; scales above lateral line at midbody nine to 12 S. yepezi sp. nov.

  6. Total anal-fin rays 193–210; lightly coloured mid-dorsal stripe present from tip of snout to origin of electroreceptive organ; mid-dorsal region of body scaled S. roseni

    Total anal-fin rays 222–245; mid-dorsal region of head and body uniformly coloured; mid-dorsal region of body lacking scales S. mormyrus

Key to the species ofSternarchorhynchusoccurring in the coastal basins of Guyana and Suriname

  1. Caudal length 8.7–10.8% of LEA; distance from anus to anal-fin insertion 6.8–12.8% of LEA; tail depth 13.8–19.2% of caudal length; premaxilla with six to ten teeth S. galibi sp. nov.

    Caudal length 15.1–16.9% of LEA; distance from anus to anal-fin insertion 13.0–17.4% of LEA; tail depth 8.0–8.9% of caudal length; premaxilla with four to five teeth S. freemani sp. nov.

Species ofSternarchorhynchusoccurring in the upper Rio Paraná basin

Only a single Sternarchorhynchus species, S. britskii, is known from the upper Rio Paraná basin or indeed across the Río de La Plata system. Sternarchorhynchus britskii can be distinguished by all congeners by the features listed in the Diagnosis for that species.

Species accounts

In order to facilitate the location of individual species accounts and cross references between them, the species descriptions are arranged alphabetically within Sternarchorhynchus rather than by region of occurrence or phylogenetic affinity. Species accounts are followed by a discussion (Possible Additional Undescribed Species) of population samples that represent likely undescribed species of Sternarchorhynchus that are not formally described at this time.

Sternarchorhynchus axelrodisp. nov. (Figs26–28; Table4)

Sternarchorhynchus axelrodi sp. nov., holotype, male, 444 mm total length, MNRJ 31279; Brazil, Goiás, Rio Tocantins.

Sternarchorhynchus axelrodi sp. nov., holotype, male, 444 mm total length, MNRJ 31279; Brazil, Goiás, Rio Tocantins.

Sternarchorhynchus axelrodi sp. nov., paratype, female, 493 mm total length, MNRJ 12168; Brazil, Goiás, Rio Tocantins; showing more pronounced ventral curvature of snout in mature females of species.

Sternarchorhynchus axelrodi sp. nov., paratype, female, 493 mm total length, MNRJ 12168; Brazil, Goiás, Rio Tocantins; showing more pronounced ventral curvature of snout in mature females of species.

Map of central and northern South America showing geographical distribution of Sternarchorhynchus axelrodi sp. nov.(1 = holotype locality), Sternarchorhynchus britskii, Sternarchorhynchus caboclo, Sternarchorhynchus chaoi sp. nov.(2 = holotype locality), and Sternarchorhynchus cramptoni sp. nov.(3 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus axelrodi sp. nov.(1 = holotype locality), Sternarchorhynchus britskii, Sternarchorhynchus caboclo, Sternarchorhynchus chaoi sp. nov.(2 = holotype locality), and Sternarchorhynchus cramptoni sp. nov.(3 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Morphometrics for examined specimens of Sternarchorhynchus axelrodi, Sternarchorhynchus britskii, and Sternarchorhynchus caboclo(paratypes, INPA 10594)

 S. axelrodi S. britskii S. caboclo 
Paratypes Mean Range Mean Paratypes Mean 
Total length (mm) 444 433–530 (N= 9) – 261 209–250 (N= 14) – 225 46.3–226 (N= 57) – 
Length to end of anal fin (mm) 420 410–510 (N= 9) – 242 179–232 (N= 11) – 206 41.1–206 (N= 57) – 
Head length (mm) 99.9 93.7–115.9 (N= 9) – 47.5 33.1–46.9 (N= 14) – 39.2 8.7–39.2 (N= 57) – 
Caudal length (mm) 23.3 24.9–30.5 (N= 9) – 19.2 12.0–192 (N= 10) – 16.8 8.7–21.6 (N= 57) – 
Per cent of length to end of anal fin 
Anal-fin base 83.3 76.9–97.1 (N= 9) 85.1 87.6 86.4–93.0 (N= 11) 88.3 84.0 79.3–86.4 (N= 57) 83.1 
Distance snout to anus 8.5 4.6–8.1 (N= 9) 6.9 – – – 11.2 11.2–14.9 (N= 10) 12.9 
Preanal-fin distance 16.5 14.1–18.5 (N= 9) 15.9 13.3 12.5–15.8 (N= 11) 13.5 16.4 12.1–20.0 (N= 57) 16.6 
Prepectoral-fin distance 24.3 22.1–25.7 (N= 9) 24.0 20.1 19.4–23.6 (N= 11) 21.1 21.1 17.4–25.1 (N= 57) 21.5 
Greatest body depth 14.4 13.5–17.6 (N= 9) 15.1 14.1 12.1–15.7 (N= 13) 14.3 11.0 11.3–14.7 (N= 57) 12.7 
Head length 23.7 21.8–25.1 (N= 9) 23.4 19.6 18.9–23.1 (N= 14) 20.1 19.0 15.6–24.5 (N= 57) 20.9 
Caudal length 5.5 4.9–6.7 (N= 9) 6.1 7.9 5.4–8.5 (N= 14) 7.8 8.2 8.8–23.9 (N= 57) 12.8 
Per cent of head length 
Anus to anal-fin insertion 27.6 29.5–48.2 (N= 9) 37.1 24.8 14.4–23.9 (N= 14) 18.7 25.1 2.9–19.3 (N= 57) 11.2 
Pectoral-fin length 35.7 35.3–38.7 (N= 9) 37.5 39.3 35.5–41.3 (N= 14) 38.6 39.5 31.3–46.9 (N= 57) 39.0 
Head depth at eye 31.0 27.9–33.2 (N= 9) 30.1 33.3 18.2–36.5 (N= 14) 29.1 24.4 23.3–31.1 (N= 10) 26.5 
Head depth at nape 50.5 46.4–60.7 (N= 9) 53.5 53.9 44.9–57.7 (N= 14) 49.2 46.6 45.4–60.3 (N= 57) 50.9 
Head width 24.8 22.9–27.6 (N= 9) 25.1 28.4 15.6–28.4 (N= 14) 22.7 19.1 18.1–23.5 (N= 10) 20.7 
Snout length 54.7 51.8–55.6 (N= 9) 54.0 63.8 59.8–65.7 (N= 14) 62.4 58.3 44.3–61.3 (N= 57) 54.6 
Posterior naris to snout 10.0 8.7–10.5 (N= 9) 9.6 53.4 41.1–58.4 (N= 14) 50.9 8.1 6.4–9.1 (N= 10) 8.0 
Posterior naris to eye 44.6 42.6–45.5 (N= 9) 44.4 55.8 53.9–57.9 (N= 14) 56.1 61.8 29.2–77.2 (N= 57) 56.4 
Mouth length 9.1 8.0–9.7 (N= 9) 8.9 7.3 5.7–7.8 (N= 14) 6.7 7.1 5.6–12.4 (N= 57) 8.0 
Internarial distance 3.9 2.5–4.2 (N= 9) 3.7 2.7 2.5–3.6 (N= 14) 3.1 3.7 3.0–6.4 (N= 57) 4.3 
Eye diameter 2.9 2.6–3.5 (N= 9) 3.0 3.6 2.9–4.6 (N= 14) 3.9 6.3 4.1–10.5 (N= 57) 6.7 
Interocular width 5.4 5.0–6.6 (N= 9) 5.90 6.2 4.1–7.9 (N= 14) 6.3 6.1 7.0–19.7 (N= 57) 10.0 
Postocular distance 47.3 46.1–49.3 (N= 9) 47.8 42.7 40.0–45.9 (N= 14) 43.7 42.2 37.1–53.9 (N= 57) 45.4 
Branchial opening 15.8 11.2–16.9 (N= 9) 15.2 15.5 12.1–17.9 (N= 14) 16.3 13.1 11.8–25.2 (N= 57) 15.6 
Per cent of caudal length 
Tail depth 33.0 25.9–35.7 (N= 9) 30.7 22.3 18.9–25.9 (N= 10) 22.2 17.6 8.7–21.3 (N= 57) 15.3 
Caudal-fin length 58.7 49.7–62.5 (N= 9) 54.1 37.1 33.3–46.4 (N= 10) 39.4 38.2 17.2–38.9 (N= 57) 28.6 
 S. axelrodi S. britskii S. caboclo 
Paratypes Mean Range Mean Paratypes Mean 
Total length (mm) 444 433–530 (N= 9) – 261 209–250 (N= 14) – 225 46.3–226 (N= 57) – 
Length to end of anal fin (mm) 420 410–510 (N= 9) – 242 179–232 (N= 11) – 206 41.1–206 (N= 57) – 
Head length (mm) 99.9 93.7–115.9 (N= 9) – 47.5 33.1–46.9 (N= 14) – 39.2 8.7–39.2 (N= 57) – 
Caudal length (mm) 23.3 24.9–30.5 (N= 9) – 19.2 12.0–192 (N= 10) – 16.8 8.7–21.6 (N= 57) – 
Per cent of length to end of anal fin 
Anal-fin base 83.3 76.9–97.1 (N= 9) 85.1 87.6 86.4–93.0 (N= 11) 88.3 84.0 79.3–86.4 (N= 57) 83.1 
Distance snout to anus 8.5 4.6–8.1 (N= 9) 6.9 – – – 11.2 11.2–14.9 (N= 10) 12.9 
Preanal-fin distance 16.5 14.1–18.5 (N= 9) 15.9 13.3 12.5–15.8 (N= 11) 13.5 16.4 12.1–20.0 (N= 57) 16.6 
Prepectoral-fin distance 24.3 22.1–25.7 (N= 9) 24.0 20.1 19.4–23.6 (N= 11) 21.1 21.1 17.4–25.1 (N= 57) 21.5 
Greatest body depth 14.4 13.5–17.6 (N= 9) 15.1 14.1 12.1–15.7 (N= 13) 14.3 11.0 11.3–14.7 (N= 57) 12.7 
Head length 23.7 21.8–25.1 (N= 9) 23.4 19.6 18.9–23.1 (N= 14) 20.1 19.0 15.6–24.5 (N= 57) 20.9 
Caudal length 5.5 4.9–6.7 (N= 9) 6.1 7.9 5.4–8.5 (N= 14) 7.8 8.2 8.8–23.9 (N= 57) 12.8 
Per cent of head length 
Anus to anal-fin insertion 27.6 29.5–48.2 (N= 9) 37.1 24.8 14.4–23.9 (N= 14) 18.7 25.1 2.9–19.3 (N= 57) 11.2 
Pectoral-fin length 35.7 35.3–38.7 (N= 9) 37.5 39.3 35.5–41.3 (N= 14) 38.6 39.5 31.3–46.9 (N= 57) 39.0 
Head depth at eye 31.0 27.9–33.2 (N= 9) 30.1 33.3 18.2–36.5 (N= 14) 29.1 24.4 23.3–31.1 (N= 10) 26.5 
Head depth at nape 50.5 46.4–60.7 (N= 9) 53.5 53.9 44.9–57.7 (N= 14) 49.2 46.6 45.4–60.3 (N= 57) 50.9 
Head width 24.8 22.9–27.6 (N= 9) 25.1 28.4 15.6–28.4 (N= 14) 22.7 19.1 18.1–23.5 (N= 10) 20.7 
Snout length 54.7 51.8–55.6 (N= 9) 54.0 63.8 59.8–65.7 (N= 14) 62.4 58.3 44.3–61.3 (N= 57) 54.6 
Posterior naris to snout 10.0 8.7–10.5 (N= 9) 9.6 53.4 41.1–58.4 (N= 14) 50.9 8.1 6.4–9.1 (N= 10) 8.0 
Posterior naris to eye 44.6 42.6–45.5 (N= 9) 44.4 55.8 53.9–57.9 (N= 14) 56.1 61.8 29.2–77.2 (N= 57) 56.4 
Mouth length 9.1 8.0–9.7 (N= 9) 8.9 7.3 5.7–7.8 (N= 14) 6.7 7.1 5.6–12.4 (N= 57) 8.0 
Internarial distance 3.9 2.5–4.2 (N= 9) 3.7 2.7 2.5–3.6 (N= 14) 3.1 3.7 3.0–6.4 (N= 57) 4.3 
Eye diameter 2.9 2.6–3.5 (N= 9) 3.0 3.6 2.9–4.6 (N= 14) 3.9 6.3 4.1–10.5 (N= 57) 6.7 
Interocular width 5.4 5.0–6.6 (N= 9) 5.90 6.2 4.1–7.9 (N= 14) 6.3 6.1 7.0–19.7 (N= 57) 10.0 
Postocular distance 47.3 46.1–49.3 (N= 9) 47.8 42.7 40.0–45.9 (N= 14) 43.7 42.2 37.1–53.9 (N= 57) 45.4 
Branchial opening 15.8 11.2–16.9 (N= 9) 15.2 15.5 12.1–17.9 (N= 14) 16.3 13.1 11.8–25.2 (N= 57) 15.6 
Per cent of caudal length 
Tail depth 33.0 25.9–35.7 (N= 9) 30.7 22.3 18.9–25.9 (N= 10) 22.2 17.6 8.7–21.3 (N= 57) 15.3 
Caudal-fin length 58.7 49.7–62.5 (N= 9) 54.1 37.1 33.3–46.4 (N= 10) 39.4 38.2 17.2–38.9 (N= 57) 28.6 

H, holotype; range includes nontype specimens. Number of specimens indicated in parentheses.

Sternarchorhynchus cf. mormyrus, Campos-da-Paz, 2000: 533 [Brazil, Rio Tocantins; as probable undescribed species].

Sternarchorhynchus mormyrus, not of Steindachner, dos Santos et al., 2004: 103, unnumbered figure [Brazil, lower Rio Tocantins; food habits].

Diagnosis:

Sternarchorhynchus axelrodi is distinguished from all congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through the anterior nares, the absence of scales along the mid-dorsal region of the body as far posteriorly as the origin of the electroreceptive filament, and the possession of 200–217 anal-fin rays.

Description:

Morphometric data for examined specimens in Table 4.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and curved ventrally, and with sexual dimorphism in overall form across examined specimens of large body sizes. Snout more anteroventrally aligned in mature females than in mature males of comparable sizes as evidenced by specimens of each sex with mature gonads. Sexual dimorphism in form of lower jaw and associated dentition found in many congeners not apparent in examined samples. Mouth terminal, rictus situated anterior of vertical through anterior naris. Anus and urogenital papilla located ventral to head, with openings situated anterior to vertical through anterior margin of eye (in specimens of 433–510 mm SL). Combined opening for anus and urogenital papilla distinctly elongate longitudinally and bordered laterally by raised fleshy ridges.

Premaxilla with six to ten teeth apparent in whole specimens (N= 4). Dentary with one tooth row consisting of nine to 15 curved conical teeth in whole specimens (N= 4), with second row of apparent replacement teeth located internal to functional row.

Branchiostegal rays five; with first to third rays apparently narrow and elongate in radiographs, and with fourth and fifth rays clearly very large and broad. Precaudal vertebrae 18–19 (15 anterior; three to four transitional; N= 10).

Pectoral-fin rays ii + 13–17 [ii + 16] (N= 10). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 22–34 [33] (N= 5). Total anal-fin rays 200–217 [205] (N= 10). Scales above lateral line at midbody nine to 11 [11] (N= 10). Scales absent along much of mid-dorsal line in region anterior to origin of midsaggital electroreceptive filament. Areas lacking scales sometimes separated by regions covered by mid-dorsal scales, but with position and extent of regions lacking scales variable across examined specimens. Origin of midsaggital electroreceptive filament located approximately at 65% of TL. Filament extending posteriorly to point four scales anterior of vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, elongate, pointed caudal fin. Caudal-fin rays 16–20 [19] (N= 10).

Coloration in alcohol:

Overall ground coloration brown. Head dark overall with more concentrated band of dark chromatophores present along dorsal portion of snout in region from vertical through posterior naris to slightly posterior of eye; dark pigmentation in some individuals extending anteriorly nearly to tip of snout. Body brown overall, with pigmentation darker dorsally, but without any distinct pigmentation pattern other than for dark mid-dorsal coloration in those regions that lack scales.

Pectoral fin dark, more so distally and sometimes nearly completely very darkly pigmented. Anal fin dark overall with distal pigmentation distinctly darker and forming irregular band. Caudal fin largely unpigmented, but with some individuals having variable dark pigmentation approximately midway along its length.

Distribution:

Most examined specimens of S. axelrodi originated at one location in the upper portions of the Rio Tocantins basin; however, a single lot of the species collected in the lower portions of that river system at Tucuruí demonstrates that the species is broadly distributed along the Rio Tocantins (Fig. 28).

Ecology:

The type locality of S. axelrodi was reported as a clear water system with the bottom formed by small stones and rocks (Vari & Harold, 2001: 164). dos Santos et al.(2004: 103; species cited therein as S. mormyrus) report that the species grows to 600 mm TL and feeds on insects and other aquatic invertebrates.

Secondary sexual dimorphism:

Sternarchorhynchus axelrodi demonstrates pronounced secondary sexual dimorphism in the overall form of the snout. Large males with mature testes have the snout slightly curved ventrally, whereas mature females with well-developed ovaries have a more pronounced ventral curvature of the snout (compare Figs 26, 27). No secondary sexual dimorphism of the dentary and lower jaw dentition comparable to that present in some congeners was apparent in the available samples of the species.

Etymology:

The species name, axelrodi, is in honour of Dr Herbert Axelrod, whose generous support of ichthyological research assisted with the completion of this project.

Remarks:

Sternarchorhynchus axelrodi was collected at the same locality as the type series of S. mesensis. The two species differ in various features including the total number of anal-fin rays (200–217 versus 171–189, respectively), caudal-fin rays (16–20 versus 13–15, respectively), coloration of the mid-dorsal portion of the body anterior to the origin of the electroreceptive filament (with distinct, narrow, lightly coloured stripe versus dark or sometimes with indistinct lightly coloured region, respectively), the greatest body depth (13.5–17.6 versus 10.0–12.3% of LEA, respectively), the caudal length (4.9–6.7 versus 9.0–14.3% of LEA, respectively), and other features.

Material examined

Holotype:

– BRAZIL. Goiás: Município de Minaçu/Cavalcante, Rio Tocantins, at future site of Usina Hidroeléctrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected by D. F. Moraes, C. N. Ricci, O. Roppa, vii.1988; MNRJ 31279 (444).

Paratypes:

– BRAZIL. Goiás: Município de Minaçu/Cavalcante, Rio Tocantins, at future site of Usina Hidroelétrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected with holotype by D. F. Moraes, C. N. Ricci, O. Roppa, vii.1988; MNRJ 12168, 8 (433–530), USNM 391570, 1 (475).

Nontype specimens:

– BRAZIL. Goiás: Município de Minaçu/Cavalcante, Rio Tocantins, at future site of Usina Hidroeléctrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected with holotype; MNRJ 31244,1 (not measured); MNRJ 31245, 1 (not measured); MNRJ 31246, 1 (not measured). Pará: INPA 27484, 2 (300–360); Rio Tocantins, Tucuruí, Mercado de Cametá.

Sternarchorhynchus britskii Campos-da-Paz(Figs28, 29; Table4)

Sternarchorhynchus britskii, holotype, 272 mm total length, MZUSP 52923; Brazil, Mato Grosso do Sul, Rio Paraná.

Sternarchorhynchus britskii, holotype, 272 mm total length, MZUSP 52923; Brazil, Mato Grosso do Sul, Rio Paraná.

Sternarchorhynchus sp. Britski, 1972: 91 [first citation of Sternarchorhynchus from Rio Paraná basin, Brazil].

Sternarchorhynchus curvirostris, not of Boulenger, Triques, 1993: 91 [details of osteology].

Sternarchorhynchus britskiiCampos-da-Paz, 2000: 529, fig. 9 [type locality: Brazil, Mato Grosso do Sul, Rio Paraná at Ilha Solteira dam (right margin), approximately 20°30′S 51°00′W]. –Albert, 2003: 500 [in listing of members of genus]. –Campos-da-Paz, 2005: 399 [additional locality information; threatened status]. –Triques, 2007: 125 [threatened status]. –da Graça & Pavanelli, 2007: 193, unnumbered fig. [Brazil, upper Rio Paraná; threatened status]. –Crampton, 2007: 289 [Brazil, Rio Paraná].

