Abstract

In this article we review the phylogenetic distribution of major chaetal types within the Polychaeta, discuss what has been demonstrated about chaetal function in modern worms, and examine what is known about the evolution of chaete through the fossil record. We conclude with specific cautions about how chaetae are treated in phylogenetic analyses and make suggestions about how they could be used to provide a stronger phylogenetic signal.

Introduction

Despite the facts that the Polychaeta take their name from chaetae and that these ubiquitous structures are used as major tools in identifying and classifying worms, there is a plethora of questions about them. The fundamental conundrum about the evolution of chaetae is whether a particular morphological innovation (e.g., “hookedness” or “jointedness”) arose once and thus truly marks the descendents of a given lineage or whether it has occurred multiple times as a result of convergence. What insight does chaetal function give us into the lives of extant and fossil species, and how might understanding what these structures do for a worm modify our way of seeing them and using them in phylogenetic analyses? And, in association with the themes of this symposium, what genetic and developmental processes give rise to patterns of chaetae found within the Polychaeta, and how might knowing these processes help our understanding of polychaete evolution? To begin to answer these questions we examine the distribution and function of capillary, hooked, and jointed chaetae within the context of the current morphology-based phylogeny of the Polychaeta (Rouse and Fauchald 1997). We compare those patterns with what is found in the fossil record and conclude with specific cautions about how chaetae are treated in phylogenetic analyses and make suggestions as to how they could be used to provide a stronger phylogenetic signal.

Phylogenetic distribution and function of chaetae

One way to explore the evolution of chaetae is to examine the distribution of chaetal types in reference to proposed relationships among modern worm lineages. In the past decade, an increasingly rich selection of phylogenetic hypotheses has been developed. Fauchald and Rouse (1997) and Rouse and Fauchald (1997) provided an enormous service to the polychaete community by collecting and coding the morphological data for both the polychaetes and associated taxa. This synthesis has brought clarity, stimulated debate, and helped define areas of confusion that need to be resolved. Rouse and Fauchald's basic classification of the polychaetes is redrawn here as Figure 1 (their Figure 73) and their coding of chaetal characters (with modification, see footnotes associated with the tables) forms the basis of our Tables 1 and 2 (Rouse and Fauchald 1997). For purposes of clarity and brevity in this article we will confine our discussion to the 54 polychaete taxa that were included in their restricted analysis, and Tables 1 and 2 are arranged in the order suggested by that analysis (Fig. 1). The majority of the taxa that were excluded from their restricted analyses are characterized as being interstitial, pelagic, or symbiotic. We have examined the chaetal distributions in those taxa as well and have found no major exceptions to the general patterns we describe below.

Fig. 1

After cladogram produced by Rouse and Fauchald (1997, their Figure 73) based on morphological traits.

Fig. 1

After cladogram produced by Rouse and Fauchald (1997, their Figure 73) based on morphological traits.

Table 1

Distribution of capillary, compound, and pseudocompound chaetae and mobility patterns within the polychaetes

 Notopodium Neuropodium  
 
 

 
 
 Capillary Compound Pseudocompound Capillary Compound Pseudocompound Mobility 
Siboglinidae1 caps?   caps?   
Sabellariidae2 caps   caps   
Sabellidae2 caps   caps   
Serpulidae2 caps   caps   
Oweniidae caps      d, s 
Acrocirridae caps  pscmpd   pscmpd 
Flabelligeridae3 caps  pscmpd caps  pscmpd 
Cirratulidae caps   caps   
Alvinellidae caps      d, s 
Ampharetidae caps      d, s 
Pectinariidae caps      d, s 
Terebellidae caps      d, s 
Tricobranchidae caps      d, s 
Apistobranchidae    caps   
Spionidae caps   caps   d,s 
Trochochaetidae caps   caps   
Longosomatidae caps   caps   d, s 
Magelonidae caps   caps   
Poecilochaetidae caps   caps   
Chaetopteridae caps      
Acoetidae caps   caps   
Aphroditidae4 caps      
Eulepethidae caps   caps   
Polynoidae5 caps   caps   
Sigalionidae caps   caps 1-lig  
Pholoidae caps    1-lig  
Chrysopetalidae6 caps    1-lig  m, d 
Glyceridae caps    1-lig  
Goniadidae caps    1-lig  
Paralacydoniidae caps    1-lig  
Pisionidae    caps 1-lig  
Lacydoniidae caps    1-lig  
Phyllodocidae7 caps   caps 1-lig  
Nephtyidae caps   caps   
Nereididae8  1-lig   1-lig  m, d 
Hesionidae caps    1-lig  m, d 
Pilargidae9 caps   caps   
Sphaerodoridae10    caps? 1-lig, 2-lig?  m, d 
Syllidae11 caps? 1-lig  caps 1-lig  m, d 
Amphinomidae caps   caps   
Euphrosinidae12 caps   caps   m, d 
Dorvilleidae    caps 2-lig  
Lumbrineridae13    caps 2-lig pscmpd m, d 
Eunicidae13    caps 2-lig pscmpd m, d 
Onuphidae13    caps 2-lig pscmpd m, d 
Arenicolidae caps      d, s 
Maldanidae caps      
Capitellidae14 caps   caps   m, d 
Opheliidae caps   caps   
Scalibregmatidae caps   caps   m, d 
Orbiniidae caps   caps   
Paraonidae caps   caps   m, d 
Questidae caps   caps   m, d 
Cossuridae caps   caps   m, d 
 Notopodium Neuropodium  
 