Diagnosis:

Sternarchorhynchus britskii is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the hyaline anal fin, the presence of a definite series of scales along the mid-dorsal region of the body, the lateral line that extends to the base of the caudal fin, the possession of five branchiostegal rays, seven teeth on the premaxilla, eight teeth in the outer row of the dentary, two teeth in the inner row of dentary, 15 precaudal vertebrae, 16–22 anterior unbranched anal-fin rays, 166–178 total anal-fin rays, 11–15 branched pectoral-fin rays, 13–15 caudal-fin rays, 16–17 precaudal vertebrae, 11–13 scales above the lateral line at the midbody, the greatest body depth (12.1–15.7% of LEA), the length of the base of the anal fin (86.4–93.0% of LEA), the prepectoral-fin distance (19.4–23.6% of LEA), the preanal distance (12.2–15.8% of LEA), the distance from the anus to the anal-fin insertion (14.4–24.8% of HL), the head length (18.9–23.9% of LEA), the snout length (59.8–65.7% of HL), the eye diameter (2.9–4.6% of HL), the distance from the posterior naris to the eye (53.9–57.9% of HL), the head depth at the nape (44.9–57.7% of HL), the caudal length (5.4–8.5% of HL), the mouth length (5.7–7.8% of HL), the postocular distance (40.0–45.9% of HL), the pectoral-fin length (35.5–46.4% of HL), the caudal-fin length (33.3–45.8% of caudal length), and the tail depth (18.9–25.9% of caudal length).

Diagnosis:

Morphometric data for examined specimens in Table 4.

Lateral line extending posteriorly to point between two scales short of base of caudal fin, but absent on fin. Snout elongate, compressed and curved ventrally anteriorly. Sexual dimorphism in form of lower jaw and associated dentition found in many congeners not apparent in examined samples. Anus and urogenital papilla located ventral to head. Openings shift anteriorly ontogenetically, with larger specimens having those structures situated along vertical three orbital diameters anterior of vertical through anterior margin of eye. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with seven teeth apparent in whole specimens (N= 1). Dentary with two tooth rows posteriorly and one sometimes somewhat irregular row anteriorly in whole specimens. Teeth in outer row eight and in inner row two (N= 1). Mouth small and terminal, rictus situated anterior of vertical through anterior naris.

Branchiostegal rays five; with first and second rays narrow and third to fifth rays large and broad. Precaudal vertebrae 16–17 (13–14 anterior; three transitional; N= 5).

Pectoral-fin rays ii + 11–13 [ii + 13] (N= 13). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 16–22 [19] (N= 13). Total anal-fin rays 166–178 [173] (N= 13). Scales above lateral line at midbody 11–13 [13] (N= 13). Scales present along mid-dorsal line in region anterior to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately at 70% of TL. Filament extending to within region delimited by verticals through point one scale short of vertical through posterior terminus of base of anal fin or point slightly beyond terminus of the base of that fin. Tail compressed and short, ending in small, elongate, slightly pointed caudal fin. Caudal-fin rays 13–15 [14] (N= 13).

Coloration in alcohol:

Overall ground coloration ranging from light brown up to grey to brown. Head with anteriorly attenuating band of more concentrated dark chromatophores along dorsolateral region of snout in region from eye to vertical through posterior naris. Dark pigmentation on snout forms lateral border of narrow, lightly coloured, mid-dorsal stripe extending from snout to rear of head and widening slightly laterally along postocular portion of head (stripe indistinct, particularly in postocular region in some specimens). Remainder of head light brown other than for lightly coloured region forming anteriorly convex arch overlying preopercle in some individuals. Body pigmentation darker dorsally, but with irregular and to varying degrees obscure, narrow, more lightly coloured mid-dorsal region. Stripe sometimes continuous anteriorly with mid-dorsal stripe on head and extending posteriorly in some specimens to origin of electroreceptive filament. Pectoral fin with dark chromatophores distributed over rays and otherwise hyaline. Anal fin lightly coloured overall, with small dark chromatophores overlying rays. Caudal fin with dusky central basal spot in some individuals but otherwise hyaline over rays.

Distribution:

Sternarchorhynchus britskii is only known from the upper Río Paraná basin in the states of Mato Grosso do Sul, Minas Gerais, and São Paulo, Brazil (Fig. 28).

Secondary sexual dimorphism:

Sternarchorhynchus britskii is not known to demonstrate sexual dimorphism.

Remarks:

Britski (1972: 91) reported that S. britskii(identified therein as Sternarchorhynchus sp.) was rare in sampled habitats with the type series having been collected in a cofferdam during the building of the Ilha da Solteira hydroelectric dam across the upper Rio Paraná. More recently, Campos-da-Paz (2005: 399) proposed that the insect larvae utilized by S. britskii may be associated with a specific habitat in flowing deep river channels. The extensive modifications of the upper Rio Paraná associated with the construction of major impoundments along the length of the main river channels throughout the basin and the resultant destruction of flowing deep river habitats favoured by S. britskii has undoubtedly adversely affected many populations of the species. As a consequence, S. britskii is now listed as threatened (Campos-da-Paz, 2005; Triques, 2007).

Material examined

BRAZIL. Mato Grosso do Sul: Rio Paraná, at Ilha Solteira dam, cofferdam at right margin of river (approximately 20°30′S, 51°00′W), MZUSP 24468 52923, 1 (262; holotype); MZUSP 24464, 8 [163–254; six specimens from this lot designated as paratypes by Campos-da-Paz (2000), but not possible to now determine which specimens were so designated]; MBUCV 13398, 1 (244; formerly MZUSP 24464, in part; nontype specimen collected with holotype). Rio Paraná, Ilha Solteira (approximately 20°30′S, 51°00′W), MZUSP 23097, 3 (125–240). Rio Paraná, ROM 37960, 1 (210). São Paulo: Rio Grande, Represa de Água Vermelha along boundary between state of São Paulo and Minas Gerais, MZUSP 39956, 7 (128–232). Fernandópolis (approximately 20°15′36″S, 50°13′59″W), MZUSP 50188, 1 (265).

Sternarchorhynchus caboclode Santana& Nogueira(Figs28, 30; Table4)

Sternarchorhynchus caboclo, holotype, male, 225 mm total length, INPA 22894; Brazil, Roraima, Rio Mucajaí.

Sternarchorhynchus caboclo, holotype, male, 225 mm total length, INPA 22894; Brazil, Roraima, Rio Mucajaí.

Sternarchorhynchus oxyrhynchus, not of Müller & Troschel, Ferreira, dos Santos & Jégu, 1988: 344 [in listing of species from Rio Mucajaí in vicinity of Ilha Paredão, Roraima, Brazil].

Sternarchorhynchus caboclode Santana & Nogueira, 2006: 89: figs 1b, 2b, c [type locality: Brazil, Roraima, Rio Mucajaí, below Cachoeira Paredão].

Diagnosis:

Sternarchorhynchus caboclo is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the absence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament, the lateral line that extends posteriorly to the base of the caudal fin, the dusky anal fin, the possession of four branchiostegal rays, six premaxillary teeth, 13–15 branched pectoral-fin rays, 153–174 total anal-fin rays, ten to 12 scales above the lateral line at the midbody, the greatest body depth (11.0–14.7% of LEA), the length of the base of the anal fin (79.3–86.4% of LEA), the mouth length (5.6–12.4% of HL), the snout length (44.3–61.3% of HL), the head depth at the nape (45.4–60.3% of HL), the internarial distance (3.0–6.4% of HL), the eye diameter (4.1–10.5% of HL), the pectoral-fin length (31.3–46.9% of HL), the interocular distance (6.1–19.7% of HL), and the caudal-fin length (17.2–38.9% of caudal length).

Description:

Morphometric data for examined specimens in Table 4.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and ranging from nearly straight to slightly curved ventrally. Mouth terminal and relatively small, with rictus located slightly anterior to, or along, vertical through anterior naris. Anus and urogenital papilla located ventral to head, with their position apparently somewhat ontogenetically variable in available sample. Anus and urogenital papilla in smaller individuals located along vertical slightly less than one orbital diameter posterior of rear margin of eye and very slightly anterior of vertical through anterior margin of eye in larger individuals. Combined opening for anus and urogenital papilla longitudinally ovoid in all specimens.

Premaxilla with eight to ten teeth (N= 8) apparent in whole specimens. Dentary with two tooth rows; outer row with seven to eight teeth and inner row with three to four teeth (N= 7).

Branchiostegal rays four; with first two rays narrow and elongate and fourth and fifth rays large and broad and posteriorly forked (de Santana & Nogueira, 2006: fig. 3). Precaudal vertebrae 16 (13–14 anterior; two to three transitional; N= 18).

Pectoral-fin rays ii + 13–15 [ii + 13] (N= 58). Anal-fin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 20–40 [23] (N= 58). Total anal-fin rays 153–174 [168] (N= 58). Scales above lateral line at midbody 10–12 [10] (N= 58). Scales present along mid-dorsal line to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located relatively far posteriorly, approximately at 67% of TL. Filament extending posteriorly to point two to four scales beyond vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate caudal fin. Caudal-fin rays 12–15 [12] (N= 33).

Coloration in alcohol:

Overall coloration dark brown. Head dark overall with stripe of slightly darker pigmentation extending anteriorly from orbit to, or nearly to, tip of snout. Slightly more lightly coloured region present on lateral surface of anterior one-third to one-half of snout. Ventral margin of snout dark. Mid-dorsal region of head dark. Body dark brown overall with mid-dorsal region uniformly dark. Pectoral fin dusky with rays covered with small dark chromatophores. Anal fin dusky with rays overlain by small, dark chromatophores. Basal portions of caudal fin dusky, but with distal portions of fin hyaline.

Distribution:

Sternarchorhynchus caboclo is only known from the Rio Mucajaí basin of Roraima, Brazil (Fig. 28).

Ecology:

The types of S. caboclo were collected above and below a waterfall along the Rio Mucajaí. According to Ferreira et al.(1988: 341) the river in that region is characterized by steep banks and with dense forest along the shore. Large amounts of aquatic plans of the family Podostemonacease were present in the area, with the water being acidic (pH 5.8) and well oxygenated (91% saturation).

Secondary sexual dimorphism:

de Santana & Nogueira (2006: 90: fig. 2b, c) noted apparent differences in the angle of the snout and depth of the posterior portion of the snout in males versus females of S. caboclo. A definitive statement on the generality of this difference requires examination of more extensive samples of the species.

Remarks:

Sternarchorhynchus caboclo and S. severii were collected at the type location. The two species are readily distinguishable by the number of branchiostegal rays (four versus five, respectively), pigmentation of the mid-dorsal region of the head and body (head and body dark versus with narrow, lightly coloured stripe extending posteriorly to the origin of the electroreceptive filament, respectively), the pectoral-fin length (31.3–46.9 versus 48.7–66.6% of HL, respectively), and the depth at the eye (23.3–31.1 versus 31.7–36.0% of HL, respectively).

Material examined

BRAZIL. Roraima: Rio Branco basin, Rio Mucajaí, below Cachoeira Paredão 2 (approximately 2°57′N, 61°27′W), INPA 22894, 1 (225.0, holotype, male); INPA 10594, 60 (46.3–226, 2 CS specimens, paratypes). Rio Branco basin, Rio Mucajaí, above Cachoeira Paredão 2 (approximately 2°57′N, 61°27′W), INPA 10578, 1 (178, paratype). Rio Branco basin, Rio Mucajaí, Estrada between Fé and Esperança, INPA 10592, 2 (156–160, paratypes). Rio Branco basin, side channel of Rio Mucajaí, INPA 10541, 1 (36). Rio Branco basin, Rio Mucajaí, approximately 2 km from Cachoeira Paredão 2, mouth of tributary igarapé (approximately 2°57′N, 61°27′W); INPA 10580, 1 (193). Rio Branco basin, mouth of Igarapé Traíra, INPA 10590, 1 (200).

Sternarchorhynchus chaoisp. nov. (Figs28, 31; Table5)

Sternarchorhynchus chaoi sp. nov., holotype, presumably male, 155 mm total length, INPA 28357; Brazil, Rondônia, Rio Jamari.

Sternarchorhynchus chaoi sp. nov., holotype, presumably male, 155 mm total length, INPA 28357; Brazil, Rondônia, Rio Jamari.

Morphometric data for holotype (H), and paratypes of Sternarchorhynchus chaoi, Sternarchorhynchus cramptoni, and Sternarchorhynchus curumim

 S. chaoi S. cramptoni S. curumim 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 155 126–172 (N= 10) – 290 133–315 (N= 14) – 163 181–211 (N= 3) – 
Length to end of anal fin (mm) 148 117–163 (N= 9) – 270 126–294 (N= 13) – 146 168–187 (N= 3) – 
Head length (mm) 30.9 26.0–32.5 (N= 10) – 46.4 24.1–51.3 (N= 14) – 30.0 31.5–38.1 (N= 3) – 
Caudal length (mm) 5.5 8.4–12.1 (N= 8) – 19.7 6.3–24.4 (N= 12) – 16.0 20.1–22.9 (N= 2) – 
Per cent of length to end of anal fin 
Anal-fin base 83.7 80–86.5 (N= 9) 82.6 87.7 83.0–88.8 (N= 13) 86.7 85.6 83.9–94.7 (N= 3) 88.6 
Distance snout to anus 12.0 11.0–14.7 (N= 9) 12.7 10.2 8.9–12.0 (N= 13) 10.5 – – – 
Preanal-fin distance 15.6 14.2–17.6 (N= 9) 15.7 11.8 10.9–14.9 (N= 13) 12.6 13.3 12.4–14.4 (N= 3) 13.7 
Prepectoral-fin distance 20.8 19.9–23.4 (N= 9) 21.2 16.8 16.5–20.3 (N= 13) 18.0 20.6 18.3–20.4 (N= 3) 19.1 
Greatest body depth 11.2 10.0–11.7 (N= 9) 11.2 10.5 9.7–12.1 (N= 13) 10.8 9.9 9.0–10.0 (N= 3) 9.6 
Head length 20.8 19.7–22.2 (N= 9) 20.9 17.1 16.0–19.5 (N= 13) 17.9 20.6 18.7–19.7 (N= 3) 19.0 
Caudal length 3.7 5.7–8.9 (N= 8) 7.4 7.3 5.0–8.4 (N= 12) 6.8 10.9 12.0–12.2 (N= 2) 12.1 
Per cent of head length 
Anus to anal-fin insertion 9.7 7.6–15.5 (N= 10) 11.0 9.7 6.7–12.3 (N= 14) 9.4 8.3 12.4–13.5 (N= 3) 12.8 
Pectoral-fin length 34.5 27.5–38.3 (N= 10) 34.4 42.4 38.0–46.5 (N= 14) 42.9 34.5 36.9–37.8 (N= 3) 37.3 
Head depth at eye 25.9 25.0–32.0 (N= 10) 27.8 27.9 25.6–35.4 (N= 14) 30.3 – 25.6–30.3 (N= 2) 28.0 
Head depth at nape 47.7 41.4–46.6 (N= 10) 44.3 51.3 46.2–58.8 (N= 14) 51.9 41.8 42.8–48.3 (N= 3) 45.2 
Head width 20.6 19.5–23.1 (N= 10) 20.9 23.6 21.3–25.9 (N= 14) 23.4 – 19.0–19.8 (N= 2) 19.5 
Snout length 60.1 58.8–62.9 (N= 10) 60.9 66.3 60.6–67.0 (N= 14) 64.8 60.2 60.6–62.5 (N= 3) 61.8 
Posterior naris to snout 8.1 7.2–9.1 (N= 10) 8.4 6.8 6.0–8.5 (N= 14) 7.6 – 5.9–6.2 (N= 2) 6.0 
Posterior naris to eye 50.0 46.3–52.1 (N= 10) 50.1 57.6 50.9–59.3 (N= 14) 55.7 – 54.9–59.8 (N= 2) 57.4 
Mouth length 5.6 5.5–6.7 (N= 10) 6.1 5.5 5.2–8.8 (N= 14) 6.5 5.3 3.5–5.6 (N= 3) 4.2 
Internarial distance 3.3 2.5–3.6 (N= 10) 2.9 3.2 2.3–4.0 (N= 14) 3.2 2.8 2.4–2.6 (N= 3) 2.5 
Eye diameter 3.4 3.0–4.2 (N= 10) 3.5 2.6 2.5–4.2 (N= 14) 3.1 3.9 4.1–5.0 (N= 3) 4.7 
Interocular width 6.4 5.5–7.2 (N= 10) 6.5 6.1 5.2–8.5 (N= 14) 6.0 6.5 4.8–6.4 (N= 3) 5.5 
Postocular distance 40.3 39.4–41.3 (N= 10) 40.5 39.3 37.4–43.7 (N= 14) 41.0 36.9 33.6–39.2 (N= 3) 37.2 
Branchial opening 12.7 10.0–14.8 (N= 10) 12.2 15.4 11.7–17.7 (N= 14) 14.6 11.8 11.7–12.9 (N= 3) 12.1 
Per cent of caudal length 
Tail depth 34.2 14.0–21.8 (N= 8) 16.6 25.3 23.6–29.6 (N= 12) 26.9 11.9 10.6–11.7 (N= 2) 11.1 
Caudal-fin length 69.5 29.6–51.3 (N= 8) 36.4 35.5 30.9–61.4 (N= 12) 40.9 24.3 19.5–25.2 (N= 2) 22.3 
 S. chaoi S. cramptoni S. curumim 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 155 126–172 (N= 10) – 290 133–315 (N= 14) – 163 181–211 (N= 3) – 
Length to end of anal fin (mm) 148 117–163 (N= 9) – 270 126–294 (N= 13) – 146 168–187 (N= 3) – 
Head length (mm) 30.9 26.0–32.5 (N= 10) – 46.4 24.1–51.3 (N= 14) – 30.0 31.5–38.1 (N= 3) – 
Caudal length (mm) 5.5 8.4–12.1 (N= 8) – 19.7 6.3–24.4 (N= 12) – 16.0 20.1–22.9 (N= 2) – 
Per cent of length to end of anal fin 
Anal-fin base 83.7 80–86.5 (N= 9) 82.6 87.7 83.0–88.8 (N= 13) 86.7 85.6 83.9–94.7 (N= 3) 88.6 
Distance snout to anus 12.0 11.0–14.7 (N= 9) 12.7 10.2 8.9–12.0 (N= 13) 10.5 – – – 
Preanal-fin distance 15.6 14.2–17.6 (N= 9) 15.7 11.8 10.9–14.9 (N= 13) 12.6 13.3 12.4–14.4 (N= 3) 13.7 
Prepectoral-fin distance 20.8 19.9–23.4 (N= 9) 21.2 16.8 16.5–20.3 (N= 13) 18.0 20.6 18.3–20.4 (N= 3) 19.1 
Greatest body depth 11.2 10.0–11.7 (N= 9) 11.2 10.5 9.7–12.1 (N= 13) 10.8 9.9 9.0–10.0 (N= 3) 9.6 
Head length 20.8 19.7–22.2 (N= 9) 20.9 17.1 16.0–19.5 (N= 13) 17.9 20.6 18.7–19.7 (N= 3) 19.0 
Caudal length 3.7 5.7–8.9 (N= 8) 7.4 7.3 5.0–8.4 (N= 12) 6.8 10.9 12.0–12.2 (N= 2) 12.1 
Per cent of head length 
Anus to anal-fin insertion 9.7 7.6–15.5 (N= 10) 11.0 9.7 6.7–12.3 (N= 14) 9.4 8.3 12.4–13.5 (N= 3) 12.8 
Pectoral-fin length 34.5 27.5–38.3 (N= 10) 34.4 42.4 38.0–46.5 (N= 14) 42.9 34.5 36.9–37.8 (N= 3) 37.3 
Head depth at eye 25.9 25.0–32.0 (N= 10) 27.8 27.9 25.6–35.4 (N= 14) 30.3 – 25.6–30.3 (N= 2) 28.0 
Head depth at nape 47.7 41.4–46.6 (N= 10) 44.3 51.3 46.2–58.8 (N= 14) 51.9 41.8 42.8–48.3 (N= 3) 45.2 
Head width 20.6 19.5–23.1 (N= 10) 20.9 23.6 21.3–25.9 (N= 14) 23.4 – 19.0–19.8 (N= 2) 19.5 
Snout length 60.1 58.8–62.9 (N= 10) 60.9 66.3 60.6–67.0 (N= 14) 64.8 60.2 60.6–62.5 (N= 3) 61.8 
Posterior naris to snout 8.1 7.2–9.1 (N= 10) 8.4 6.8 6.0–8.5 (N= 14) 7.6 – 5.9–6.2 (N= 2) 6.0 
Posterior naris to eye 50.0 46.3–52.1 (N= 10) 50.1 57.6 50.9–59.3 (N= 14) 55.7 – 54.9–59.8 (N= 2) 57.4 
Mouth length 5.6 5.5–6.7 (N= 10) 6.1 5.5 5.2–8.8 (N= 14) 6.5 5.3 3.5–5.6 (N= 3) 4.2 
Internarial distance 3.3 2.5–3.6 (N= 10) 2.9 3.2 2.3–4.0 (N= 14) 3.2 2.8 2.4–2.6 (N= 3) 2.5 
Eye diameter 3.4 3.0–4.2 (N= 10) 3.5 2.6 2.5–4.2 (N= 14) 3.1 3.9 4.1–5.0 (N= 3) 4.7 
Interocular width 6.4 5.5–7.2 (N= 10) 6.5 6.1 5.2–8.5 (N= 14) 6.0 6.5 4.8–6.4 (N= 3) 5.5 
Postocular distance 40.3 39.4–41.3 (N= 10) 40.5 39.3 37.4–43.7 (N= 14) 41.0 36.9 33.6–39.2 (N= 3) 37.2 
Branchial opening 12.7 10.0–14.8 (N= 10) 12.2 15.4 11.7–17.7 (N= 14) 14.6 11.8 11.7–12.9 (N= 3) 12.1 
Per cent of caudal length 
Tail depth 34.2 14.0–21.8 (N= 8) 16.6 25.3 23.6–29.6 (N= 12) 26.9 11.9 10.6–11.7 (N= 2) 11.1 
Caudal-fin length 69.5 29.6–51.3 (N= 8) 36.4 35.5 30.9–61.4 (N= 12) 40.9 24.3 19.5–25.2 (N= 2) 22.3 

Number of specimens indicated in parentheses.

Diagnosis:

Sternarchorhynchus chaoi is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the absence of scales along the mid-dorsal region of the body as far posteriorly as the vertical through posterior limit of the pectoral fin, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body that extends posteriorly to the origin of the electroreceptive filament and sometimes beyond that point, the presence of eight to 13 premaxillary teeth, 15 precaudal vertebrae, 15–23 anterior unbranched anal-fin rays, 165–178 total anal-fin rays, and ten to 13 scales above the lateral line at the midbody, the greatest body depth (10.0–11.7% of LEA), the eye diameter (3.0–4.2% of HL), the head width (19.5–23.1% of SL), the postocular distance (39.1–41.3% of HL), and the caudal length (5.7–8.9% of LEA).

Description:

Morphometric data for examined specimens in Table 5.

Lateral line extending posteriorly to point approximately three scales short of base of caudal fin and absent on remainder of tail and fin. Snout elongate, compressed and nearly straight to slightly curved ventrally distally. Mouth terminal and relatively small, with rictus located within area delimited by verticals running slightly anterior to and slightly posterior of anterior naris. Anus and urogenital papilla located ventral to head, with their position somewhat ontogenetically variable in available sample. Anus and urogenital papilla located along vertical approximately three orbital diameters posterior of rear margin of eye in smaller specimens and very slightly posterior of, or along, vertical through eye in larger individuals. Combined opening for anus and urogenital papilla ovoid in one apparent female and somewhat more horizontally elongate in juveniles and apparent males.

Premaxilla with eight to 13 teeth (N= 11) apparent in whole specimens. Dentary with two tooth rows; outer row with seven to nine teeth and inner row with three to five teeth (N= 11).

Branchiostegal rays five; with first three rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15 (12 anterior; three transitional; N= 11).

Pectoral-fin rays ii + 11–13 [13] (N= 11). Anal-fin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 15–23 [22] (N= 10). Total anal-fin rays 165–178 [176] (N= 10). Scales above lateral line at midbody 10–13 [12] (N= 11). Scales absent along mid-dorsal line for distance of one-quarter to one-third of distance between rear of head and origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located approximately at 67% of TL. Filament extending posteriorly to vertical through posterior terminus of base of anal fin or one to two scales beyond that point. Tail compressed and short, ending in small, moderate, lanceolate caudal fin. Caudal-fin rays 13–16 [14] (N= 8).

Coloration in alcohol:

Overall coloration dark brown. Snout dark overall with stripe of darker pigmentation extending anteriorly from orbit for distance one-half to two-thirds length of snout. Dorsal region of dark pigmentation on head forms lateral margin of narrow, somewhat obscure, lightly coloured mid-dorsal stripe. Stripe expands very slightly laterally over postocular region, but separated from lightly coloured mid-dorsal stripe on body by region of dark pigmentation. Mid-dorsal lightly coloured stripe on body commences somewhat posterior of terminus of comparable stripe on head and extends varying degrees posteriorly, reaching to origin of electroreceptive filament in some individuals. Pectoral fin distinctly dusky with rays covered with small, dark chromatophores. Anal fin dusky with rays overlain by small, dark chromatophores. Basal portions of caudal fin dusky to distinctly dark, but with distal portions of fin hyaline.

Distribution:

Sternarchorhynchus chaoi is only known from one locality in the Rio Jamari basin of the state of Rondônia, Brazil (Fig. 28).

Secondary sexual dimorphism:

No clear sexual dimorphism is apparent in the limited available samples of S. chaoi.

Etymology:

The species name, chaoi, is in honour of Ning Labish Chao of the Universidade Federal do Amazonas, who provided invaluable assistance and financial support to the first author during his studies of gymnotiforms in Manaus, Brazil.

Material examined

Holotype:

BRAZIL. Rondônia: Rio Jamari, Porto Velho, canal de desvio da (diversion canal of ) UHE (Usina Hidroeléctica) Samuel (approximately 8°27′S, 63°30′W), collected by G. M. dos Santos, 7.vi.1998; INPA 28357 (155).

Paratypes:

– BRAZIL. Rondônia: Rio Jamari, Porto Velho, Canal de desvio da (diversion canal of) UHE (Usina Hidroeléctica) Samuel (approximately 8°27′S, 63°30′W), collected with holotype by G. M. dos Santos, 7.vi.1998; INPA 20851, 11 (126–174); USNM 391713, 1 (164).

Nontype specimens:

– BRAZIL. Rondônia: amongst rocks above UHE (Usina Hidroeléctrica) Samuel (approximately 8°27′S, 63°30′W); INPA uncat., field number Polo 896, 7 (48–168).

Sternarchorhynchus cramptonisp. nov. (Figs28, 32, 33; Table5)

Sternarchorhynchus cramptoni sp. nov., holotype, female, 290 mm total length, INPA 28376; Brazil, Amazonas, Rio Solimões, downstream from mouth of Rio Purus.

Sternarchorhynchus cramptoni sp. nov., holotype, female, 290 mm total length, INPA 28376; Brazil, Amazonas, Rio Solimões, downstream from mouth of Rio Purus.

Sternarchorhynchus cramptoni sp. nov., sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 331 mm total length, MCP 41638, demonstrating expanded sexually dimorphic dentary and associated dentition; Brazil, Amazonas, Rio Solimões, Ilha do Prego.

Sternarchorhynchus cramptoni sp. nov., sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 331 mm total length, MCP 41638, demonstrating expanded sexually dimorphic dentary and associated dentition; Brazil, Amazonas, Rio Solimões, Ilha do Prego.

Sternarchorhynchus cf. roseni, Rapp Py-Daniel & Cox-Fernandes, 2005: 105, fig. 5. [Brazil, Amazon Basin; secondary sexual dimorphics in form of lower jaw].

Diagnosis:

Sternarchorhynchus cramptoni is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament and sometimes beyond that point, the presence of a dark band along the distal one-fifth to one-half of much of the anal fin and over most of the posterior rays of that fin, the presence of eight to 12 teeth on the premaxilla, six to eight teeth on the outer tooth row of the dentary, 180–221 total anal-fin-rays, 16 precaudal vertebrae, the preanal distance (10.9–14.9 of LEA), the greatest body depth (9.7–12.1% of LEA), the caudal length (5.0–8.4% of LEA), the head length (16.0–19.5% of LEA), the distance from the anus to the anal-fin insertion (6.7–12.3% of HL), the snout length (60.6–67.0% of HL), the head depth at the nape (46.2–58.8% of HL), the postocular distance (37.4–43.7% of HL), the prepectoral-fin distance (16.5–20.3% of HL), the tail depth (23.6–29.6% of caudal length), and the caudal-fin length (30.9–61.4% of caudal length).

Description:

Morphometric data for holotype and paratypes in Table 5.

Lateral line extending to base of caudal fin, but absent on fin. Snout elongate, compressed and curved ventrally distally. Posterior naris located closer to tip of snout than to anterior margin of eye. Branchial opening restricted and situated slightly anterior to vertical through pectoral-fin origin. Location of anus and urogenital papilla ontogenetically variable and apparently sexually dimorphic. Anus and urogenital opening in smaller individuals positioned along vertical located slight distance posterior of vertical through rear margin of eye. Larger specimens lacking secondary male characters with two openings located at vertical running slightly anterior of anterior margin of eye. Males with patch of large teeth at anterior of expanded dentary with anus and urogenital papilla positioned more anteriorly at vertical about one-third of distance between anterior margin of eye and tip of snout. Aperture for anus and urogenital opening rounded in juveniles and apparent females; longitudinally elongate in males demonstrating sexually dimorphic modifications of dentary and associated dentition.

Premaxilla of small size, somewhat rounded, with eight to 12 teeth (N= 6). Dentary elongate with fleshy pad on anteroventral surface in larger specimens. Dentaries approximately of same length in males and females, but with form sexually dimorphic. Anterior portion of dentary of males somewhat widened laterally into moderately dorsally bulbous structure bearing enlarged teeth (degree of expansion of dentary and of enlargement of teeth less pronounced than that present in some congeners). Dentary with two rows of curved conical teeth. Outer row with six to eight teeth (N= 10) and inner row with three to four teeth (N= 8). Mouth terminal with rictus located anterior to vertical through posterior naris in juveniles. Rictus slightly more posteriorly positioned in larger individuals.

Branchiostegal rays five; with first to third rays relatively narrow and elongate and remaining rays large and broad. Precaudal vertebrae 16 (12–13 anterior; three to four transitional, N= 11).

Pectoral-fin rays ii + 11–14 [ii + 14] (N= 30). Anal-fin origin located slightly posterior of vertical through anterior margin of opercle. Anterior unbranched anal-fin rays 17–29 [24] (N= 15). Total anal-fin rays 180–221 [221] (N= 24); number of rays apparently increases ontogenetically given evident correlation between greater body size and increased numbers of anal-fin rays amongst examined specimens. Scales above lateral line at midbody 9–12 [9] (N= 33). Scales along mid-dorsal region of body readily apparent. Origin of midsaggital electroreceptive filament located approximately at 62% of TL. Filament extending posteriorly to vertical running through to posterior terminus of base of anal fin. Tail compressed and short, ending in small, caudal fin with rounded distal margin. Caudal-fin rays 13–16 [15] (N= 21).

Coloration in life:

A photo of a recently collected specimen of S. cramptoni from the Rio Negro, Brazil, shows a whitish-pink fish with the dark chromatophores on the head appearing as very small dots.

Coloration in alcohol:

Overall ground coloration ranging from brown to dark brown. Head and body with dark chromatophores relatively densely scattered over surfaces. Size of chromatophores and intensity of their pigmentation greater in overall darker specimens. Snout with variably distinct, narrow band of darker pigmentation extending anteriorly from region somewhat anterior of eye and reaching anterior portion of snout in many specimens. Band of dark pigmentation forms lateral border of narrow, lightly coloured mid-dorsal band on head. Ventral margin of snout somewhat darker than lateral surface of that region in some specimens, more so in individuals with overall dark coloration of head and body. Mid-dorsal region of head with narrow, lightly coloured stripe apparent even in overall darkly pigmented individuals. Head stripe continuous posteriorly with narrow, lightly coloured mid-dorsal stripe on body. Body pigmentation slightly darker dorsally, but with lightly pigmented mid-dorsal stripe extending posteriorly onto basal portions of electroreceptive filament and to varying degrees posteriorly from that point in different individuals.

Pectoral-fin coloration ranging from dusky to distinctly dark, with dark pigmentation overlying pectoral-fin rays and more developed distally. Anal fin with distinct band of dark pigmentation. Dark band covering approximately distal one-fifth to one-half of fin in most examined specimens, but with dark coloration extending over all, or nearly all, of lateral surface of fin in some darkly coloured individuals. Caudal-fin rays ranging from slightly dusky to distinctly dark with extent and intensity of pigmentation greater in overall more darkly pigmented individuals.

Distribution:

Sternarchorhynchus cramptoni is widely distributed along the mainstream of the Amazon River from the region of Iquitos in north-eastern Peru downstream to the Rio Trombetas, a left bank tributary of the Amazon in the state of Pará, Brazil (Fig. 28).

Ecology:

Juveniles of S. cramptoni have been captured along the margins of main river channels in várzea, whereas adults were captured in the main channels of the sampled rivers (C. D. de Santana, pers. observ.).

Electrical organ discharge:

Crampton & Albert (2006: 689) reported that S. cramptoni(the Sternarchorhynchus n. sp. C of those authors and Crampton, 2007) has a complex EOD pattern comprising four phases of alternating polarities with the weak negative phase apparently flat, but on closer examination exhibiting a gentle curve around a single peak (the type F EOD of those authors). They also noted that the multiphasic nature of this EOD form diverts the energy of the signal away from the fundamental, with the first harmonic representing the peak power frequency.

Secondary sexual dimorphism:

Examined samples of S. cramptoni demonstrated sexual dimorphism in the form of the lower jaw and associated dentition. Although males of the species have a somewhat enlarged anterior portion of the dentary, that region is not as relatively elongate or bulbous as in males of some congeners. Dentary dentition in that region is also proportionally less developed than are the teeth in mature males of many species of Sternarchorhynchus.

Etymology:

The species name, cramptoni, is in recognition of the many contributions of William Crampton, University of Central Florida, to our knowledge of the biology and systematics of gymnotiforms.

Material examined

Holotype:

– BRAZIL. Amazonas: Rio Solimões, downstream from mouth of Río Purus (3°27′27″S, 60°45′26″W); collected by Angela Zanata et al., 1.viii.1996; INPA 28376 (290, female; formerly FMNH 115489).

Paratypes:

– BRAZIL. Amazonas: Rio Solimões, south bank of Ilha do Jaraqui, Alvarães (3°09.51'S, 64°48.76′W), collected by W. G. R. Crampton, 9.xii.1999; MCP 41637, 1 (236). Río Solimões, Ilha do Prego, opposite town of Alvarães (3°12.63'S 64°47.38′W); collected by W. G. R. Crampton, 19.ii.2001; MCP 41638, 1 (331). Rio Solimões, downstream from mouth of Rio Purus (3°27′27″S, 60°45′26″W); collected by Angela Zanataet al., 1.viii.1996, collected with holotype, FMNH 115489, 3 (155–273). Rio Solimões, downstream from mouth of Río Purus (3°26′46″S, 60°45′00″W); collected by Angela Zanata et al., 31.vii.1996; FMNH 115488, 4 (231–275). Rio Solimões, downstream from mouth of Rio Purus (3°27′22″S, 60°45′21″W); collected by Angela Zanata et al., 1.viii.1996; FMNH 115490, 7 (133–315).

Nontype specimens:

– BRAZIL. Amazonas: Rio Solimões, between mouths of Rio Iça and Río Tonantins, between towns of São Antônio do Iça and Nova Tonantins (2°55′00″S, 67°50′48″W); FMNH 115483, 1 (222). Rio Solimões, between mouth of Rio Iça and Río Tonantins, between towns of São Antônio do Iça and Nova Tonantins (2°55′24″S, 67°51′23″W); FMNH 115482, 1 (280). Rio Amazonas, between mouth of Rio Madeira and Paraná do Serpa, between Manaus and Itacoatiara (3°19′44″S, 58°35′28″W); FMNH 115494, 1 (128). Rio Amazonas, downstream from mouth of Rio Madeira, upstream from Itacoatiara (3°20′22″S, 58°36′31″W); FMNH 115496, 1 (267). Rio Amazonas, upstream from mouth of Rio Madeira and upstream from Itacoatiara (3°15′42″S, 58°58′24″W); FMNH 115491, 2 (154–293). Río Amazonas, downstream from mouth of Río Madeira and upstream from Itacoatiara (3°20′09″S, 58°36′11″W); FMNH 115492, 1 (240). Rio Amazonas, downstream from mouth of Rio Madeira, upstream from Itacoatiara (3°20′59″S, 58°39′32″W); FMNH 115495, 1 (307). Rio Amazonas, between mouths of Rio Negro and Rio Madeira, between towns of Nova Oriente and Itacoatiara (3°16′40″S, 58°56′56″W); FMNH 115493, 1 (215). Rio Purus, Itapuru (4°17′S, 61°54′10″W); INPA 17098, 1 (180). Rio Solimões, Ilha da Marchantaria; INPA 27491 (255); INPA 27114, 1 (245). Rio Purus, marginal lago on shore opposite Lago Surara (4°07′47S, 61°34′50″W); INPA 17099, 1 (265). Rio Purus at Beruri; INPA 17602, 1 (380). Rio Purus, Baia da Resaca, at Beruri; INPA 17603, 2 (328–358). Rio Solimões, Ilha do Careiro; INPA 17601, 3 (327–334). Rio Solimões; INPA uncat. 1 (166). Rio Solimões (3°36′19″S, 61°18′39″W); USNM 373328, 1 (163). Rio Solimões, below Purus (3°36′25″S 61°19′40″W); MZUSP 56882, 2 (150–164). Pará: Río Trombetas, between tributaries Lago Iripixi and Cachoeiri, between towns Oriximiná and Fazenda Paraíso; FMNH 115487, 1 (170).

PERU. Loreto: Iquitos, Río Amazonas, obtained from aquarium trade; UF 116761, 4 (99–121), UF 123454, 2 (167–310). Iquitos, Río Amazonas; UF 116760, 1 (241).

Sternarchorhynchus curumimde Santana& Crampton(Figs34, 35; Table5)

Sternarchorhynchus curumim, paratype, female, 211 mm total length, MCP 38305; Brazil, Amazonas, Río Tefé.

Sternarchorhynchus curumim, paratype, female, 211 mm total length, MCP 38305; Brazil, Amazonas, Río Tefé.

Map of central and northern South America showing geographical distribution of Sternarchorhynchus curumim, Sternarchorhynchus curvirostris, Sternarchorhynchus freemani sp. nov.(1 = holotype locality), Sternarchorhynchus galibi sp. nov.(2 = holotype locality), and Sternarchorhynchus gnomus(some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus curumim, Sternarchorhynchus curvirostris, Sternarchorhynchus freemani sp. nov.(1 = holotype locality), Sternarchorhynchus galibi sp. nov.(2 = holotype locality), and Sternarchorhynchus gnomus(some symbols represent more than one locality and/or lot of specimens).

Sternarchorhynchus curumimde Santana & Crampton, 2006: 59, fig. 1 [type locality: Brazil, Amazonas, Rio Tefé, Toco Preto, Municipality of Tefé, 3°47.31′S, 64°59.91′W]. –Crampton, 2007: 289, 297 [western Amazon; occurrence in blackwater but not whitewater habitats].

Diagnosis:

Sternarchorhynchus curumim is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the absence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament, the lateral line that extends to the base of the caudal fin, the dusky anal fin without a distinct dark band along its distal margin, the possession of seven to ten teeth on the premaxillary, seven to eight teeth in the outer row of the dentary, 15 precaudal vertebrae, 23–24 anterior unbranched anal-fin rays, 179–189 total anal-fin rays, ten to 11 scales above the lateral line at the midbody, 13–15 caudal-fin rays, the greatest body depth (9.0–10.0% of LEA), the distance from the anus to the anal-fin insertion (8.3–13.5% of HL), the preanal distance (12.4–14.4% of LEA), the caudal length (10.9–12.2% of LEA), the head length (18.7–20.6% of LEA), the snout length (60.2–62.5% of HL), the mouth length (3.5–5.6% of HL), the eye diameter (4.1–5.0% of HL), the internarial distance (2.4–2.8% of HL), the postocular distance (33.6–39.2% of HL), the pectoral-fin length (34.5–37.8% of HL), the tail depth (10.6–11.9% of caudal length), and the caudal length (10.9–12.2% of LEA).

Description:

Morphometric data for examined specimens in Table 5.

Lateral line extending posteriorly to point approximately at base of caudal fin but absent on fin. Snout elongate, compressed and slightly curved ventrally distally. Mouth terminal and relatively small, with rictus located slightly posterior of vertical through anterior naris. Anus and urogenital papilla located ventral to head, with position apparently sexually dimorphic in limited available sample of species. Anus and urogenital papilla positioned along vertical situated approximately one orbital diameter posterior of rear margin of eye in single examined mature female and positioned at vertical one orbital diameter anterior of eye in single mature male. Combined opening for anus and urogenital papilla ovoid in one examined mature male and somewhat more horizontally elongate in mature female.

Premaxilla with seven to ten teeth (N= 3) apparent in whole specimens. Dentary with two rows of conical teeth; outer row with seven to eight teeth and inner row with three to four teeth (N= 2).

Branchiostegal rays five; with first three rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15 (12 anterior; three transitional; N= 4).

Pectoral-fin rays ii + 11–12 [ii + 12] (N= 4). Anal-fin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 23–24 (N= 2). Total anal-fin rays 179–189 [189] (N= 3). Scales above lateral line at midbody ten to 11 [10] (N= 4). Scales clearly present along mid-dorsal line to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located relatively far posteriorly, approximately at 60% of TL. Filament extending posteriorly to vertical located approximately three scales posterior of vertical through terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate caudal fin. Caudal-fin rays 13–15 [15] (N= 3).

Coloration in alcohol:

Overall coloration of head and body dark brown. Snout dark overall with stripe of slightly darker pigmentation extending anteriorly from orbit to, or nearly to, tip of snout. Pectoral fin distinctly dusky with rays covered with small dark chromatophores. Anal fin dusky with rays overlain by dark chromatophores. Basal portions of caudal fin distinctly dark, but with distal portions of fin hyaline.

Distribution:

Sternarchorhynchus curumim is only known from one locality in the Rio Tefé in the state of Amazonas, Brazil (Fig. 35).