 

 
 
 Capillary Compound Pseudocompound Capillary Compound Pseudocompound Mobility 
Siboglinidae1 caps?   caps?   
Sabellariidae2 caps   caps   
Sabellidae2 caps   caps   
Serpulidae2 caps   caps   
Oweniidae caps      d, s 
Acrocirridae caps  pscmpd   pscmpd 
Flabelligeridae3 caps  pscmpd caps  pscmpd 
Cirratulidae caps   caps   
Alvinellidae caps      d, s 
Ampharetidae caps      d, s 
Pectinariidae caps      d, s 
Terebellidae caps      d, s 
Tricobranchidae caps      d, s 
Apistobranchidae    caps   
Spionidae caps   caps   d,s 
Trochochaetidae caps   caps   
Longosomatidae caps   caps   d, s 
Magelonidae caps   caps   
Poecilochaetidae caps   caps   
Chaetopteridae caps      
Acoetidae caps   caps   
Aphroditidae4 caps      
Eulepethidae caps   caps   
Polynoidae5 caps   caps   
Sigalionidae caps   caps 1-lig  
Pholoidae caps    1-lig  
Chrysopetalidae6 caps    1-lig  m, d 
Glyceridae caps    1-lig  
Goniadidae caps    1-lig  
Paralacydoniidae caps    1-lig  
Pisionidae    caps 1-lig  
Lacydoniidae caps    1-lig  
Phyllodocidae7 caps   caps 1-lig  
Nephtyidae caps   caps   
Nereididae8  1-lig   1-lig  m, d 
Hesionidae caps    1-lig  m, d 
Pilargidae9 caps   caps   
Sphaerodoridae10    caps? 1-lig, 2-lig?  m, d 
Syllidae11 caps? 1-lig  caps 1-lig  m, d 
Amphinomidae caps   caps   
Euphrosinidae12 caps   caps   m, d 
Dorvilleidae    caps 2-lig  
Lumbrineridae13    caps 2-lig pscmpd m, d 
Eunicidae13    caps 2-lig pscmpd m, d 
Onuphidae13    caps 2-lig pscmpd m, d 
Arenicolidae caps      d, s 
Maldanidae caps      
Capitellidae14 caps   caps   m, d 
Opheliidae caps   caps   
Scalibregmatidae caps   caps   m, d 
Orbiniidae caps   caps   
Paraonidae caps   caps   m, d 
Questidae caps   caps   m, d 
Cossuridae caps   caps   m, d 

The distribution of chaetal types follows Rouse and Fauchald, (1997), except as noted. See text for definitions of capillary, compound, and pseudocompound chaetae. caps, capillary chaetae present; 1-lig, 1 ligament; 2-lig, 2 ligament; pscmpd, pseudocompound. Mobility patterns based on generalized descriptions of activity within a family. m, mobile, walking or swimming worms, active burrowers; d, discretely mobile, worms move in isolated bouts, may extend partially or wholly from tube to get food, may be commensal on the body of host; s, sessile, reside within a tube with no excursions outside of it.

1Siboglinidae—analyzed by Rouse and Fauchald separately as the Frenulata and Vestimentifera. Chaetae reported by Southward (1993); Gardiner and Jones (1993); Schulze (2001); Sasayama and others (2003) and Rousett et al. (2004).