Secondary sexual dimorphism:

As documented by de Santana & Crampton (2006: 1166), the single mature male of S. curumim has the posterior region of the snout distinctly more developed vertically than do the three known females. No other sexually dimorphic features are present other than for the relatively minor difference in the positions of the anus and urogenital papilla described above.

Material examined

BRAZIL. Amazonas: Rio Tefé, Toco Preto, Município de Tefé (3°47.31′S, 64°59.91′W); MCP 38304, 1 (holotype; female, 163 mm); MCP 38305, 2 (paratypes: female 211 mm, male 183 mm); INPA 25256, 1 (cleared and stained paratype; female, 181 mm).

Sternarchorhynchus curvirostris(Boulenger) (Figs35, 36, 37; Table6)

Sternarchorhynchus curvirostris, illustration of syntype from Boulenger (1887).

Sternarchorhynchus curvirostris, illustration of syntype from Boulenger (1887).

Sternarchorhynchus curvirostris, female, 235 mm total length, MCZ 46676; Ecuador, Pastaza, Río Putuno.

Sternarchorhynchus curvirostris, female, 235 mm total length, MCZ 46676; Ecuador, Pastaza, Río Putuno.

Morphometrics for examined specimens of Sternarchorhynchus curvirostris, Sternarchorhynchus freemani, and Sternarchorhynchus galibi

 S. curvirostris S. freemani S. galibi 
Range Mean Paratypes Mean Paratypes Mean 
Total length (mm) 203–237 (N= 4) – 201 163–172 (N= 2) – 168 80–202 (N= 9) – 
Length to end of anal fin (mm) 188–220 (N= 4) – 171 142–146 (N= 2) – 153 75–183 (N= 9) – 
Head length (mm) 38.5–41.0 (N= 5) – 37.9 31.3–32.7 (N= 2) – 32.3 16.3–38.9 (N= 9) – 
Caudal length (mm) 17.0–20.3 (N= 3) – 28.9 21.5–24.2 (N= 2) – 14.7 8.7–18.4 (N= 8) – 
Per cent of length to end of anal fin 
Anal-fin base 83.1–90.2 (N= 4) 85.7 88.8 84.5–86.9 (N= 2) 85.7 86.2 79.5–86.2 (N= 9) 82.4 
Distance snout to anus 9.5–12.0 (N= 4) 10.7 11.9 12.2–12.7 (N= 2) 12.4 11.4 11.3–15.4 (N= 9) 13.7 
Preanal-fin distance 11.3–13.4 (N= 3) 12.6 15.0 16.0–16.7 (N= 2) 16.2 14.3 14.2–18.3 (N= 9) 16.3 
Prepectoral-fin distance 18.7–22.5 (N= 4) 20.3 21.6 22.6–23.2 (N= 2) 22.9 21.8 19.9–24.6 (N= 9) 22.7 
Greatest body depth 12.5–15.0 (N= 4) 13.4 11.9 11.0–11.2 (N= 2) 11.1 12.6 11.1–12.7 (N= 9) 11.8 
Head length 18.5–21.8 (N= 4) 20.0 22.1 21.4–23.0 (N= 2) 22.2 21.1 20.2–23.7 (N= 9) 22.1 
Caudal length 7.9–10.4 (N= 6) 9.0 16.9 15.1–16.6 (N= 2) 15.8 9.6 8.7–10.8 (N= 8) 9.9 
Per cent of head length 
Anus to anal-fin insertion 1.6–6.8 (N= 4) 3.2 15.0 13.0–17.4 (N= 2) 15.2 11.1 6.8–12.8 (N= 9) 9.5 
Pectoral-fin length 38.5–45.7 (N= 4) 42.4 34.8 35.6–37.0 (N= 2) 36.3 31.5 31.5–40.7 (N= 9) 34.6 
Head depth at eye 27.2–35.4 (N= 5) 30.2 27.7 25.6–27.0 (N= 2) 26.3 30.4 26.2–30.8 (N= 9) 28.6 
Head depth at nape 50.9–67.3 (N= 3) 57.4 41.4 39.5–42.2 (N= 2) 40.9 43.2 39.5–47.6 (N= 9) 43.0 
Head width 18.7–30.3 (N= 5) 23.1 20.4 18.8–19.9 (N= 2) 19.4 20.9 17.9–23.3 (N= 9) 21.1 
Snout length 64.5–66.1 (N= 5) 65.5 66.0 63.8–64.6 (N= 2) 64.2 63.4 57.2–66.0 (N= 9) 61.8 
Posterior naris to snout 6.9–7.4 (N= 2) 7.2 6.5 7.2–7.5 (N= 2) 7.4 9.6 7.9–10.7 (N= 9) 9.4 
Posterior naris to eye 54.5–57.1 (N= 2) 55.8 58.0 53.0–54.6 (N= 2) 53.8 52.5 42.5–55.5 (N= 9) 50.0 
Mouth length 5.1–5.7 (N= 2) 5.4 5.0 4.8–5.3 (N= 2) 5.0 6.6 5.8–7.6 (N= 9) 6.5 
Internarial distance 3.0–4.5 (N= 2) 3.8 2.0 2.3–3.1 (N= 2) 2.7 2.9 2.2–3.5 (N= 9) 2.7 
Eye diameter 3.6–5.8 (N= 5) 4.6 4.0 3.8–4.3 (N= 2) 4.1 4.6 3.9–6.7 (N= 9) 5.0 
Interocular width 5.6–6.8 (N= 5) 6.2 5.2 5.4–6.2 (N= 2) 5.8 6.3 5.6–13.5 (N= 9) 8.3 
Postocular distance 36.7–40.3 (N= 5) 38.2 38.7 36.1–38.6 (N= 2) 37.3 39.8 35.6–39.8 (N= 9) 37.9 
Branchial opening 13.8–16.6 (N= 4) 15.5 9.5 10.2–10.8 (N= 2) 10.5 11.5 9.8–14.0 (N= 9) 11.8 
Per cent of caudal length 
Tail depth 12.7–17.5 (N= 4) 14.7 8.9 8.0–8.6 (N= 2) 8.3 17.1 13.8–19.2 (N= 8) 15.8 
Caudal-fin length 30.3–56.9 (N= 2) 43.6 19.7 17.2–21.7 (N= 2) 19.1 29.4 26.6–45.0 (N= 8) 34.3 
 S. curvirostris S. freemani S. galibi 
Range Mean Paratypes Mean Paratypes Mean 
Total length (mm) 203–237 (N= 4) – 201 163–172 (N= 2) – 168 80–202 (N= 9) – 
Length to end of anal fin (mm) 188–220 (N= 4) – 171 142–146 (N= 2) – 153 75–183 (N= 9) – 
Head length (mm) 38.5–41.0 (N= 5) – 37.9 31.3–32.7 (N= 2) – 32.3 16.3–38.9 (N= 9) – 
Caudal length (mm) 17.0–20.3 (N= 3) – 28.9 21.5–24.2 (N= 2) – 14.7 8.7–18.4 (N= 8) – 
Per cent of length to end of anal fin 
Anal-fin base 83.1–90.2 (N= 4) 85.7 88.8 84.5–86.9 (N= 2) 85.7 86.2 79.5–86.2 (N= 9) 82.4 
Distance snout to anus 9.5–12.0 (N= 4) 10.7 11.9 12.2–12.7 (N= 2) 12.4 11.4 11.3–15.4 (N= 9) 13.7 
Preanal-fin distance 11.3–13.4 (N= 3) 12.6 15.0 16.0–16.7 (N= 2) 16.2 14.3 14.2–18.3 (N= 9) 16.3 
Prepectoral-fin distance 18.7–22.5 (N= 4) 20.3 21.6 22.6–23.2 (N= 2) 22.9 21.8 19.9–24.6 (N= 9) 22.7 
Greatest body depth 12.5–15.0 (N= 4) 13.4 11.9 11.0–11.2 (N= 2) 11.1 12.6 11.1–12.7 (N= 9) 11.8 
Head length 18.5–21.8 (N= 4) 20.0 22.1 21.4–23.0 (N= 2) 22.2 21.1 20.2–23.7 (N= 9) 22.1 
Caudal length 7.9–10.4 (N= 6) 9.0 16.9 15.1–16.6 (N= 2) 15.8 9.6 8.7–10.8 (N= 8) 9.9 
Per cent of head length 
Anus to anal-fin insertion 1.6–6.8 (N= 4) 3.2 15.0 13.0–17.4 (N= 2) 15.2 11.1 6.8–12.8 (N= 9) 9.5 
Pectoral-fin length 38.5–45.7 (N= 4) 42.4 34.8 35.6–37.0 (N= 2) 36.3 31.5 31.5–40.7 (N= 9) 34.6 
Head depth at eye 27.2–35.4 (N= 5) 30.2 27.7 25.6–27.0 (N= 2) 26.3 30.4 26.2–30.8 (N= 9) 28.6 
Head depth at nape 50.9–67.3 (N= 3) 57.4 41.4 39.5–42.2 (N= 2) 40.9 43.2 39.5–47.6 (N= 9) 43.0 
Head width 18.7–30.3 (N= 5) 23.1 20.4 18.8–19.9 (N= 2) 19.4 20.9 17.9–23.3 (N= 9) 21.1 
Snout length 64.5–66.1 (N= 5) 65.5 66.0 63.8–64.6 (N= 2) 64.2 63.4 57.2–66.0 (N= 9) 61.8 
Posterior naris to snout 6.9–7.4 (N= 2) 7.2 6.5 7.2–7.5 (N= 2) 7.4 9.6 7.9–10.7 (N= 9) 9.4 
Posterior naris to eye 54.5–57.1 (N= 2) 55.8 58.0 53.0–54.6 (N= 2) 53.8 52.5 42.5–55.5 (N= 9) 50.0 
Mouth length 5.1–5.7 (N= 2) 5.4 5.0 4.8–5.3 (N= 2) 5.0 6.6 5.8–7.6 (N= 9) 6.5 
Internarial distance 3.0–4.5 (N= 2) 3.8 2.0 2.3–3.1 (N= 2) 2.7 2.9 2.2–3.5 (N= 9) 2.7 
Eye diameter 3.6–5.8 (N= 5) 4.6 4.0 3.8–4.3 (N= 2) 4.1 4.6 3.9–6.7 (N= 9) 5.0 
Interocular width 5.6–6.8 (N= 5) 6.2 5.2 5.4–6.2 (N= 2) 5.8 6.3 5.6–13.5 (N= 9) 8.3 
Postocular distance 36.7–40.3 (N= 5) 38.2 38.7 36.1–38.6 (N= 2) 37.3 39.8 35.6–39.8 (N= 9) 37.9 
Branchial opening 13.8–16.6 (N= 4) 15.5 9.5 10.2–10.8 (N= 2) 10.5 11.5 9.8–14.0 (N= 9) 11.8 
Per cent of caudal length 
Tail depth 12.7–17.5 (N= 4) 14.7 8.9 8.0–8.6 (N= 2) 8.3 17.1 13.8–19.2 (N= 8) 15.8 
Caudal-fin length 30.3–56.9 (N= 2) 43.6 19.7 17.2–21.7 (N= 2) 19.1 29.4 26.6–45.0 (N= 8) 34.3 

Number of specimens indicated in parentheses. H, holotype; range includes nontype specimens.

Sternarchus(Rhamphosternarchus) curvirostrisBoulenger, 1887: 282, pl. 24 [type locality; Ecuador, Canelos]. –Eigenmann & Eigenmann, 1891: 62 [assignment to Sternarchorhynchus]. –Ellis, 1913: 141 [Sternarchorhynchus curvirostris placed in synonymy of S. oxyrhynchus]. –Fernández-Yépez, 1967: 18 [Sternarchorhynchus curvirostris resurrected from synonymy of S. oxyrhynchus]

Sternarchorhynchus curvirostris, Eigenmann & Bean, 1907: 666 [S. curvirostris as possible synonym of S. mormyrus]. –Fernández-Yépez, 1967: 18 [S. curvirostris resurrected from synonymy of S. oxyrhynchus]. –Mago-Leccia, 1994: 37, fig. 55 [as valid species in listing of members of genus]. –Campos-da-Paz, 2000: 528, fig. 3 [in key to species of Sternarchorhynchus; syntype illustrated]. –Albert, 2003: 500 [in listing of members of genus].

Diagnosis:

Sternarchorhynchus curvirostris is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the lateral line that extends posteriorly to the base of the caudal fin, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and sometimes the mid-dorsal region of the body anterior of the origin of the electroreceptive filament, the darkly coloured anal fin with darker basal coloration but without a distinct, dark, distal band, the presence of eight to 11 teeth on the premaxilla, six teeth in the outer row of the premaxilla, four teeth on the inner row of the premaxilla, 25 unbranched anal-fin rays, 173–180 total anal-fin rays, 16 precaudal vertebrae, 15–16 caudal-fin rays, the preanal distance (11.3–13.4% of LEA), the greatest body depth (12.5–15.0% of LEA), the caudal length (7.9–10.4% of LEA), the distance from the anus to the anal-fin insertion (6.0–6.8% of HL), the head depth at the eye (27.2–35.4% of HL), the head depth at the nape (50.9–67.3% of HL), the snout length (64.5–66.1% of HL), the distance from the posterior naris to the snout (6.9–7.4% of HL), the internarial distance (3.0–4.5% of HL), the eye diameter (3.6–5.8% of HL), the interocular distance (5.6–6.8% of HL), the postocular length (36.7–40.3% of HL), the height of the branchial opening (13.8–16.6% of HL), the pectoral-fin length (38.5–45.7% of HL), and the tail depth (12.7–17.5% of caudal length).

Description:

Morphometric data for examined specimens in Table 6.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and distinctly curved ventrally. Mouth terminal and relatively small, with rictus located anterior to vertical through posterior naris. Anus and urogenital papilla located ventral to head, situated along vertical through eye. Combined opening for anus and urogenital papilla ovoid, with intraspecific proportional variation in width of opening.

Premaxilla with eight to 11 teeth (N= 2) apparent in whole specimen. Dentary with two tooth rows; outer row with six teeth and inner row with four teeth (N= 2).

Branchiostegal rays five; with first three rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16 (13 anterior; three transitional; N= 1).

Pectoral-fin rays ii + 11–14 [ii + 11] (N= 5). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 25 (N= 1). Total anal-fin rays 173–180 [180] (N= 4). Scales above lateral line at midbody nine to 11 [10] (N= 5). Scales present along mid-dorsal line to origin of electroreceptive filament. Origin of midsaggital electroreceptive filament located approximately at 59% of TL. Filament extending posteriorly to vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, elongate, pointed caudal fin. Caudal-fin rays 15–16 (N= 3).

Coloration in alcohol:

Available specimens including syntypes variably depigmented. Overall coloration tan to brown, but missing on some regions in some nontype specimens as consequence of apparent postmortem damage. Head slightly lighter midventrally and with narrow, lightly coloured mid-dorsal stripe that commences midway along length of snout and expands laterally in region above orbit into longitudinally ovoid, more lightly pigmented area on dorsal surface of postocular portion of head. Lightly coloured region terminates posteriorly approximately at vertical through anterior margin of opercle. Stripe obscure in some individuals. Body tan to dark overall and lacking distinct, narrow, lightly coloured mid-dorsal stripe present in some congeners. Pectoral fin dusky to dark. Anal fin nearly hyaline to dark, more so basally. Caudal fin dusky to dark.

Distribution:

Sternarchorhynchus curvirostris is known from the Río Bobanaza, eastern Ecuador and perhaps from north-eastern Peru (Fig. 35; under Remarks below).

Remarks:

The original description of Sternarchus(Rhamphosternarchus) curvirostris by Boulenger (1887: 282) reported the length as 125 mm TL. The two specimens now identified as the syntype series (BMNH 1880.12.890-891) are, however, distinctly longer than that length [note: Mago-Leccia (1994: fig. 55) and Campos-da-Paz (2000: fig. 3) both labelled a specimen as the ‘type’, whereas there are two syntypes]. Searches through the collections of the BMNH failed to yield any other specimens of S. curvirostris that correspond to the length reported in the species description. It is impossible to determine whether the reported original length was a lapsus or the purported types are not the material on which the species was based. This question notwithstanding, the specimens identified as the syntypes of Sternarchus curvirostris(= Sternarchorhynchus curvirostris) otherwise conform to the details in the original description of that species.

Eigenmann & Bean (1907: 666) proposed that S. curvirostris might be a synonym of S. mormyrus on the basis of specimens that originated in the lower Amazon River between Manaus and Pará (= Belém). They believed that those eastern Amazonian specimens bridged the differences in morphometrics and meristics purported to differentiate S. curvirostris from S. mormyrus. Sternarchorhynchus curvirostris along with S. mormyrus were, in turn, considered to be junior synonyms of S. oxyrhynchus by Ellis (1913: 141), who recognized the latter species as a monotypic, morphologically highly variable form.

Fernández-Yépez (1967: 18) resurrected S. curvirostris(and S. mormyrus) from the synonymy of S. oxyrhynchus on the basis of external features of the head. The recognition of S. curvirostris as a distinct species continued with Mago-Leccia (1994: 37), Campos-da-Paz (2000: 528), and other authors. Based on the information provided by Fernández-Yépez (1967), the specimens that he considered to be S. mormyrus were misidentified and it is highly questionable whether the specimen he identified as S. curvirostris was that species. Notwithstanding the fact that Fernández-Yépez based his conclusions on erroneous information, our results confirm that S. curvirostris is a distinct species. Various authors followed Fernández-Yépez (1967) in considering S. curvirostris a component of the Venezuelan ichthyofauna, but we have not examined any specimens of the species that originated in that country or regions proximate to it. It is likely that the original citation represents a misidentification.

We located only five specimens of S. curvirostris in museum collections, the two apparent syntypes (BMNH 1880.12.8.90-91) and three other specimens (MCZ 48676; USNM 163887) that were also collected in the Río Bobonaza, Ecuador, the river system from which the type series originated. Three other specimens from portions of north-eastern Peru proximate to the type region (LACM 36324-2, LACM 36327-3) would be considered conspecific with S. curvirostris based on counts and overall form of the head and body, but differ from Ecuadorian samples in the degree of pigmentation on the snout. Although this difference may be a consequence of differential preservation or geographical variation, we defer from formally equating these specimens with S. curvirostris at this time.

Secondary sexual dimorphism:

Sternarchorhynchus curvirostris is not known to demonstrate sexual dimorphism.

Material examined

ECUADOR. Pastaza: Canelos (1°35′S, 77°45′W); BMNH 1880.12.8.90-91, 2 (length undetermined because of missing tail-203.2; syntypes of Sternarchorhynchus(Rhamphosternarchus) curvirostris; See remarks concerning the reported type status of these specimens). Río Putuno, tributary of Río Bobonaza; MCZ 48676, 1 (235). Upper Río Pastaza basin, Río Bobonaza, Chicirota (2°22′S, 76°38′W), USNM 163887, 2 (217–240).

The following specimens may be S. curvirostris(see Remarks above).

PERU. Amazonas: Río Santiago at La Poza (4°01′S, 75°18W); LACM 36324-2, 1 (189), LACM 36327-3, 2 (150–238).

Sternarchorhynchus freemanisp. nov. (Figs35, 38; Table6)

Sternarchorhynchus freemani sp. nov., holotype, male, 201 mm total length, CAS 72246; Guyana, Amatuk, Lower Potaro River.

Sternarchorhynchus freemani sp. nov., holotype, male, 201 mm total length, CAS 72246; Guyana, Amatuk, Lower Potaro River.

Sternarchorhynchus oxyrhynchus, not of Müller & Troschel, Eigenmann, 1912: 438 [in part, specimens from Guyana, Warraputa, and Amatuk]. –Ellis, 1913: 141, 174 [in part, specimens from Guyana, Warraputa, and Amatuk; diet].

Diagnosis:

Sternarchorhynchus freemani is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the absence of scales along the mid-dorsal region of the body as far posteriorly as the vertical through posterior limit of the pectoral fin, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body extending as far posteriorly as the origin of the electroreceptive filament and sometimes beyond that point, the presence of four to five premaxillary teeth, 16–17 precaudal vertebrae, 26–28 anterior unbranched anal-fin rays, 170–177 total anal-fin rays, eight to nine scales above the lateral line at the midbody, the termination of the electroreceptive filament at the vertical running through a point four scales posterior of the posterior limit of the base of the anal fin, the termination of the lateral line at a point four scales anterior of the base of the caudal fin, the greatest body depth (11.0–11.9% of LEA), the distance from the anus to the anal-fin insertion (13.0–17.4% of HL), the caudal length (15.1–16.9% of LEA), the head width (18.8–20.4% of HL), the distance from the posterior naris to the snout (6.5–7.5% of HL), the interocular width (5.2–6.2% of HL), the postocular distance (36.1–38.7% of HL), the height of the branchial opening (9.5–10.8% of HL), the head depth at the eye (25.6–27.7% of HL), and the tail depth (8.0–8.9% of caudal length).

Description:

Morphometric data for examined specimens in Table 6.

Lateral line extending posteriorly to point four scales from base of caudal fin and absent on remainder of tail and on caudal fin. Snout elongate, compressed and slightly curved ventrally along its anterior portion. Mouth terminal. Rictus located slightly anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head, either situated approximately at vertical through eye or along vertical one eye diameter anterior to margin of eye. Combined opening for anus and urogenital papilla longitudinally ovoid in all specimens.

Premaxilla with four to five teeth (N= 2) apparent in whole specimens. Dentary with two tooth rows; outer row with eight teeth and inner row with one to two teeth (N= 2).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16–17 (12–14 anterior; two to four transitional; N= 4).

Pectoral-fin rays ii + 11–12 [ii + 11] (N= 4). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 22–28 [22] (N= 4). Total anal-fin rays 170–177 [176] (N= 4). Scales above lateral line at midbody eight to nine [8] (N= 4). Scales absent along mid-dorsal region of body from rear of head posteriorly to point approximately along vertical through posterior limit of pectoral fin. Scaleless region followed by area with sparse mid-dorsal scales and then fully scaled region extending to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately 60% of TL. Filament extending posteriorly four scales beyond vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate, pointed caudal fin. Caudal-fin rays 13–16 [13] (N= 4).

Coloration in alcohol:

Overall coloration of head and body ranging from tan to brown. More lightly pigmented specimen with narrow band of darker pigmentation extending anteriorly from eye nearly to tip of snout and with second darker band extending along ventral portions of snout. Two bands of darker pigmentation border more lightly pigmented region extending along lateral surface of snout. Dark bands and intervening more lightly coloured area on snout less apparent in overall darkly pigmented individuals. Dorsal dark band forms lateral margin of narrow, lightly coloured stripe running along mid-dorsal region of snout; mid-dorsal stripe terminates above eye, or slightly posterior of that point. Mid-dorsal stripe on anterior portion of head separated from mid-dorsal stripe on body by dark pigmentation on dorsal surface of head in postocular region. Body with mid-dorsal more lightly pigmented stripe extending from rear of head to origin of electroreceptive filament or to varying degrees posterior of that point. Pectoral fin in overall more lightly coloured specimens hyaline but somewhat dusky with small dark chromatophores overlying fin rays in darker individuals. Anal fin ranging from hyaline to dusky, with darker individuals having small, dark chromatophores overlying fin rays. Caudal fin dark, more so in overall darker individuals, with distal regions somewhat more lightly coloured.

Distribution:

Sternarchorhynchus freemani is only known from locations in the lower portions of the Essequibo River basin in Guyana (Fig. 35).

Ecology:

Ellis (1913: 174) reported that S. freemani(specimens identified as S. oxyrhynchus by that author) feeds on various types of aquatic larvae and annelids.

Secondary sexual dimorphism:

No sexual dimorphism was apparent in the limited available sample of S. freemani.

Etymology:

The species name, freemani, is in honour of Bryon J. Freeman of the University of Georgia who provided the senior author with invaluable assistance at the Georgia Museum of Natural History.

Remarks:

Eigenmann (1912: 438) and Ellis, 1913: 141) reported S. oxyrhynchus from Amatuk and Warraputa in the Essequibo River system of Guyana. More recently Watkins et al.(2004) cited that species from streams of the Iwokrama forest along the left bank of the Essequibo. As discussed under Remarks for S. oxyrhynchus, none of the examined samples of Sternarchorhynchus from Guyana proved to be S. oxyrhynchus. Examination of the material that served at least in part as the basis of the Eigenmann and Ellis citations of S. oxyrhynchus(CAS 72248 formerly CM 1807; CAS 72246, formerly IU 12590) have shown that they are S. freemani, a new species described in this study.

Müller & Troschel (1848) reported that the material on which they based their description of S. oxyrhynchus originated in the Essequibo River, Guyana. As mentioned within the Remarks section under S. oxyrhynchus, none of the samples of that species we examined originated in the Essequibo River or any other river in the Guianas. Rather, all examined specimens of S. oxyrhynchus were captured in the Río Orinoco basin. Samples of Sternarchorhynchus that originated in the Essequibo River are all S. freemani, a species distinguished from S. oxyrhynchus by numerous features. One other species of Sternarchorhynchus was discovered in samples from the easterly flowing rivers of the Guianas; that being S. galibi, a species apparently endemic to the Marowijne River (= Fleuve Maroni) system along the border between Suriname and French Guiana. Sternarchorhynchus galibi and S. oxyrhynchus differ in diverse features, most notably in mouth form.

Material examined

Holotype:

GUYANA. Essequibo River basin, Lower Potaro River at Amatuk (approximately 5°18′N, 59°18W), collected by C. H. Eigenmann, 10.x.1908; CAS 72246, (201, male).

Paratypes:

GUYANA. Essequibo River basin, Lower Potaro River at Amatuk (approximately 5°18′N, 59°18W), collected by C. H. Eigenmann with the holotype, CAS 227622 ex CAS 72246, 1 (163, female; originally two specimens, but one female was lost during the clearing and staining process but photographed and radiographed), Essequibo River basin, lower Potaro River at Warraputa cataract; CAS 72248, 1 (185).

Sternarchorhynchus galibisp. nov. (Figs35, 39; Table6)

Sternarchorhynchus galibi sp. nov., holotype, female, 168 mm total length, ZMA 109.366; Suriname, Marowijne District, Lawa River at Stroomafwaarts Rapids near Anapaikekondre.

Sternarchorhynchus galibi sp. nov., holotype, female, 168 mm total length, ZMA 109.366; Suriname, Marowijne District, Lawa River at Stroomafwaarts Rapids near Anapaikekondre.

Diagnosis:

Sternarchorhynchus galibi is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the absence of scales along the mid-dorsal region of the body as far posteriorly as the vertical through the posterior limit of the pectoral fin, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body extending as far posteriorly as the origin of the electroreceptive filament and sometimes beyond that point, the presence of six to ten premaxillary teeth, 26–31 anterior unbranched anal-fin rays, 169–179 total anal-fin rays, and seven to ten scales above the lateral line at the midbody, the termination of the electroreceptive filament at the vertical running through a point one or two scales anterior of the posterior limit of the base of the anal fin, the termination of the lateral line at a point three or four scales anterior of the base of the caudal fin, the greatest body depth (11.1–12.7% of LEA), the caudal length (8.7–10.8% of LEA), the distance from the anus to the anal-fin insertion (6.8–12.8% of HL), the head width (17.9–23.3% of HL), eye diameter (3.9–6.7 of HL), the head depth at the eye (26.2–30.8% of HL), the head depth at the nape (39.5–47.6% of HL), the postocular distance (35.6–39.8% of HL), the pectoral-fin length (31.5–40.7% of HL), and the tail depth (13.8–19.2% of caudal length).

Description:

Morphometric data for examined specimens in Table 6.

Lateral line extending posteriorly to point three or four scales anterior of base of caudal fin but absent on remainder of tail and on caudal fin. Snout elongate, compressed and slightly curved ventrally along anterior portion. Mouth terminal. Rictus located along vertical through anterior naris. Anus and urogenital papilla located ventral to head, with position in smaller specimens located within area delimited anteriorly by vertical running approximately to eye and posteriorly one orbital diameter posterior of eye. Openings positioned one eye diameter anterior to vertical through eye in larger individuals. Combined opening for anus and urogenital papilla circular in smaller individuals and longitudinally ovoid in larger specimens.

Premaxilla with six to ten teeth (N= 4) apparent in whole specimens. Dentary with two tooth rows; outer row with six to nine teeth and inner row with one to three teeth (N= 4).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15–16 (11–12 anterior; four transitional; N= 2).

Pectoral-fin rays ii + 12–13 [ii + 12] (N= 10). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 26–31 [26] (N= 8). Total anal-fin rays 169–179 [175] (N= 10). Scales above lateral line at midbody seven to ten [10] (N= 10). Scales absent along mid-dorsal region of body from rear of head posteriorly to point approximately one-third of distance along length of body and three times length of pectoral fin from rear of head. Scaleless region followed by fully scaled mid-dorsal area to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately 70% of TL. Filament extending to point one or two scales short of vertical through posterior terminus of base of anal fin to vertical through latter landmark. Tail compressed and short, ending in small, elongate, pointed caudal fin. Caudal-fin rays 15–17 [15] (N= 9).

Coloration in alcohol:

Overall coloration of head and body dark brown. Slightly darker bands of pigmentation extending anteriorly from eye along dorsolateral portion of snout and along ventral portions of snout. Dark bands border more lightly pigmented region on midlateral surface of anterior half of snout. Dorsal dark band forms lateral margin of narrow, lightly coloured stripe running along mid-dorsal region of snout; stripe terminates above eye, or slightly posterior of that point. Mid-dorsal stripe on snout separated from mid-dorsal stripe on body by dark pigmentation on dorsal surface of head in postocular region. Body with more lightly pigmented, mid-dorsal stripe extending from rear of head to origin of electroreceptive filament or to varying degrees posterior of that point. Pectoral fin dusky with small, dark chromatophores overlying fin rays. Anal fin dusky with small, dark chromatophores overlying fin rays. Caudal fin dark with distal regions hyaline.

Distribution:

Sternarchorhynchus galibi is known from the Lawa River, Marowijne River (= Fleuve Maroni) basin along the border between Suriname and French Guiana (Fig. 35).

Secondary sexual dimorphism:

No sexual dimorphism was apparent in the limited available sample of S. galibi.

Etymology:

The species name, galibi, is in reference to the town of that name begun as a major settlement of the indigenous Caribs. The town is at the mouth of the Marowijne, the drainage system that includes the type locality of S. galibi.

Remarks:

Sternarchorhynchus oxyrhynchus was cited by Meunier (2004: 89) as occurring in the upper portions of the Fleuve Maroni (= Marowijne River in French Guiana); however, it is likely that this citation was based on S. galibi.

Material examined

Holotype:

SURINAME. Marowijne District: Lawa River at Stroomafwaarts Rapids near Anapaikekondre (= Anapaike country; Anapaike at 3°34′N, 54°39′W), collected by J. P. Gosse, 18.xi.1966, ZMA 109.366 (168, originally part of IRSCNB 16793, apparently female).

Paratypes:

SURINAME. Marowijne District: Lawa River at Stroomafwaarts Rapids near Anapaikekondre (= Anapaike country; Anapaike at 3°34′N, 54°39′W), ZMA 124.524, 1 (121), IRSCNB 16793, 7 (80–202); collected with the holotype by J. P. Gosse, 18.xi.1966. Lawa River base camp, approximately 8 km south-south-west of Anapaike/Kawahaken (airstrip) (3°19′31″N, 54°03′48″W), collected by J. Lundberg, M. Sabaj, P. Willink, and J. Mol, 18.iv.2007, ANSP 187155, 1 (200).

Nontype specimen:

FRENCH GUIANA. Upper Maroni River (no specific locality), MHNG 2167.45, 1 (143).

Sternarchorhynchus gnomusde Santana& Taphorn(Figs35, 40; Table7)

Sternarchorhynchus gnomus, holotype, female, 133.5 mm total length, MCNG 53963; Venezuela, Bolivar, Río Caroní.

Sternarchorhynchus gnomus, holotype, female, 133.5 mm total length, MCNG 53963; Venezuela, Bolivar, Río Caroní.

Morphometric data for holotype (H), and paratypes of Sternarchorhynchus gnomus, Sternarchorhynchus goeldii, and Sternarchorhynchus hagedornae

 S. gnomus S. goeldii S. hagedornae 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 133.5 76.5–152 (N= 9) – 306.6 193–333.4 (N= 9) – 249 162–206 (N= 5) – 
Length to end of anal fin (mm) 116.5 69.1–137 (N= 8) – 280 165–293 (N= 7) – 233 150–193 (N= 5) – 
Head length (mm) 23.7 15.6–30.7 (N= 9) – 48.8 34.9–50.0 (N= 5) – 40.9 28.7–35.0 (N= 5) – 
Caudal length (mm) 17.3 9.9–14.9 (N= 5) – 27.4 26.7–41.9 (N= 3) – 17.1 10.0–14.4 (N= 5) – 
Per cent of length to end of anal fin 
Anal-fin base 81.8 82.4–86.9 (N= 7) 84.5 87.1 80.4–87.3 (N= 7) 85.3 87.5 86.6–89.6 (N= 5) 88.2 
Distance snout to anus 13.4 11.8–17.4 (N= 8) 15.0 15.6 9.5–15.4 (N= 5) 12.9 6.4 8.4–12.0 (N= 5) 10.0 
Preanal-fin distance 14.7 14.1–17.7 (N= 7) 16.6 14.2 12.8–18.9 (N= 5) 16.0 11.6 10.4–13.9 (N= 5) 11.9 
Prepectoral-fin distance 20.0 20.0–23.2 (N= 7) 22.3 17.3 17.1–20.9 (N= 5) 19.0 17.6 17.9–20.8 (N= 5) 19.1 
Greatest body depth 9.6 9.3–11.4 (N= 7) 10.4 7.5 8.2–10.8 (N= 7) 9.3 11.4 12.9–13.3 (N= 5) 13.1 
Head length 20.3 20.1–24.3 (N= 9) 22.2 17.4 17.0–21.1 (N= 4) 19.2 17.5 17.7–20.0 (N= 5) 18.8 
Caudal length 14.8 10.1–17.4 (N= 5) 14.6 9.8 14.3–16.2 (N= 3) 14.9 7.3 5.5–8.3 (N= 5) 7.1 
Per cent of head length 
Anus to anal-fin insertion 15.0 7.8–19.9 (N= 8) 12.3 23.1 10.1–20.1 (N= 5) 14.5 23.3 8.0–10.1 (N= 5) 8.9 
Pectoral-fin length 38.8 34.8–41.0 (N= 9) 36.9 42.7 33.7–37.8 (N= 5) 36.4 43.8 39.1–46.9 (N= 5) 42.9 
Head depth at eye – 37.4–39.0 (N= 3) 38.3 26.5 20.2–25.3 (N= 5) 23.7 39.8 29.6–37.2 (N= 5) 34.0 
Head depth at nape 41.7 39.7–44.9 (N= 9) 41.8 45.4 35.0–44.8 (N= 5) 39.4 55.4 53.1–59.2 (N= 5) 55.8 
Head width 20.3 20.0–21.5 (N= 5) 20.6 21.0 17.5–20.7 (N= 5) 19.0 26.8 24.1–28.3 (N= 5) 25.8 
Snout length 62.8 57.6–65.4 (N= 9) 61.1 61.3 61.6–64.7 (N= 5) 62.7 59.6 57.4–58.9 (N= 5) 58.1 
Posterior naris to snout 11.4 10.9–11.5 (N= 5) 11.3 4.9 5.6–6.8 (N= 5) 6.2 9.0 8.7–9.8 (N= 5) 9.0 
Posterior naris to eye 52.7 47.6–55.7 (N= 9) 50.3 54.2 53.3–57.5 (N= 5) 55.3 48.9 44.6–47.9 (N= 5) 46.5 
Mouth length 6.3 5.5–9.2 (N= 9) 7.3 9.2 6.5–7.9 (N= 5) 7.2 8.6 6.4–7.9 (N= 5) 7.0 
Internarial distance 5.9 2.7–7.0 (N= 9) 4.4 3.3 1.8–3.6 (N= 5) 2.6 2.7 2.3–3.5 (N= 5) 3.1 
Eye diameter 5.4 4.1–7.6 (N= 9) 5.4 4.5 3.8–5.1 (N= 5) 4.2 4.3 4.1–5.9 (N= 5) 4.8 
Interocular width 8.8 4.2–9.6 (N= 9) 6.9 5.6 6.1–7.3 (N= 5) 5.0 7.5 6.0–8.5 (N= 5) 7.7 
Postocular distance 39.2 36.9–41.6 (N= 9) 38.7 39.5 34.3–37.1 (N= 5) 35.7 44.1 43.6–46.2 (N= 5) 45.3 
Branchial opening 11.8 9.5–12.3 (N= 9) 10.9 16.4 11.0–13.8 (N= 5) 12.6 14.8 11.9–14.9 (N= 5) 13.6 
Per cent of caudal length 
Tail depth 10.9 8.3–13.8 (N= 5) 9.9 22.6 13.2–16.3 (N= 3) 14.8 22.0 21.3–30.4 (N= 5) 25.0 
Caudal-fin length 25.4 19.6–31.8 (N= 5) 24.9 46.3 22.3 (N= 1) – 40.3 25.8–54.3 (N= 5) 38.3 
 S. gnomus S. goeldii S. hagedornae 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 133.5 76.5–152 (N= 9) – 306.6 193–333.4 (N= 9) – 249 162–206 (N= 5) – 
Length to end of anal fin (mm) 116.5 69.1–137 (N= 8) – 280 165–293 (N= 7) – 233 150–193 (N= 5) – 
Head length (mm) 23.7 15.6–30.7 (N= 9) – 48.8 34.9–50.0 (N= 5) – 40.9 28.7–35.0 (N= 5) – 
Caudal length (mm) 17.3 9.9–14.9 (N= 5) – 27.4 26.7–41.9 (N= 3) – 17.1 10.0–14.4 (N= 5) – 
Per cent of length to end of anal fin 
Anal-fin base 81.8 82.4–86.9 (N= 7) 84.5 87.1 80.4–87.3 (N= 7) 85.3 87.5 86.6–89.6 (N= 5) 88.2 
Distance snout to anus 13.4 11.8–17.4 (N= 8) 15.0 15.6 9.5–15.4 (N= 5) 12.9 6.4 8.4–12.0 (N= 5) 10.0 
Preanal-fin distance 14.7 14.1–17.7 (N= 7) 16.6 14.2 12.8–18.9 (N= 5) 16.0 11.6 10.4–13.9 (N= 5) 11.9 
Prepectoral-fin distance 20.0 20.0–23.2 (N= 7) 22.3 17.3 17.1–20.9 (N= 5) 19.0 17.6 17.9–20.8 (N= 5) 19.1 
Greatest body depth 9.6 9.3–11.4 (N= 7) 10.4 7.5 8.2–10.8 (N= 7) 9.3 11.4 12.9–13.3 (N= 5) 13.1 
Head length 20.3 20.1–24.3 (N= 9) 22.2 17.4 17.0–21.1 (N= 4) 19.2 17.5 17.7–20.0 (N= 5) 18.8 
Caudal length 14.8 10.1–17.4 (N= 5) 14.6 9.8 14.3–16.2 (N= 3) 14.9 7.3 5.5–8.3 (N= 5) 7.1 
Per cent of head length 
Anus to anal-fin insertion 15.0 7.8–19.9 (N= 8) 12.3 23.1 10.1–20.1 (N= 5) 14.5 23.3 8.0–10.1 (N= 5) 8.9 
Pectoral-fin length 38.8 34.8–41.0 (N= 9) 36.9 42.7 33.7–37.8 (N= 5) 36.4 43.8 39.1–46.9 (N= 5) 42.9 
Head depth at eye – 37.4–39.0 (N= 3) 38.3 26.5 20.2–25.3 (N= 5) 23.7 39.8 29.6–37.2 (N= 5) 34.0 
Head depth at nape 41.7 39.7–44.9 (N= 9) 41.8 45.4 35.0–44.8 (N= 5) 39.4 55.4 53.1–59.2 (N= 5) 55.8 
Head width 20.3 20.0–21.5 (N= 5) 20.6 21.0 17.5–20.7 (N= 5) 19.0 26.8 24.1–28.3 (N= 5) 25.8 
Snout length 62.8 57.6–65.4 (N= 9) 61.1 61.3 61.6–64.7 (N= 5) 62.7 59.6 57.4–58.9 (N= 5) 58.1 
Posterior naris to snout 11.4 10.9–11.5 (N= 5) 11.3 4.9 5.6–6.8 (N= 5) 6.2 9.0 8.7–9.8 (N= 5) 9.0 
Posterior naris to eye 52.7 47.6–55.7 (N= 9) 50.3 54.2 53.3–57.5 (N= 5) 55.3 48.9 44.6–47.9 (N= 5) 46.5 
Mouth length 6.3 5.5–9.2 (N= 9) 7.3 9.2 6.5–7.9 (N= 5) 7.2 8.6 6.4–7.9 (N= 5) 7.0 
Internarial distance 5.9 2.7–7.0 (N= 9) 4.4 3.3 1.8–3.6 (N= 5) 2.6 2.7 2.3–3.5 (N= 5) 3.1 
Eye diameter 5.4 4.1–7.6 (N= 9) 5.4 4.5 3.8–5.1 (N= 5) 4.2 4.3 4.1–5.9 (N= 5) 4.8 
Interocular width 8.8 4.2–9.6 (N= 9) 6.9 5.6 6.1–7.3 (N= 5) 5.0 7.5 6.0–8.5 (N= 5) 7.7 
Postocular distance 39.2 36.9–41.6 (N= 9) 38.7 39.5 34.3–37.1 (N= 5) 35.7 44.1 43.6–46.2 (N= 5) 45.3 
Branchial opening 11.8 9.5–12.3 (N= 9) 10.9 16.4 11.0–13.8 (N= 5) 12.6 14.8 11.9–14.9 (N= 5) 13.6 
Per cent of caudal length 
Tail depth 10.9 8.3–13.8 (N= 5) 9.9 22.6 13.2–16.3 (N= 3) 14.8 22.0 21.3–30.4 (N= 5) 25.0 
Caudal-fin length 25.4 19.6–31.8 (N= 5) 24.9 46.3 22.3 (N= 1) – 40.3 25.8–54.3 (N= 5) 38.3 

Number of specimens indicated in parentheses.