2Sabellariidae, Sabellidae and Serpulidae all share the characteristic of segmental inversion where the distribution of chaetal types in the notopodia and neuropodia in the anterior segments is reversed compared with the orientation in the posterior segments, for example, in the thorasic segments the variously ornamented capillaries are found in the notopodial position, in the abdominal segments they are found in the neuropodial position (Dales 1962; Hartman 1969; Knight-Jones 1981; Fitzhugh 1989).

3Flabelligeridae—capillaries in notopodium (always) and neuropodium (sometimes), and are known as “barred chaetae” which Rouse and Pleijel (2001) suggest may be pseudocompound.

4Aphroditidae—various authors describe capillary chaetae in different positions, we follow here Pettibone (1963).

5Polynoidae—scored as having capillaries by Rouse and Fauchald (1997), there are robust simple chaetae associated with both noto and neuropodium, described as capillaries by Hutchings (2000a), although in other contexts, chaetae as thick as these would be considered to be spines.

6Chrysopetalidae—scored with capillaries by Rouse and Fauchald (1997), however, Russell (2000) and Rouse and Pleijel (2001) describe simple setae but imply that they are too stout to be called capillaries.

7Phyllodocidae—when notochaetae are present they are capillaries, neurochaetae are usually compound spinagers sometimes with long fine capillaries (Wilson 2000a; Rouse and Pleijel 2001).

8Nereididae—all setae compound spinagers and falcigers. Wilson (2000b) reports that some posterior segments of some species may have setae fused at the articulartion. Fauchald (1977) reports Cirronereis as having simple falcigers.

9Pilargidae—Notopodium reduced, with spines or rarely capillaries (Rouse and Pleijel 2001), neuropodium with capillaries or spines (Glasby 2000a).

10Sphaerodoridae—Fauchald and Rouse (1997) say variously ornamented capillaries are present but do not specify whether notopodial or neuropodial, (although in general the notopdium is reduced or absent). Fauchald (1977) and Rouse and Pleijel (2001) describe the parapodia as uniramous. Neither the text nor illustrations in Wilson (2000c) or Rouse and Pleijel (2001) describe capillaries. Although scored by Rouse and Fauchald (1997) as having compound chaetae with a single ligament, Kudenov (1987a, b) illustrates what may be double ligaments similar to what is described for the Eunicidae (Rouse and Pleijel 2001).

11Syllidae—scored with capillaries by Rouse and Fauchald (1997) although they are limited to only some genera (Glasby 2000a). The notopodium is reduced and without chaetae except in reproductive forms where some epitokous species develop natatory chaetae (fused blade and shaft) in the notopodia of middle and posterior segments (Glasby 2000b). Pettibone (1963) describes and illustrates these swimming chaetae as simple.

12Euphrosinidae—majority of chaetae have a distal asymmetrical bifurcation and are termed “ringent”; there are simple, slender tapering chaetae (capillaries) associated with the ventral portion of the neuropodium.

13In the Lumbrineridae, Eunicidae, and Onuphidae the parapodia are described as either uniramous or subbiramous with very reduced notopodia although embedded aciculae may be present (Fauchald 1977; Pettibone 1963).

14Capitellidae have capillary chaetae only on the anterior portion of the worm, the point at which the capillary chaetae stop depends on species, age or state of regeneration of worm (Hutchings 2000b).

Table 2

Type, distribution, and lifestyle association of hooked chaetae within the polychaetes

 Type of hooked chaetae, position, distribution  
 
 
 
 Simple hooks Compound (jointed) hooks  
 
 

 
 
 Uncini Dentate Falcate Hooked Dentate Falcate “Anchored” lifestyles 
Siboglinidae1 rw, ptch, no, ne      
Sabellariidae2 rw, no, ne      
Sabellidae2 rw, no, ne      
Serpulidae2 rw, no, ne      
Oweniidae  ptc, ne     
Acrocirridae    fas, ne    
Flabelligeridae3   fas, ne fas, ne   
Cirratulidae4  fas, ne     
Alvinellidae rw, ne      
Ampharetidae5 rw, ne  sol, no    
Pectinariidae5 rw, ne  fas, no    
Terebellidae rw, ne      
Trichobranchidae rw, ne rw, ne     
Apistobranchidae        
Spionidae6  rw, no, ne     
Trochochaetidae7   fas, ne    
Longosomatidae8   fas, sol, ne    
Magelonidae9  rw, fas, no, ne     
Poecilochaetidae10   fas, ne, no    
Chaetopteridae rw, ne      
Acoetidae11       
Aphroditidae        
Eulepethidae12   fas, no    
Polynoidae13   fas, ne    
Sigalionidae     fas, ne  
Pholoidae     fas, ne  
Chrysopetalidae      fas, ne 
Glyceridae        
Goniadidae        
Paralacydoniidae        
Pisionidae      fas, ne 
Lacydoniidae        
Phyllodocidae        
Nephtyidae        
Nereididae      fas, no, ne t, s 
Hesionidae       
Pilargidae        
Sphaerodoridae14   fas, ne   fas, ne 
Syllidae15   fas, ne?  fas, ne  t, s, i 
Amphinomidae        
Euphrosinidae        
Dorvilleidae     fas, ne  
Lumbrineridae16  fas, ne   fas, ne  
Eunicidae17  fas, ne   fas, ne  
Onuphidae18  fas, ne   fas ne  
Arenicolidae19  rw, ne     
Maldanidae  rw, ne     
Capitellidae20  rw, ne, no     
Opheliidae        
Scalibregmatidae21   fas, no, ne    
Orbiniidae22  fas, ne     
Paraonidae23  fas, ne     
Questidae  fas, ne     
Cossuridae        
 Type of hooked chaetae, position, distribution  
 
 
 
 Simple hooks Compound (jointed) hooks  
 
 

 
 
 Uncini Dentate Falcate Hooked Dentate Falcate “Anchored” lifestyles 
Siboglinidae1 rw, ptch, no, ne      
Sabellariidae2 rw, no, ne      
Sabellidae2 rw, no, ne      
Serpulidae2 rw, no, ne      
Oweniidae  ptc, ne     
Acrocirridae    fas, ne    
Flabelligeridae3   fas, ne fas, ne   
Cirratulidae4  fas, ne     
Alvinellidae rw, ne      
Ampharetidae5 rw, ne  sol, no    
Pectinariidae5 rw, ne  fas, no    
Terebellidae rw, ne      
Trichobranchidae rw, ne rw, ne     
Apistobranchidae        
Spionidae6  rw, no, ne     
Trochochaetidae7   fas, ne    
Longosomatidae8   fas, sol, ne    
Magelonidae9  rw, fas, no, ne     
Poecilochaetidae10   fas, ne, no    
Chaetopteridae rw, ne      
Acoetidae11       
Aphroditidae        
Eulepethidae12   fas, no    
Polynoidae13   fas, ne    
Sigalionidae     fas, ne  
Pholoidae     fas, ne  
Chrysopetalidae      fas, ne 
Glyceridae        
Goniadidae        
Paralacydoniidae        
Pisionidae      fas, ne 
Lacydoniidae        
Phyllodocidae        
Nephtyidae        
Nereididae      fas, no, ne t, s 
Hesionidae       
Pilargidae        
Sphaerodoridae14   fas, ne   fas, ne 
Syllidae15   fas, ne?  fas, ne  t, s, i 
Amphinomidae        
Euphrosinidae        
Dorvilleidae     fas, ne  
Lumbrineridae16  fas, ne   fas, ne  
Eunicidae17  fas, ne   fas, ne  
Onuphidae18  fas, ne   fas ne  
Arenicolidae19  rw, ne     
Maldanidae  rw, ne     
Capitellidae20  rw, ne, no     
Opheliidae        
Scalibregmatidae21   fas, no, ne    
Orbiniidae22  fas, ne     
Paraonidae23  fas, ne     
Questidae  fas, ne     
Cossuridae        

The designation of chaetal types follows Rouse and Fauchald (1997). except as noted, see the text for definitions of simple, compound, uncini, dentate, falcate. “Anchored” lifestyles are ones in which the ability to anchor is necessary and a family was indicated with a particular lifestyle if there is a report of at least one species within that family having such a lifestyle. rw, row; ptc, patch; fas, fascicle; sol, solitary; no, notopodium; ne, neuropodium; t, tube or burrow-dwelling; s, symbiotic; i, interstitial.