Sternarchorhynchus gnomusde Santana & Taphorn, 2006: 2, figs 1, 2 [type locality: Venezuela, Bolivar, Río Caroni, at confluence with Río Claro, 7°54′30″N, 63°02′50″W].

Diagnosis:

Sternarchorhynchus gnomus is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the lateral line that extends to the base of the caudal fin, the absence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament or with the stripe very obscure, the hyaline anal fin lacking a dark band along its distal margin, the possession of six to eight premaxillary teeth, one row of dentary dentition with seven to eight teeth, 12 branched pectoral-fin rays, 15 precaudal vertebrae, 21–25 anterior unbranched anal-fin rays, 156–177 total anal-fin rays, six to seven scales above the lateral line at the midbody, ten to 12 caudal fin-rays, the greatest body depth (9.3–11.4% of LEA), the distance from the snout to the anus (11.8–17.4% of LEA), the preanal distance (14.1–17.7% of LEA), the prepectoral-fin distance (20.0–23.2% of LEA), the caudal length (10.1–17.4% of LEA), the head length (20.1–24.3% of LEA), the snout length (57.6–65.4% of HL), the head depth at the nape (39.7–44.9% of HL), the postocular distance (36.9–41.6% of HL), the pectoral-fin length (34.8–41.0% of HL), and the tail depth (8.3–13.8% of caudal length).

Description:

Morphometric data for examined specimens in Table 7.

Lateral line extending posteriorly to approximately three to four scales anterior of base of caudal fin, but absent fin. Snout elongate, compressed and slightly curved ventrally along anterior portion. Mouth terminal. Rictus located distinctly anterior of vertical through anterior naris. Anus and urogenital papilla located ventral to head. Openings located at vertical approximately two eyes diameter posterior of margin of orbit in largest specimens. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with six teeth (N= 1) in cleared and stained specimen, seven to eight apparent in whole specimens (N= 4). Dentary with one tooth row with seven functional and five replacement teeth in cleared and stained specimen (N= 1), six to eight functional plus three replacement teeth apparent in whole specimens (N= 4).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15 (13 anterior; two transitional; N= 4).

Pectoral-fin rays ii + 12 [ii + 12] (N= 10). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 21–25 (N= 5). Total anal-fin rays 159–177 [175] (N= 9). Scales above lateral line at midbody six to seven [7] (N= 10). Scales present along mid-dorsal region of body to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately 60% of TL. Filament extending posteriorly to vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate, pointed caudal fin. Caudal-fin rays ten to 12 (N= 4).

Coloration in alcohol:

Overall coloration of head and body dark brown. Slightly darker bands of pigmentation extending from anterior of orbit anteriorly along dorsolateral portion of snout and along ventral portions of snout. Two dark bands border more lightly pigmented region on midlateral surface of anterior half of snout. Mid-dorsal region of head and body with narrow, more lightly coloured, irregularly interrupted stripe extending to origin of midsaggital electroreceptive filament. Pectoral and anal fins hyaline. Caudal fin dark with distal regions hyaline.

Distribution:

Sternarchorhynchus gnomus is known from the lower portions of the Río Caroni basin, Venezuela (Fig. 35). The type locality is now submerged by the waters of the Caruachi Reservoir (de Santana & Taphorn, 2006: 1).

Ecology:

The type locality of S. gnomus was a side channel of the Río Caroni near its confluence with the Río Claro. At that site, the water was clear with a visibility of approximately 2 m and the pH ranging from 6.6–7.6. Stomach contents of the species included chironomids, ephemeropterans, and trichopterans.

Secondary sexual dimorphism:

S exually mature males of S. gnomus appear to have the ventral curvature of the snout more pronounced than do females of comparable sizes (de Santana & Taphorn, 2006: fig. 2).

Material examined

VENEZUELA. Bolivar: Río Caroní, at its confluence with Río Claro (7°54′30″N 63°02′50″W); ANSP 182798, 1 (108.8; paratype); INPA 182798, 3 (one cleared and stained, 145.0–152.0; paratypes); MCNG 53963, 1 (133.5; holotype, female); MCNG 18435, 5 (76.5–152.0; paratypes).

Sternarchorhynchus goeldiisp. nov. (Figs41, 42; Table7)

Sternarchorhynchus goeldii sp. nov., holotype, female, 306 mm total length, INPA 28378; Brazil, Amazonas, Rio Purus.

Sternarchorhynchus goeldii sp. nov., holotype, female, 306 mm total length, INPA 28378; Brazil, Amazonas, Rio Purus.

Diagnosis:

Sternarchorhynchus goeldii is distinguished from all congeners with the exception of S. oxyrhynchus by having the gape of the mouth aligned with main axis of snout, with the gape elongate and extending posteriorly distinctly beyond the vertical through posterior naris contrary to a shorter gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares. Sternarchorhynchus goeldii differs from S. oxyrhynchus in the number of total vertebrae (92–94 versus 97–108, respectively), the greatest body depth (8.2–10.8 versus 6.3–8.2% of TL, respectively), and to a large degree in the head depth at the nape (35.0–44.8 versus 26.8–37.7% of SL, respectively).

Description:

Morphometric data for holotype and paratypes specimens in Table 7.

Lateral line extending posteriorly to base of caudal fin, but not extending onto fin. Snout elongate, compressed and very slightly curved ventrally along its distal portion, but having overall anteroventral orientation. Anus and urogenital papilla located ventral to head, with position ranging from slightly anterior of vertical through anterior margin of orbit to vertical located two orbital diameters posterior of vertical through rear of orbit. Anus and urogenital papilla generally more anteriorly positioned in larger specimens. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with ten to 11 teeth with six replacement teeth in cleared and stained specimen (N= 6). No sexual dimorphism in form of dentary or dentary dentition apparent in examined samples. Dentary with two rows of teeth. Outer row consisting of 13–20 curved conical teeth and inner row with three to five teeth (N= 7). Mouth terminal. Gape of mouth aligned with main axis of snout, elongate and extending posteriorly distinctly beyond vertical through posterior naris (Fig. 1A). Lower jaw with terminal fleshy pad that extends anteriorly beyond vertical through anterior limit of snout. Fleshy pad directed dorsally and overlaps tip of snout when mouth closed.

Branchiostegal rays five; with first and second ray narrow and elongate, third to fifth rays becoming progressively wider. Precaudal vertebrae 16–17 (three anterior; three to four transitional; N= 15). Total vertebrae 92–94 (N= 10).

Pectoral-fin rays ii + 12–14 [ii + 13] (N= 29). Anal-fin origin located anterior to vertical through opercle. Anterior unbranched anal-fin rays 21–40 [26] (N= 17). Total anal-fin rays 204–237 [215] (N= 29). Scales above lateral line at midbody three to six [5] (N= 21). Scales absent along mid-dorsal line from rear of head to origin of electroreceptive filament. Origin of midsaggital electroreceptive filament located approximately at 62% of TL. Filament extending posteriorly beyond vertical through posterior terminus of base of anal fin for distance of approximately five scales. Tail compressed and short, ending in small, elongate caudal fin with rounded posterior margin. Caudal-fin rays 16–22 [20] (N= 13).

Coloration in alcohol:

Overall ground coloration tan to dark brown. Head and body with small dark chromatophores relatively densely scattered over surfaces. Chromatophores more densely packed and more intensely pigmented in darker specimens. Portion of head anterior of orbit somewhat darker than remaining region of head in some specimens. Some specimens with posteriormost portion of body and all of tail distinctly darker than remainder of body.

Pectoral fin ranging from nearly completely hyaline to overall darkly pigmented, but with dark pigmentation most developed on distal portions and along dorsal margin of fin. Very lightly pigmented specimens with dark pigmentation limited to distalmost portions and dorsal margin of fin. Overall darkly pigmented specimens with dark pigmentation covering distal half to two-thirds of pectoral fin. Distal portions of anal fin variably dark. Overall more lightly pigmented specimens often with distal dark pigmentation barely apparent on anteriormost portion of anal fin and with obvious dark coloration present only as narrow band along distal portion of fin. Some other individuals with distal dark pigmentation extending forward to anterior limit of fin and with broad irregular band of dark pigmentation extending along most of distal portion of fin. Caudal fin dark even in overall lightly pigmented individuals and very dark in most specimens.

Distribution:

Sternarchorhynchus goeldii is distributed along the main stream of the Amazon from the region of Óbidos, Brazil, in the eastern portions of the Amazon basin westwards to north-eastern Peru. The species also occurs to varying degrees in the lower courses of tributary rivers of the main Amazon (Fig. 42).

Electrical organ discharge:

Crampton (1998) and Crampton & Albert (2006) reported that individuals of S. goeldii(identified therein as Sternarchorhynchus oxyrhynchus) have a type D EOD pattern with a fundamental frequency of 1321–1390 Hz.

Habits:

Crampton (2007: 330) reported on the activity pattern of S. goeldii(identified by that author as S. oxyrhynchus) in an aquarium setting under subdued natural light. In that setting the species demonstrated increased activity levels at night, but only sporadic foraging activities in the course of the day.

Secondary sexual dimorphism:

No secondary sexual dimorphism comparable to that found in many congeners was apparent in exampled samples of S. goeldii.

Etymology:

The species name, goeldii, is in honour of Emilio Goeldi, who made many contributions to our knowledge of many groups of Amazonian animals including fishes.

Material examined

Holotype:

BRAZIL. Amazonas: Rio Purus (3°50′06″S, 61°23′59″W); collected by M. Toledo-Piza et al., 26.vii.1996; INPA 28378 (306, formerly USNM 375467).

Paratypes:

BRAZIL. Amazonas: Rio Solimões, downstream from mouth of Río Purus, upstream from town of Manacapuru (3°27′22″S, 60°45′21″W) FMNH 115520, 1 (193). Rio Purus (4°53′47″S, 62°54′53″W); INPA 17104, 2 (175–202). Rio Amazonas, 20.6 km above Itacoatiara, 35.5 km below Santa Antonia (3°15′46″S, 58°36′02″W), collected by F. Langeani et al., 20.x.1994; USNM 373081, 1 (212). Rio Solimões, Costa do Marimba, Ilha do Careiro, collected by C. Cox-Fernandes et al.; INPA 17633, 4 (285–336). Rio Solimões, near ‘encontro das águas’ (area of mixing of waters of Rio Negro and Rio Amazonas, collected by C. Cox-Fernandes et al.; INPA 24109, 5 (335–367).

Nontype specimens:

BRAZIL. Amazonas: Rio Negro, between tributary Rios Cuieras and Taruma-Açu, between towns of Santa Maria and Leprosário (3°01′45″S, 60°24′7″W); FMNH 115505, 4 (213–263), FMNH 115506, 4 (162–244). Rio Jaú, curve of Jaú, Novo Airão; INPA 11355, 1 (184). Rio Jaú, mouth of Río Preto, Novo Airão; INPA 12548, 1 (133), INPA 12551, 2 (176–217), INPA 12656, 1 (194). Rio Negro, 12 km above Leprosário, 16.3 km below Santa Maria (3°02′58″S, 60°22′10″W); USNM 373069, 3 (221–257). Rio Negro 4 km above Manaus (3°08′S, 60°03′W); USNM 373044, 2 (231–251). Rio Negro, between Paraná da Floresta and Paraná da Cotia, between towns of Moura and São Francisco (1°33′48″S, 61°33′3″W); FMNH 115507, 1 (231). Rio Negro, 18.5 km above São Francisco (1°33′45″S, 61°33′01″W); USNM 373075, 1 (221). Rio Negro, USNM 373046, 2 (225–243). Rio Negro, above Manaus; MCZ 98360, 1 (210). Rio Negro, between tributary drainages Curidique and Paraná Jacaré, between towns of Novo Caioe and São Francisco de Assis (1°58′30″S, 61°14′20″W); FMNH 115508, 4 (180–256). Rio Negro, upstream from tributary drainage Tarumã-Mirim and upstream from city of Manaus (3°04′11″S, 60°11′03″W); FMNH 115517, 1 (272). Rio Negro, upstream from Manaus (3°1′18″S, 60°23′29″W); FMNH 115516, 1 (326). Rio Negro, Praia Grande, Manaus; INPA 27495 1 (235). Rio Solimões, Manaus; INPA 24111, 2 (335–339). Rio Negro, near Manaus; INPA 4435, 3 (180–303). Rio Negro, 5.6 km below São Francisco de Assis (1°42′42″S 61°24′27″W); USNM 373106, 3 (173–207). Rio Negro, 33 km below Novo Caioe, 7 km above São Francisco de Assis (1°58′24″S, 61°14′30″W); USNM 373886, 1 (214). Rio Jaú, mouth of Jaú; INPA 12551, 2 (176–217). Rio Jaú, mouth of Rio Preto; INPA 11355, 1 (184), INPA 12656, 1 (191). Rio Amazonas, Paraná do Tapará; INPA 19991, 1 (203). Rio Solimões, Paraná do Xiborena, INPA 17636, 1 (354). Rio Solimões, Paraná do Xiborena; INPA 24110 1 (285). Rio Solimões, Ilha do Careiro, Paraná do Rei; INPA 27497, 1 (353). Rio Solimoes, Lago Januacá, Manaquiri, INPA 27499, 5 (345–360). Rio Solimões, Paraná do Tapará, Autazes; INPA 19991, 1 (200). Rio Purus, Beabá (4°53′47″S, 62°54′51″W); INPA 17104, 2 (173–211). Rio Solimões, Ilha do Careiro, Paraná do Rei; INPA 17634 1 (288), INPA 17637, 1 (347), INPA uncat., 1 (353), INPA uncat., 1 (370). Rio Japurá between mouth of Lagos Mamirauá and Joaquim; INPA 18274, 1 (334). Rio Japurá, at mouth of Lago Mamirauá; INPA uncat., 1 (214). Rio Tefé at Tefé; INPA 15821, 6 (315–370). Rio Tefé, Toco Preto; INPA 15821, 6 (311–369). Rio Amazonas, upstream from mouth of Rio Madeira/Paraná da Eva, upstream from town of Novo Oriente (3°13′37″S, 59°3′32″W); FMNH 115519, 7 (193–295). Rio Solimões, below mouth of Rio Madeira; INPA 20221, 28 (230–313). Rio Solimões, Paraná Xiborena; INPA 16636, 1 (357). Rio Amazonas downstream from Rio Madeira, upstream from town of Itacoatiara (3°20′9″S, 58°36′11″W); FMNH 115518, 1 (289). Rio Uatumã, Presidente Figueiredo at Balbina Dam; INPA 4833, 1 (341). Rio Juruá, tributaries Lago Pauapixuna and Rio Solimões, between towns of Pauapixuna and Vitória (2°41′11″S, 65°48′30″W); FMNH 115503, 2 (257–282). Rio Juruá between Lago Pauapixuna and Rio Solimões, between towns of Pauapixuna and Vitória (2°41′11″S, 65°48′30″W); FMNH 115503, 2 (257–282). Rio Juruá, Lago Pauapixuna and Lago Meneroa, between towns of Pauapixuna and Tamanicoa (2°46′31″S, 65°50′00″W); FMNH 115509, 1 (283). Rio Juruá, between Lago Pauapixuna and Rio Solimões, between towns of Pauapixuna and Vitória (2°41′08″S, 65°48′29″W); FMNH 115502, 1 (289). Rio Jutaí, between mouths of Rio Zinho and Rio Solimões, between towns of Porto Antunes and Copatana (2°51′16″S, 66°57′23″W); FMNH 115510, 1 (257). Rio Juruá tributaries Lago Pauapixuna and Meneroa, between towns of Pauapixuna and Tamanicoa (2°46′31″S, 65°50′00″W); FMNH 115509, 1 (283). Rio Juruá, between Lago Pauapixuna and Rio Solimões, between towns of Pauapixuna and Vitória (2°41′08″S, 65°48′29″W); FMNH 115502, 1 (289). Rio Solimões, downstream from mouth of Rio Purus, upstream from Manacapuru (3°27′22″S, 60°45′21″W); FMNH 115520, 1 (193). Rio Solimões, Lago Manacapuru; MCZ 9340, 2 (200–355), MCZ 24934, 1 (350). Rio Jutaí, between Rio Zinho and Rio Solimões, between towns of Porto Antunes and Copatana (2°51′16″S, 66°57′23″W); FMNH 115510, 1 (257). Rio Jutaí, between Porto Antunes and Foz do Jutaí (2°53′00″S, 66°58′22″W); USNM 375482, 2 (351–353). Rio Solimões (3°36′19″S, 61°18′39″W); USNM 375398, 2 (210–216). Rio Solimões (3°35′43″S, 61°07′16″W); USNM 375399, 1 (261). Pará: Rio Tocantins, Tucuruí; INPA 1501, 1 (220). Rio Amazonas between tributaries Paraná de Jurutí and Trombetas, between towns of Jurutí and Óbidos (2°00′39″S, 55°55′17″W); FMNH 115511, 1 (259). Rio Trombetas between tributaries Furo Maria Tereza and Amazonas, between towns of Fazenda Santana and Óbidos (1°52′53″S, 55°38′13″W); FMNH 115512, 1 (183). Rio Trombetas, between tributaries Lago Bacabal and Lago Samauma (1°31′3″S, 56°9′57″W); FMNH 115513, 2 (not measured). Rio Trombetas, between Rio Cumina and Lago Axipica, between towns of Bom Jesus and Santa Cecilia (1°32′40″S; 56°01′07″W); FMNH 115514, 1 (245). Rio Amazonas, 58.5 km below Jurutí, 21.1 km above Óbidos (1°55′48″S, 55°40′41″W); USNM 373040, 4 (329–368). Roraima: Rio Negro, at confluence with Rio Branco or along lower 30 mi (48 km) below Rio Branco (approximately 1°24′S, 61°27′W); MCZ 46887, 3 (1 CS, 190–215).

PERU. Loreto: Rio Amazonas, beach along east bank of east channel of river, opposite Iquitos; INHS 55418, 3 (224–283).

Sternarchorhynchus hagedornaesp. nov. (Figs42–44; Table7)

Sternarchorhynchus hagedornae sp. nov., holotype, sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 249 mm total length, MUSM 30534; Peru, Madre de Dios, Río Manu.

Sternarchorhynchus hagedornae sp. nov., holotype, sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 249 mm total length, MUSM 30534; Peru, Madre de Dios, Río Manu.

Sternarchorhynchus hagedornae sp. nov., female, approximately 192 mm total length, ANSP 180637; Peru, Madre de Dios, Quebrada Pachija, Río Manu.

Sternarchorhynchus hagedornae sp. nov., female, approximately 192 mm total length, ANSP 180637; Peru, Madre de Dios, Quebrada Pachija, Río Manu.

Sternarchorhynchus sp., Ortega, 1996: 469 [Peru, Manu Biosphere Reserve, Rio Manu and Quebrada de Pachija, tributary to Rio Manu]. –Hagedorn & Keller, 1996: 493, fig. 2f [Peru, Manu Biosphere Reserve, Quebrada de Pachija, tributary to Rio Manu; habitat, EOD, secondary sexual dimorphism].

Diagnosis:

Sternarchorhynchus hagedornae is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the distinctly longitudinally elongate combined opening for the anus and the urogenital opening, the sparse scales covered by skin along the mid-dorsal portion of the body anterior to the origin of the electroreceptive filament, the consistently dark overall coloration of the head and body, the presence of a more lightly coloured, narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament and sometimes posterior of that point, the absence of a very dark band of pigmentation at the base of the enlarged patch of dentition on the dentary in mature males, the possession of five to six teeth in the outer tooth row of the dentary, 27–35 anterior unbranched anal-fin rays, 173–193 total anal-fin rays, nine to 13 scales above the lateral line at the midbody, the greatest body depth (11.4–13.3% of LEA), the length of the anal-fin base (86.6–89.6% of LEA), the prepectoral-fin distance (17.6–20.8% of LEA), the caudal length (5.5–8.3% of LEA), the head depth at the nape (53.1–59.2% of HL), the head width (24.1–28.3% of HL), the distance from the posterior naris to the snout (8.7–9.8% of HL), the mouth length (6.4–8.6% of HL), the interocular width (6.0–8.5% of HL), the distance from the posterior naris to the eye (44.6–48.9% of HL), the postocular distance (43.6–46.2% of HL), the pectoral-fin length (39.1–46.9% of HL), and the tail depth (21.3–30.4% of caudal length).

Description:

Morphometric data for holotype and paratypes in Table 7.

Lateral line extending to base of caudal fin, but absent on fin. Snout elongate, compressed and straight. Posterior naris closer to tip of snout than to anterior margin of orbit. Branchial opening restricted and situated slightly anterior to vertical through pectoral-fin origin. Location of anus and urogenital papilla apparently ontogenetically and perhaps sexually variable, but definite pattern uncertain because of limited sample size. Anus and urogenital papilla in smaller individual (162 mm TL) located at vertical running two orbital diameters posterior of eye. Midsized specimen (206 mm SL) with structures positioned slightly anterior of vertical through eye. Male with definite patch of enlarged dentition on expanded anterior portion of dentary (249 mm TL) with anus and urogenital papilla more anteriorly positioned, and located along vertical approximately one-quarter of distance between anterior limit of orbit and tip of snout. Combined opening for anus and urogenital papilla longitudinally elongate.