1Siboglinidae—analyzed by Rouse and Fauchald separately as the Frenulata and Vestimentifera. Chaetae reported by Southward (1993), Gardiner and Jones (1993), Schulze (2001), Sasayama et al. (2003) and Rousett et al. (2004).

2Sabellariidae, Sabellidae and Serpulidae all share the characteristic of segmental inversion where the distribution of chaetal types in the notopodia and neuropodia in the anterior segments is reversed compared with the orientation in the posterior segments for example, in the thorasic segments the uncini are found in the neuropodial position, in the abdominal segments they are found in the notopodial position (Dales 1962; Hartman 1969; Knight-Jones 1981; Fitzhugh 1989).

3Flabelligeridae—Some genera are tubiculous, some bore into coral, others live in the tubes produced by other invertebrates, others live under stones or burrow (Hutchings 2000c). Notochaetae are always capillaries, neurochaetae may be simple, compound, or pseudocompound hooks, (Rouse and Pleijel 2001).

4Cirratulidae—Rouse and Fauchald (1997) state that the hooks are not considered homologous with other hooks (although they give no explanation or reference). Rouse and Pleijel (2001) describe and illustrate multi-dentate hooks. The distribution of hooks is varied although generally neuropodial; Cirriformia is known to replace hooks with capillary chaetae as they mature (Blake 1975). Some species build tubes (Rouse and Pleijel 2001) others live in mud tubes or burrow into mollusk shell or coral Glasby 2000c).

5Ampharetidae, Pectinariidae—in addition to uncini on tori, they can have notopodial paleal hooks (Hutchings 2000d,e).

6Spionidae—hooks are most common in posterior segments and are primarily neuropodial (occurring with the parapodial bundle in short rows, Rouse and Pleijel 2001), however, Wilson (2000d) reports that notochaetae and neurochaetea include capillaries and hooks.

7Trochochaetidae—Rouse and Fauchald 1997 (and others) describe only spines, but as figured in Wilson (2000e) they are thick hooks (Fig. 1.109). Described as living in tubes.

8Longosomatidae—not indicated as having hooks by Rouse and Fauchald (1997); however there are falcate spines on posterior segments (Willson 2000f) and they have stout, hook-like spines on the neuropodia of the first segment and live in deep burrows (Rouse and Pleijel 2001).

9Magelonidae—abdominal segments have hooks in both the notopodium and neuropodium in fascicles or becoming more row-like in posterior segments and live in mucus lined burrows (Wilson 2000g).

10Poecilochaetidae—although these worms are reported to live in long, lined burrows or in branching tubes, they are not indicated by Rouse and Fauchald (1997) as having hooks; however, they are described and illustrated as having stout spines that can be sigmoidal to strongly curved, occurring in the notopodium or neuopodium (Allen 1905; Rouse and Peijel 2001). Poecilichatetidae live in long, lined burrows or in branching tubes (Rouse and Peijel 2001).

11Acoetidae—interesting because they live in flexible tubes made in part of their own woven notosetae [Pflugfelder (1934) and Pettibone (1989)], however, they but do not have hooks.

12Eulepethidae—Rouse and Fauchald (1997) record only spines for this group, but illustrations and descriptions in Hutchings (2000f) show hooked chaetae, interesting for their position in the notopodium. Their natural history is not well studied, one is commensal with a polyodontid scale worm (Pettibone 1969) others were collected within sponge tissue (Pettibone 1986).

13Polynoidae—scored only as having spines by Rouse and Fauchald (1997), but as illustrated in Hutchings (2000a), some neurochaetae have hooked tips. In general the group is described as being active predators but some species are commensal living on echinoderms and bivalves (Hutchings 2000a).

14Sphaerodoridae—scored as having compound falcate chaete by Rouse and Fauchald (1997), in the text of Fauchald and Rouse (1997) there is an additional mention of curved spines. Pettibone (1963) describes and illustrates simple and compound hooks. Several species are described as commensal (Rouse and Pleijel 2001).

15Syllidae—in atokous forms the biramous parapodium has a much reduced notopodium that can become large and bear chaetea in swimming atokes (Fauchald and Rouse 1997). Compound hooks are common in the neuropodium, Pettibone (1963) reports hooks with fused joints in some species.

16Lumbrineridae—burrowing worms with some species forming membranous tubes (Rouse and Pleijel 2001). Paxton (2000a) describes lumbrinerids as having simple and/or compound neuropodial hooks. Fauchald and Rouse (1997) report that when compound chaetae are present it is only associated with the anterior segments.