Premaxilla of small size, somewhat rounded, with five to eight teeth (N= 4). Dentary with two tooth rows. Outer tooth row with five to six curved conical teeth and inner tooth row with three to four teeth (N= 3). Mature males with dentary expanded anteriorly and widened laterally into dorsally bulbous region rounded from dorsal view, with expansion bearing patch of enlarged dentition. Mouth terminal with rictus in juveniles located slightly posterior to vertical through anterior naris. Rictus in adults shifted somewhat anteriorly and located anterior to vertical through anterior naris.

Branchiostegal rays five; with first to third rays relatively narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16–17 (13 anterior; three to four transitional; N= 4).

Pectoral-fin rays ii + 12–14 [ii + 14] (N= 6). Anal-fin origin located slightly posterior of vertical through anterior margin of opercle. Anterior unbranched anal-fin rays 27–35 [27] (N= 6). Total anal-fin rays 173–193 [173] (N= 6). Scales above lateral line at midbody nine to 13 [13] (N= 6). Scales along mid-dorsal region of body somewhat sparse and not readily apparent on body surface. Origin of midsaggital electroreceptive filament located approximately at 70% of TL. Filament in juveniles extending posteriorly to vertical one scale posterior of vertical through posterior terminus of base of anal fin; filament in adults reaching to vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small and moderate caudal fin. Caudal-fin rays 12–16 [18] (N= 6).

Coloration in alcohol:

Overall ground coloration dark brown. Series of whitish, papillae-like structures broadly distributed over lateral and dorsal portions of posterior two-thirds of snout, lateral surface of head, and dorsalmost portion of body. Lightly coloured spots limited to irregular single series to each side of electroreceptive filament on posterior portion of body. Snout with variably distinct, narrow band of darker pigmentation extending anteriorly from region somewhat anterior of orbit; and reaching anterior portion of snout in some individuals and apparent even in overall darkly pigmented specimens. Band of dark pigmentation forms lateral border of narrow, more lightly coloured mid-dorsal band present on head. Anterior portion of lightly coloured mid-dorsal stripe on head less apparent in male with well-developed patches of enlarged dentition on lower jaw. Body pigmentation very slightly darker dorsally, but with mid-dorsal more lightly coloured stripe continuous with corresponding stripe on head and extending posteriorly onto basal portions of electroreceptive filament.

Pectoral fin dark in all specimens, with dark chromatophores overlying fin rays. All examined specimens with anal fin dusky and posterior margin of fin outlined by series of dark chromatophores. Caudal fin dark overall, but with distal margin hyaline.

Distribution:

All examined specimens of S. hagedornae originated in the Manu Biosphere Reserve, in the upper Río Madeira basin in south-eastern Peru (Fig. 42).

Ecology:

Hagedorn & Keller (1996: 488, 490 493) remarked that the type locality for S. hagedornae, Quebrada Pachija, is an open-gallery, high flow stream with a flow of 0.748 m/s, a mean stream depth of 0.397 ± 0.059 m, and a mean stream width of 19.1 ± 1.0 m. According to these authors this species, (their Sternarchorhynchus sp.) ‘lived in the fastest flowing water sampled; most adult specimens were captured from a single rocky rapids’. Their collecting efforts also yielded numerous fry and small juveniles from within floating debris along the fast-flowing edges of the Río Manu that they considered to be the same species. We have not examined these samples.

Electrical organ discharge:

Sternarchorhynchus hagedornae was studied in the field by Hagedorn & Keller (1996: 493, figs 2f, 4) who reported that it is a high frequency, wave-type fish with peak-power EOD frequencies ranging from 977–2832 Hz and a mean of 1608.5 ± 160.6 Hz SE in the 14 specimens examined. They remarked on the variety of EOD types observed in the species (Hagedorn & Keller, 1996: fig. 4), but pointed out that they lacked sufficient specimens to determine whether this variation reflected sexual dimorphism in this feature; however, they did comment that the heterogeneity might reflect changes with ‘age, sex and/or activity pattern’.

Secondary sexual dimorphism:

The obvious sexually dimorphic feature in S. hagedornae is the anteriorly lengthened and transversely widened anterior portion of the dentary in males and the associated patch of enlarged, posteriorly recurved teeth on this region. The male (249 mm TL) had the anus and urogenital papilla more anteriorly positioned than did the slightly shorter, possibly female, specimen (206 mm TL) and the juveniles. In the male, the anus and the urogenital papilla are located along the vertical running approximately one-quarter of the distance between the anterior limit of the orbit and the tip of the snout. This position is distinctly anterior of the location of the anus and urogenital papilla in smaller specimens, in which these structures are positioned from slightly anterior to variably posterior of the vertical through the eye.

Etymology:

The species name, hagedornae, is in honour of Mary Hagedorn of the Smithsonian Institution who collected the type series and has made many contributions to our understanding of the diversity of gymnotiforms.

Material examined

Holotype:

PERU. Madre de Dios: Manu Biosphere Reserve, Quebrada Pachija, tributary emptying into Río Manu 3 km upstream of Pakitza (Pakitza at 11°55′48″S, 71°15′18″W), MUSM 30534, 1 (249, male); collected by M. Hagedorn et al.

Paratypes:

PERU. Madre de Dios: Manu Biosphere Reserve, Quebrada Pachija, tributary emptying into Río Manu 3 km upstream of Pakitza (Pakitza at 11°55′48″S, 71°15′18″W), collected by M. Hagedorn et al., with holotype, USNM 391574, 2 (169–206); MUSM 30535, 1 (202); MBUCV 33742, 2 (162–194). MUSM 23755, (174); Manu Biosphere Reserve, Río Los Amigos.

Nontype specimens:

PERU. Cuzco: Río Inambari and mouth of Quebrada Hondonado, Madre de Dios drainage, upstream of Puerto Mazuko (13°06′23″S, 70°24′44″W), ANSP 180637, 2 (200–208).

Sternarchorhynchus higuchiisp. nov. (Figs42, 45, 46; Table8)

Sternarchorhynchus higuchii sp. nov., holotype, sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 246 mm total length, INPA 28358; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Sternarchorhynchus higuchii sp. nov., holotype, sexually dimorphic male demonstrating modifications of dentary and dentary dentition, 246 mm total length, INPA 28358; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Sternarchorhynchus higuchii sp. nov., female, 180 mm total length, INPA 20855; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Sternarchorhynchus higuchii sp. nov., female, 180 mm total length, INPA 20855; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Morphometric data for holotype (H), and paratypes of Sternarchorhynchus higuchii, Sternarchorhynchus inpai, and Sternarchorhynchus jaimei

 S. gnomus S. goeldii S. hagedornae 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 133.5 76.5–152 (N= 9) – 306.6 193–333.4 (N= 9) – 249 162–206 (N= 5) – 
Length to end of anal fin (mm) 116.5 69.1–137 (N= 8) – 280 165–293 (N= 7) – 233 150–193 (N= 5) – 
Head length (mm) 23.7 15.6–30.7 (N= 9) – 48.8 34.9–50.0 (N= 5) – 40.9 28.7–35.0 (N= 5) – 
Caudal length (mm) 17.3 9.9–14.9 (N= 5) – 27.4 26.7–41.9 (N= 3) – 17.1 10.0–14.4 (N= 5) – 
Per cent of length to end of anal fin 
Anal-fin base 81.8 82.4–86.9 (N= 7) 84.5 87.1 80.4–87.3 (N= 7) 85.3 87.5 86.6–89.6 (N= 5) 88.2 
Distance snout to anus 13.4 11.8–17.4 (N= 8) 15.0 15.6 9.5–15.4 (N= 5) 12.9 6.4 8.4–12.0 (N= 5) 10.0 
Preanal-fin distance 14.7 14.1–17.7 (N= 7) 16.6 14.2 12.8–18.9 (N= 5) 16.0 11.6 10.4–13.9 (N= 5) 11.9 
Prepectoral-fin distance 20.0 20.0–23.2 (N= 7) 22.3 17.3 17.1–20.9 (N= 5) 19.0 17.6 17.9–20.8 (N= 5) 19.1 
Greatest body depth 9.6 9.3–11.4 (N= 7) 10.4 7.5 8.2–10.8 (N= 7) 9.3 11.4 12.9–13.3 (N= 5) 13.1 
Head length 20.3 20.1–24.3 (N= 9) 22.2 17.4 17.0–21.1 (N= 4) 19.2 17.5 17.7–20.0 (N= 5) 18.8 
Caudal length 14.8 10.1–17.4 (N= 5) 14.6 9.8 14.3–16.2 (N= 3) 14.9 7.3 5.5–8.3 (N= 5) 7.1 
Per cent of head length 
Anus to anal-fin insertion 15.0 7.8–19.9 (N= 8) 12.3 23.1 10.1–20.1 (N= 5) 14.5 23.3 8.0–10.1 (N= 5) 8.9 
Pectoral-fin length 38.8 34.8–41.0 (N= 9) 36.9 42.7 33.7–37.8 (N= 5) 36.4 43.8 39.1–46.9 (N= 5) 42.9 
Head depth at eye – 37.4–39.0 (N= 3) 38.3 26.5 20.2–25.3 (N= 5) 23.7 39.8 29.6–37.2 (N= 5) 34.0 
Head depth at nape 41.7 39.7–44.9 (N= 9) 41.8 45.4 35.0–44.8 (N= 5) 39.4 55.4 53.1–59.2 (N= 5) 55.8 
Head width 20.3 20.0–21.5 (N= 5) 20.6 21.0 17.5–20.7 (N= 5) 19.0 26.8 24.1–28.3 (N= 5) 25.8 
Snout length 62.8 57.6–65.4 (N= 9) 61.1 61.3 61.6–64.7 (N= 5) 62.7 59.6 57.4–58.9 (N= 5) 58.1 
Posterior naris to snout 11.4 10.9–11.5 (N= 5) 11.3 4.9 5.6–6.8 (N= 5) 6.2 9.0 8.7–9.8 (N= 5) 9.0 
Posterior naris to eye 52.7 47.6–55.7 (N= 9) 50.3 54.2 53.3–57.5 (N= 5) 55.3 48.9 44.6–47.9 (N= 5) 46.5 
Mouth length 6.3 5.5–9.2 (N= 9) 7.3 9.2 6.5–7.9 (N= 5) 7.2 8.6 6.4–7.9 (N= 5) 7.0 
Internarial distance 5.9 2.7–7.0 (N= 9) 4.4 3.3 1.8–3.6 (N= 5) 2.6 2.7 2.3–3.5 (N= 5) 3.1 
Eye diameter 5.4 4.1–7.6 (N= 9) 5.4 4.5 3.8–5.1 (N= 5) 4.2 4.3 4.1–5.9 (N= 5) 4.8 
Interocular width 8.8 4.2–9.6 (N= 9) 6.9 5.6 6.1–7.3 (N= 5) 5.0 7.5 6.0–8.5 (N= 5) 7.7 
Postocular distance 39.2 36.9–41.6 (N= 9) 38.7 39.5 34.3–37.1 (N= 5) 35.7 44.1 43.6–46.2 (N= 5) 45.3 
Branchial opening 11.8 9.5–12.3 (N= 9) 10.9 16.4 11.0–13.8 (N= 5) 12.6 14.8 11.9–14.9 (N= 5) 13.6 
Per cent of caudal length 
Tail depth 10.9 8.3–13.8 (N= 5) 9.9 22.6 13.2–16.3 (N= 3) 14.8 22.0 21.3–30.4 (N= 5) 25.0 
Caudal-fin length 25.4 19.6–31.8 (N= 5) 24.9 46.3 22.3 (N= 1) – 40.3 25.8–54.3 (N= 5) 38.3 
 S. gnomus S. goeldii S. hagedornae 
Paratypes Mean Paratypes Mean Paratypes Mean 
Total length (mm) 133.5 76.5–152 (N= 9) – 306.6 193–333.4 (N= 9) – 249 162–206 (N= 5) – 
Length to end of anal fin (mm) 116.5 69.1–137 (N= 8) – 280 165–293 (N= 7) – 233 150–193 (N= 5) – 
Head length (mm) 23.7 15.6–30.7 (N= 9) – 48.8 34.9–50.0 (N= 5) – 40.9 28.7–35.0 (N= 5) – 
Caudal length (mm) 17.3 9.9–14.9 (N= 5) – 27.4 26.7–41.9 (N= 3) – 17.1 10.0–14.4 (N= 5) – 
Per cent of length to end of anal fin 
Anal-fin base 81.8 82.4–86.9 (N= 7) 84.5 87.1 80.4–87.3 (N= 7) 85.3 87.5 86.6–89.6 (N= 5) 88.2 
Distance snout to anus 13.4 11.8–17.4 (N= 8) 15.0 15.6 9.5–15.4 (N= 5) 12.9 6.4 8.4–12.0 (N= 5) 10.0 
Preanal-fin distance 14.7 14.1–17.7 (N= 7) 16.6 14.2 12.8–18.9 (N= 5) 16.0 11.6 10.4–13.9 (N= 5) 11.9 
Prepectoral-fin distance 20.0 20.0–23.2 (N= 7) 22.3 17.3 17.1–20.9 (N= 5) 19.0 17.6 17.9–20.8 (N= 5) 19.1 
Greatest body depth 9.6 9.3–11.4 (N= 7) 10.4 7.5 8.2–10.8 (N= 7) 9.3 11.4 12.9–13.3 (N= 5) 13.1 
Head length 20.3 20.1–24.3 (N= 9) 22.2 17.4 17.0–21.1 (N= 4) 19.2 17.5 17.7–20.0 (N= 5) 18.8 
Caudal length 14.8 10.1–17.4 (N= 5) 14.6 9.8 14.3–16.2 (N= 3) 14.9 7.3 5.5–8.3 (N= 5) 7.1 
Per cent of head length 
Anus to anal-fin insertion 15.0 7.8–19.9 (N= 8) 12.3 23.1 10.1–20.1 (N= 5) 14.5 23.3 8.0–10.1 (N= 5) 8.9 
Pectoral-fin length 38.8 34.8–41.0 (N= 9) 36.9 42.7 33.7–37.8 (N= 5) 36.4 43.8 39.1–46.9 (N= 5) 42.9 
Head depth at eye – 37.4–39.0 (N= 3) 38.3 26.5 20.2–25.3 (N= 5) 23.7 39.8 29.6–37.2 (N= 5) 34.0 
Head depth at nape 41.7 39.7–44.9 (N= 9) 41.8 45.4 35.0–44.8 (N= 5) 39.4 55.4 53.1–59.2 (N= 5) 55.8 
Head width 20.3 20.0–21.5 (N= 5) 20.6 21.0 17.5–20.7 (N= 5) 19.0 26.8 24.1–28.3 (N= 5) 25.8 
Snout length 62.8 57.6–65.4 (N= 9) 61.1 61.3 61.6–64.7 (N= 5) 62.7 59.6 57.4–58.9 (N= 5) 58.1 
Posterior naris to snout 11.4 10.9–11.5 (N= 5) 11.3 4.9 5.6–6.8 (N= 5) 6.2 9.0 8.7–9.8 (N= 5) 9.0 
Posterior naris to eye 52.7 47.6–55.7 (N= 9) 50.3 54.2 53.3–57.5 (N= 5) 55.3 48.9 44.6–47.9 (N= 5) 46.5 
Mouth length 6.3 5.5–9.2 (N= 9) 7.3 9.2 6.5–7.9 (N= 5) 7.2 8.6 6.4–7.9 (N= 5) 7.0 
Internarial distance 5.9 2.7–7.0 (N= 9) 4.4 3.3 1.8–3.6 (N= 5) 2.6 2.7 2.3–3.5 (N= 5) 3.1 
Eye diameter 5.4 4.1–7.6 (N= 9) 5.4 4.5 3.8–5.1 (N= 5) 4.2 4.3 4.1–5.9 (N= 5) 4.8 
Interocular width 8.8 4.2–9.6 (N= 9) 6.9 5.6 6.1–7.3 (N= 5) 5.0 7.5 6.0–8.5 (N= 5) 7.7 
Postocular distance 39.2 36.9–41.6 (N= 9) 38.7 39.5 34.3–37.1 (N= 5) 35.7 44.1 43.6–46.2 (N= 5) 45.3 
Branchial opening 11.8 9.5–12.3 (N= 9) 10.9 16.4 11.0–13.8 (N= 5) 12.6 14.8 11.9–14.9 (N= 5) 13.6 
Per cent of caudal length 
Tail depth 10.9 8.3–13.8 (N= 5) 9.9 22.6 13.2–16.3 (N= 3) 14.8 22.0 21.3–30.4 (N= 5) 25.0 
Caudal-fin length 25.4 19.6–31.8 (N= 5) 24.9 46.3 22.3 (N= 1) – 40.3 25.8–54.3 (N= 5) 38.3 

Sexually dimorphic features for S. inpai are presented as two entries. First entry is data for all specimens other than sexually dimorphic mature males with information for sexually dimorphic male in second entry based on one specimen of 222 mm total length. Number of specimens indicated in parentheses.

Diagnosis:

Sternarchorhynchus higuchii is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of scales although sometimes sparse and covered by skin along the mid-dorsal region of the body to the origin of the electroreceptive filament, the presence of an obscure, more lightly coloured, narrow band of mid-dorsal pigmentation on the head and anterior mid-dorsal region of the body, the slightly dusky anal fin without a distinct dark band along its distal margin, the lateral line extending posteriorly to a point approximately four to seven scales short of the base of the anal fin, the possession of two rows of dentary teeth with the outer row having nine to ten teeth, 23–31 anterior unbranched anal-fin rays, 162–178 total anal-fin rays, eight to 12 scales above the lateral line at the midbody, ten to 12 caudal-fin rays, the caudal length (14.7–18.9% of LEA), and the tail depth (5.9–7.5% of caudal length).

Description:

Morphometric data for holotype and paratypes in Table 8.

Lateral line falling short of base of caudal fin by distance equal to four to seven scales and absent on remainder of tail and on caudal fin. Snout elongate, compressed and slightly curved ventrally distally. Posterior naris closer to tip of snout than to anterior margin of orbit. Branchial opening restricted and situated slightly anterior to vertical through pectoral-fin origin. Location of anus and urogenital papilla apparently sexually dimorphic. Anus and urogenital papilla in smaller individuals and apparently mature females located within area between vertical located two orbital diameters posterior of orbit and vertical running through eye. Males with definite patch of enlarged dentition on dentary with anus and urogenital papilla more anteriorly positioned, located along verticals two to four orbital diameters anterior of anterior margin of eye (most anterior position approximately one-quarter of distance between anterior margin of orbit and tip of snout). Combined opening for anus and urogenital papilla distinctly elongate longitudinally.

Premaxilla of small size, somewhat rounded, with seven to eight teeth (N= 5). Dentary with two tooth rows with six curved conical teeth in outer row and three to four teeth on inner row (N= 5) in juveniles and females. Mature males demonstrating sexual dimorphism in lower jaw with anterior portion of dentary extending further anteriorly than in females and juveniles and with dentary distinctly expanded laterally into dorsally bulbous structure rounded from dorsal view. Expanded portion of dentary bearing series of enlarged, slightly posteriorly recurved teeth. Mouth terminal in juveniles and females and somewhat dorsally directed in males with anteriorly and transversely expanded dentaries. Rictus located slightly anterior to vertical through anterior naris in all specimens.

Branchiostegal rays five; with first to third rays relatively narrow and elongate and remaining rays large and broad. Precaudal vertebrae 15–16 (12–13 anterior; two to four transitional; N= 16).

Pectoral-fin rays ii + 11–13 [ii + 13] (N= 14). Anal-fin origin located slightly posterior of vertical through anterior margin of opercle. Anterior unbranched anal-fin rays 23–31 [28] (N= 14). Total anal-fin rays 162–178 [171] (N= 15). Scales above lateral line at midbody eight to 12 [10] (N= 16). Scales along mid-dorsal region of body present, but somewhat sparse and not readily apparent on body surface. Origin of midsaggital electroreceptive filament located approximately at 59% of TL. Filament in specimens of all sizes extending six to eight scales posterior of vertical through posterior terminus of base of anal fin. Tail compressed and long, ending in very small, somewhat elongate caudal fin. Caudal-fin rays ten to 12 [11] (N= 11).

Coloration in alcohol:

Overall ground coloration dark brown. Snout with variably distinct, narrow band of slightly darker pigmentation extending from region somewhat in advance of eye to anterior portion of snout. Second stripe of dark pigmentation running ventrally on snout to lower jaw. Two dark stripes delimit intermediate, more lightly pigmented region running along lateral surface of snout. Dorsal band of dark pigmentation forms lateral border of narrow, lightly coloured, mid-dorsal band on head that terminates posteriorly in region dorsal to eye. Body pigmentation very slightly darker dorsally and without distinct mid-dorsal lightly coloured stripe present in that region in many congeners. Mid-dorsal region of body irregularly less intensely pigmented in some individuals, but with these lighter areas not confluent and not forming distinct, narrow stripe.

Pectoral fin coloration dark in all specimens, with dark chromatophores overlying fin rays. All examined specimens with anal fin dusky and posterior margin of fin-rays outlined by series of dark chromatophores. Caudal fin dark overall, but with distal margin hyaline.

Distribution:

All examined specimens of S. higuchii were collected at the Cachoeira do Miriti and Cachoeira do Morena in the Rio Uatumã in the eastern portions of Amazonas, Brazil (Fig. 42).

Secondary sexual dimorphism:

Mature males of S. higuchii have an anteriorly extended and laterally enlarged anterior portion of the dentary with the associated patch of enlarged teeth. Males bearing such elaborations of the lower jaw and dentary teeth have the anus and urogenital papilla more anteriorly positioned than in examined conspecifics of comparable body sizes. Both the anus and urogenital papillae in these males are located distinctly anterior of the vertical through the eye, rather than at, or posterior of, that position as in males lacking modified dentaries and associated dentition. Females and juveniles similarly have these structures in a posterior position. Examined females of S. higuchii with mature ovaries are smaller than available males demonstrating sexual dimorphism in the lower jaw and dentary dentition.

Etymology:

The species name, higuchii, is in honour of Horácio Higuchi of the Museu Paraense Emilio Goeldi, who provided invaluable assistance to the first author during his early studies of gymnotiforms.

Remarks:

Sternarchorhynchus higuchii was collected at the same type locality as S. jaimei. The two species differ in the posterior extent of the lateral line (falling short of base of caudal fin by four to seven scales versus reaching the base of the caudal fin, respectively), total anal-fin rays (162–178 versus 180–195, respectively), the pigmentation of the mid-dorsal region of the body anterior to the origin of the electroreceptive filament (dark versus with narrow, lightly coloured stripe, respectively), and caudal depth (5.9–7.5 versus 10.1–19.2% of caudal length, respectively), and nearly completely in body depth (8.7–11.0 versus 10.8–12.6 of LEA, respectively).

Material examined

Holotype:

BRAZIL. Amazonas: Rio Uatumã, Cachoeira do Miriti, Presidente Figueredo (approximately 2°01′S, 59°28′W), INPA 28358 (246), collected by E. Ferreira, R. Leite, and S. Kullander, 4.x.1987.

Paratypes:

BRAZIL. Amazonas: Rio Uatumã, Cachoeira do Miriti, Presidente Figueredo (approximately 2°01′S, 59°28′W), collected by E. Ferreira, R. Leite, and S. Kullander, with holotype, INPA 20855, 15 (88–243); USNM 391714, 1 (189).

Nontype specimens:

BRAZIL. Amazonas: Rio Uatumã, Cachoeira do Miriti (approximately 2°01′S, 59°28′W), INPA uncat., 2 (158–181). Rio Uatumã, Cachoeira do Morena, INPA 20854, 25 (125–233).

Sternarchorhynchus inpaisp. nov. (Figs42, 47, 48; Table8)

Sternarchorhynchus inpai sp. nov., holotype, female, 191 mm total length, INPA 22895; Brazil, Pará, Cachoeira Porteira, Rio Mapuera.

Sternarchorhynchus inpai sp. nov., holotype, female, 191 mm total length, INPA 22895; Brazil, Pará, Cachoeira Porteira, Rio Mapuera.

Sternarchorhynchus inpai sp. nov., paratype, male (tail broken), 222 mm total length, INPA 22899; Brazil, Pará, Cachoeira Porteira, Rio Mapuera.

Sternarchorhynchus inpai sp. nov., paratype, male (tail broken), 222 mm total length, INPA 22899; Brazil, Pará, Cachoeira Porteira, Rio Mapuera.

Sternarchorhynchus oxyrhynchusFerreira, 1995: 51 [Brazil, Pará, Rio Trombetas, Cachoeira Porteira].

Diagnosis:

Sternarchorhynchus inpai is distinguished from all congeners by having a very short first branchiostegal ray that is distinctly shorter than the second ray rather than being approximately subequal in length to that ossification. It is furthermore differentiated from all other species of Sternarchorhynchus by possessing the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through the anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the more lightly coloured mid-dorsal stripe on the head sometimes extending posteriorly to the vertical through the insertion of the pectoral fin, the dusky anal fin, the very short first branchiostegal ray, the possession of four to seven premaxillary teeth, five to six teeth in the outer row of the dentary, 15 precaudal vertebrae, the distance from the snout to the anus in mature males (13.4% of LEA), the preanal distance (15.2–16.3% of LEA; 18.9 in mature male), the caudal length (12.6–15.6% of LEA), the pectoral-fin length (27.9–42.3% of HL; 27.9 in mature male), the head depth at the eye in mature males (26.3 of HL), the head depth at the nape (40.5–48.6% of HL), the distance from the posterior naris to the eye in mature males (59.7 of HL), the postocular distance (33.1–41.1% of HL), the height of the branchial opening (9.4–13.4% of SL; 10.9% in mature males), the tail depth (8.0–11.1% of caudal length), and the caudal-fin length (20.1–23.9% of caudal length).

Description:

Morphometric data for examined specimens in Table 8.

Lateral line extending posteriorly to point four scales anterior of base of caudal fin but absent on fin. Snout elongate, compressed and slightly curved ventrally along anterior portion. Mouth terminal to very slightly anterodorsally orientated and relatively small with distinct fleshy pad at anterior of lower jaw. Rictus located anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head, with position apparently ontogenetically variable. Anus and urogenital papilla located along vertical about one orbital diameter posterior of eye in juveniles and along verticals through to one orbital diameter anterior of eye in mature males and females. Combined opening for anus and urogenital papilla longitudinally ovoid in all specimens.

Premaxilla with four to seven teeth (N= 6) apparent in whole specimens. Dentary with two tooth rows; outer row with five to six teeth and inner row with three teeth (N= 4).

Branchiostegal rays five; with first ray distinctly shorter than second and first three rays narrow. Fourth and fifth rays large and broad with fifth ray branched distally on both sides in cleared and stained specimen. Precaudal vertebrae 15 (13 anterior; two transitional; N= 6).

Pectoral-fin rays ii + 12–13 (N= 7). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 17–24 [17] (N= 3). Total anal-fin rays 156–167 [161] (N= 7). Scales above lateral line at midbody eight to 11 (N= 9). Scales present along mid-dorsal line to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately at 60% of TL. Filament extending posteriorly three to seven scales beyond vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate, pointed caudal fin. Caudal-fin rays 13–16 [14] (N= 6).

Coloration in alcohol:

Overall coloration brown. Head dark brown overall other than for more lightly coloured stripe along lateral surface of snout and very narrow, lightly coloured mid-dorsal stripe on snout that terminates posteriorly approximately at vertical through eye. Body dark overall, somewhat more so dorsally. Pectoral fin dusky with rays somewhat darker than membranes. Anal fin dusky with rays darker than membranes. Caudal fin dark with distal regions somewhat lighter.

Distribution:

Sternarchorhynchus inpai is only known from the type locality at Cachoeira Porteira on the Rio Mapuera, Pará, Brazil (Fig. 42).

Secondary sexual dimorphism:

The single mature examined male of S. inpai as evidenced by the well-developed testes has a slightly longer snout than do females of comparable sizes. This species matures at relatively small body sizes with a 154 mm TL female bearing well-developed eggs and the single mature male being at least 222 mm (a portion of the caudal is lacking in this specimen).

Etymology:

The species name, inpai, is in reference to the Instituto Nacional de Pesquisas da Amazônia (INPA), in Manaus, Brazil, that has been the centre for the study of the biodiversity of the Brazilian Amazon for over five decades.

Remarks:

Specimens reported as S. oxyrhynchus by Ferreira (1995: 51) from Cachoeira Porteira have proved to be S. inpai. Sternarchorhynchus inpai occurs in the same river basin as S. mareikeae and the two species have similar coloration patterns. They differ in the numbers of teeth on the premaxilla (four to seven versus 12–13, respectively), and in the outer row of the dentary (five to six versus 12–13, respectively), and the number of precaudal vertebrae (15 versus 16, respectively). Mature males of the two species can be furthermore distinguished on various sexually dimorphic morphometric features detailed in tables 15 and 18 of Ferreira (1995), most notably the distance from the snout to the anus (13.4% in S. inpai versus 8.8–10.8% in S. mareikeae), the preanal distance (18.9 versus 12.7–15.2%, respectively), and the height of the branchial opening (10.9 versus 12.3–15.4%, respectively).

Material examined

Holotype:

BRAZIL. Pará: Rio Mapuera, Cachoeira Porteira, last fall before Rio Trombetas (approximately 1°05′S 57°02′W); collected by E. Ferreira and M. Jégu, 19.iv.1985; INPA 22895, 191.0 mm TL, female.

Paratypes:

BRAZIL. Pará: Rio Mapuera, Cachoeira Porteira, last fall before Rio Trombetas (approximately 1°05′S 57°02′W); collected with holotype by E. Ferreira and M. Jégu, 19.iv.1985; INPA 22899, 10 (1 CS; 54–222); USNM 391715, 1 (195).

Sternarchorhynchus jaimeisp. nov. (Figs49, 50; Table8)

Sternarchorhynchus jaimei sp. nov., holotype, female, 200 mm total length, INPA 28359; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Sternarchorhynchus jaimei sp. nov., holotype, female, 200 mm total length, INPA 28359; Brazil, Amazonas, Cachoeira do Miriti, Rio Uatumã.

Map of central and northern South America showing geographical distribution of Sternarchorhynchus jaimei(1 = holotype locality), Sternarchorhynchus kokraimoro sp. nov.(2 = holotype locality), Sternarchorhynchus mareikeae sp. nov.(3 = holotype locality), Sternarchorhynchus marreroi sp. nov.(4 = holotype locality), and Sternarchorhynchus mendesi sp. nov.(5 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Map of central and northern South America showing geographical distribution of Sternarchorhynchus jaimei(1 = holotype locality), Sternarchorhynchus kokraimoro sp. nov.(2 = holotype locality), Sternarchorhynchus mareikeae sp. nov.(3 = holotype locality), Sternarchorhynchus marreroi sp. nov.(4 = holotype locality), and Sternarchorhynchus mendesi sp. nov.(5 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Diagnosis:

Sternarchorhynchus jaimei is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through the anterior nares, the absence of scales along the mid-dorsal region of the body as far posteriorly as the vertical through the posterior limit of the pectoral fin, the presence of a narrow, more lightly coloured band of mid-dorsal pigmentation on the head and mid-dorsal region of the body extending posteriorly to the origin of the electroreceptive filament and sometimes beyond that point, the lateral line that extends to the base of the caudal fin, the presence of four to seven premaxillary teeth, 23–26 anterior unbranched anal-fin rays, and 180–195 total anal-fin rays, the distance from the anus to the anal-fin insertion (4.8–12.3% of HL), the distance from the posterior naris to the snout (8.1–12.9% of HL), the head width (22.7–28.9% of HL), the head depth at the nape (41.8–43.2% of HL), the interocular distance (6.6–10.7% of HL), the postocular distance (41.5–49.1% of HL), the height of the branchial opening (11.4–17.9% of HL), the postocular distance (41.5–58.7% of HL), the pectoral-fin length (35.7–58.4% of HL), the caudal depth (10.1–19.2% of caudal length), and the caudal-fin length (17.3–46.0% of caudal length).

Description:

Morphometric data for examined specimens in Table 8.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and slightly curved ventrally along anterior portion; curvature more pronounced in larger individuals. Mouth terminal. Dentary with distinct fleshy pad anteriorly. Rictus located anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head, with position apparently ontogenetically variable. Anus and urogenital papilla located along vertical about two orbital diameters posterior of eye in juveniles and along vertical through eye in larger specimens. Combined opening for anus and urogenital papilla ranging from circular to longitudinally ovoid.

Premaxilla with four to seven teeth (N= 5) apparent in whole specimens. Dentary with two tooth rows; outer row with eight to nine teeth and inner row with two to three teeth (N= 5).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16 (12–13 anterior; three to four transitional; N= 12).

Pectoral-fin rays ii + 11–14 [ii + 11] (N= 12). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 23–36 [25] (N= 12). Total anal-fin rays 180–195 [184] (N= 7). Scales above lateral line at midbody nine to 12 [12] (N= 12). Scales along mid-dorsal midline of body absent or sparse to midlength of body in smaller specimens, and absent in larger specimen as far posteriorly as vertical through posterior limit of pectoral fin, then sparse to midlength of body and followed by regular series of scales as far posteriorly as origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, at point approximately 65% of TL. Filament extending posteriorly four to six scales anterior of vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, somewhat rounded caudal fin in juveniles and in small, somewhat lanceolate fin in large individuals. Caudal-fin rays 11–18 [14] (N= 7).

Coloration in alcohol:

Overall coloration of head and body brown. Head with variably intense band of dark pigmentation extending along dorsolateral surface of snout nearly to tip of snout. Ventrolateral portion of snout dark. Two dark bands on snout border intermediate more lightly coloured region on anterior half of snout. Dorsal band of dark coloration forms lateral margin of narrow, lightly coloured stripe extending from tip to snout to varying degrees beyond vertical through eye and onto mid-dorsal postocular portion of head. Stripe on head variably obscurely continuous with narrow, lightly coloured mid-dorsal stripe on body. Mid-dorsal stripe on body extending posteriorly to origin of electroreceptive filament. Pectoral fin dusky with rays somewhat darker than membranes. Anal fin dusky with rays darker than membranes. Caudal fin dark with distal regions somewhat lighter.

Distribution:

Sternarchorhynchus jaimei is only known from the Rio Uatumã, Amazonas, Brazil (Fig. 50).

Secondary sexual dimorphism:

No sexual dimorphism was apparent in the examined samples of S. jaimei.

Etymology:

The species name, jaimei, is in honour of Jaime Ribeiro Carvalho Júnior, of the Centro do Jovem Aquarista, in recognition of his invaluable assistance to the first author during the early phases of his studies of gymnotiforms.

Remarks:

One of the examined specimens, a female of 115 mm TL, had clearly developing eggs and it seems likely that the species matures at relatively small body sizes. Sternarchorhynchus jaimei was collected at the same type locality as S. higuchii. The species differ in various details of pigmentation, meristics, and morphometrics detailed in Remarks under S. higuchii.

Material examined

Holotype:

BRAZIL. Amazonas: Rio Uatumã, Cachoeira do Miriti, Presidente Figueredo (approximately 2°01′S, 59°28′W), collected by E. Ferreira, R. Leite, and S. Kullander, 4.x.1987; INPA 28359 (200, maturing female).

Paratypes:

BRAZIL. Amazonas: Rio Uatumã, Cachoeira do Miriti, Presidente Figueredo (approximately 2°01′S, 59°28′W), collected with holotype by E. Ferreira, R. Leite, and S. Kullander, 4.x.1987; INPA 22902, 2 (72.2–75.6). Rio Uatumã, Cachoeira Morena, Presidente Figueredo, collected by E. Ferreira, R. Leite, and S. Kullander, 7.x.1987; INPA 26122, 10 (one cleared and stained, 88–123); USNM 391716, 1 (127).

Sternarchorhynchus kokraimorosp. nov. (Figs50–52; Table9)

Sternarchorhynchus kokraimoro sp. nov., holotype, male, showing deeper snout of males, 197 mm total length, INPA 28360; Brazil, Pará, Cachoeira de Kaituká, Rio Xingu.

Sternarchorhynchus kokraimoro sp. nov., holotype, male, showing deeper snout of males, 197 mm total length, INPA 28360; Brazil, Pará, Cachoeira de Kaituká, Rio Xingu.

Sternarchorhynchus kokraimoro sp. nov., paratype, female, 171 mm total length, INPA 3932; Brazil, Pará, Cachoeira de Kaituká, Rio Xingu.

Sternarchorhynchus kokraimoro sp. nov., paratype, female, 171 mm total length, INPA 3932; Brazil, Pará, Cachoeira de Kaituká, Rio Xingu.

Morphometric data for holotype (H), and paratypes of Sternarchorhynchus kokraimoro, Sternarchorhynchus mareikeae, and Sternarchorhynchus marreroi

 S. kokraimoro S. mareikeae S. marreroi 
Paratype Paratypes Mean Paratypes Mean 
Total length (mm) 197 171 192 131–221 (N= 9) – 182 231–278 (N= 2) – 
Length to end of anal fin (mm) 172 148 165 120–192 (N= 9) – 166 209–252 (N= 2) – 
Head length (mm) 39.4 34.2 31.2 25.8–35.1 (N= 9) – 27.1 34.3–41.3 (N= 2) – 
Caudal length (mm) 26.7 21.2 22.1 19.8–27.8 (N= 8) – 14.6 22.8–26.4 (N= 2) – 
Per cent of length to end of anal fin 
Anal-fin base 86.0 85.1 85.4 82.0–86.9 (N= 9) 85.1 85.5 88.4–92.3 (N= 2) 90.4 
Distance snout to anus 13.6 15.0 9.6 8.8–13.5 (N= 9)
8.8–10.8 11.4
9.7 9.0 5.8–7.7 (N= 2) 6.7 
Preanal-fin distance 16.9 17.1 13.9 12.2–17.8 (N= 9)
12.7–15.2 15.2
13.7 10.8 9.2–10.1 (N= 2) 9.7 
Prepectoral-fin distance 24.0 23.1 19.0 17.9–23.1 (N= 9)
37.5–40.9 20.6
39.1 17.6 17.0–17.0 (N= 2) 17.0 
Greatest body depth 10.1 12.3 10.0 10.9–13.6 (N= 9)
10.9–11.8 11.9
11.2 13.4 11.2–14.1 (N= 2) 12.6 
Head length 22.9 23.1 18.9 18.3–23.2 (N= 9)
18.3–20.2 20.6
19.11 16.6 16.4–16.4 (N= 2) 16.4 
Caudal length 15.5 14.3 13.4 13.4–17.4 (N= 8) 15.6 8.8 10.4–10.9 (N= 2) 10.6 
Per cent of head length 
Anus to anal-fin insertion 12.5 8.2 73.7 66.9–78.3 (N= 9) 73.5 10.1 16.2–17.6 (N= 2) 16.9 
Pectoral-fin length 31.0 31.1 38.5 35.0–40.9 (N= 9)
37.5–40.9 38.6 47.5 52.0–75.8 (N= 2) 63.9 
Head depth at eye 26.3 26.7 30.0 28.7–38.4 (N= 9)
29.9–38.4 32.5
33.5 32.9 34.1–35.4 (N= 2) 34.8 
Head depth at nape 43.6 46.3 49.5 45.1–53.4 (N= 9) 49.5 62.6 55.1–65.5 (N= 2) 60.3 
Head width 20.2 18.6 24.4 19.7–25.4 (N= 9) 22.5 29.9 25.9–28.7 (N= 2) 27.3 
Snout length 66.2 63.7 61.0 51.2–65.4 (N= 9) 60.9 62.4 62.3–63.6 (N= 2) 62.9 
Posterior naris to snout 8.3 7.4 – – – 7.8 6.4–7.9 (N= 2) 7.2 
Posterior naris to eye 55.0 55.0 50.5 48.2–53.7 (N= 9)
52.3–53.7 51.8
52.9 48.8 50.7–53.4 (N= 2) 52.1 
Mouth length 6.5 5.9 6.8 4.3–9.4 (N= 9) 5.7 7.0 5.3–6.4 (N= 2) 5.8 
Internarial distance 1.8 2.0 3.1 2.9–4.6 (N= 9)
3.2–3.5 3.5
3.4 4.3 3.0–3.5 (N= 2) 3.3 
Eye diameter 3.9 3.8 4.8 3.5–5.3 (N= 9) 4.5 4.5 3.3–4.4 (N= 2) 3.9 
Interocular width 5.1 4.6 9.0 6.6–8.1 (N= 9)
6.6–7.4 7.3
7.0 8.1 6.8–8.1 (N= 2) 7.4 
Postocular distance 38.8 37.6 37.7 36.0–41.3 (N= 9) 38.6 49.0 45.3–45.4 (N= 2) 45.4 
Branchial opening 12.0 12.1 10.9 11.9–15.4 (N= 9)
12.3–15.4 12.9
13.2 15.3 15.6–19.6 (N= 2) 17.6 
Per cent of caudal length 
Tail depth 10.1 12.3 9.7 7.1–10.2 (N= 9) 8.8 20.2 15.7–19.2 (N= 2) 17.5 
Caudal-fin length 19.8 22.5 – 18.5–24.7 (N= 7) 20.8 39.0 32.4 (N= 1) – 
 S. kokraimoro S. mareikeae S. marreroi 
Paratype Paratypes Mean Paratypes Mean 
Total length (mm) 197 171 192 131–221 (N= 9) – 182 231–278 (N= 2) – 
Length to end of anal fin (mm) 172 148 165 120–192 (N= 9) – 166 209–252 (N= 2) – 
Head length (mm) 39.4 34.2 31.2 25.8–35.1 (N= 9) – 27.1 34.3–41.3 (N= 2) – 
Caudal length (mm) 26.7