17Eunicidae—many are tubicolous, some burrow in coral, others are free-living (Rouse and Pleijel 2001), and Paxton (2000b) describes eunicids with inferior compound falcigers and subacicular hooks within the parapodial bundle.

18Onuphidae—most are tubicolous, and Rouse and Fauchald (1997) score them as having compound dentate chaetae. Paxton (2000c) reports hooks (simple, compound, or pseudocompound) in small fascicles or solitary, varying with species and position on body.

19Arenicolidae—typically live in burrows, Rouse and Pleijel (2001) describe at least one genus, Branchiomaldane, that is tubiculous.

20Capitellidae—Fauchald and Rouse (1997) say that uncini are present in both noto- and neuropodia, however, in their scoring of the taxa in Rouse and Fauchald (1997) uncini are not listed for Capitellidae.

21Scalibregmidae—listed with uncini in descriptive text of Rouse and Fauchald (1997), however, they are not coded as such in the table on which the analysis was based nor are they typically described with uncini elsewhere. On the first segment scalibregmids have hooked spines associated with the notopodium. They live in crevices in rock, galleries in the sediment (Rouse and Pleijel 2001) and tubes of other animals (Hutchings 2000g).

22Orbiniidae—have hooks that closely resemble others in the Scolecida and are found worldwide, typically burrowing in sandy or silty sediments. According to Rouse and Pleijel (2001), they do not make any sort of tube. In contrast to this report, Myers (1977a, b) describes Scoloplos robustus as having a reinforced tail shaft tube that is ‘semi-permanent’, as well as numerous temporary head shafts in which it feeds.

23Paraonidae—Rouse and Fauchald (1997) do not mention or score hooked setae as being associated with the Paronidae, but in Rouse and Pleijel (2001) and in Glasby (2000d) they are described and illustrated as having hooks. Rouse and Pleijel (2001) report paranoids with burrows lined with mucus and temporary tubes.

Capillary chaetae

Capillary chaetae are simple (unjointed), long, tapering, pointed chaetae (Fig. 2) and are found in most modern families of polychaetes (and all but the Nereididae of those listed in Fig. 1, Table 1). Despite the otherwise ubiquitous distribution of capillary chaetae among the families of polychaetes, there are only a few relatively taxon-specific observations about how they might be used by the worms and even fewer experimental studies of their functions. The capillary chaetae of the Amphinomidae, for example, are reported to be brittle and function as a defense in fire worms (Hutchings 2000h). Dorsal capillaries form a fine “felt” that presumably has a protective function in the Aphroditidae (Westheide 1997; Hutchings 2000i). The commonness of capillary chaetae suggests, however, that more general functions are likely for these structures. At a minimum, they are important in locomotion, stabilization during peristalsis, and sensing the environment. Their role in locomotion can be seen in the elegant description of Mettam (1971) for Aphrodite aculeatea. The effective stroke of each parapodial step is increased as the neuropodial capillary chaetae are protruded farther, increasing “the span of the limb.” For worms living in burrows or tubes, capillary chaetae have been implicated in assisting movement and stabilizing body segments within the tube (for example, Mettam 1969; Roy 1974; Knight-Jones and Fordy 1979; Sendall and others 1995). This bracing function is especially important during irrigation of tubes, and we experimentally demonstrated the role of capillary chaetae in this function in the maldanid Clymenella torquata (Woodin and others 2003). By using pumping efficiency (mm3 water moved/s) as a measure of the role of capillary chaetae in stabilizing worms in their tubes during peristalsis, we showed that worms with ablated capillary chaetae were only about 60% as effective at moving water within their tubes as those with intact chaetae (Woodin and others 2003).

Fig. 2

Capillary chaetae (C) of the sabellid polychaete Sabellastarte magnifica, scale 200 μm.

Fig. 2

Capillary chaetae (C) of the sabellid polychaete Sabellastarte magnifica, scale 200 μm.

We also suspect that capillary chaetae have a major role as mechano-receptors in the sensory array of polychaetes. Capillary chaetae typically are associated with the majority of body segments. Their position exposes their free ends to water movement in epifaunal forms, while in sediment-dwellers they contact the inside walls of tubes or burrows. The cantilever nature of capillary chaetae and their astounding breadth of flexural stiffness (8 orders of magnitude: Merz and Woodin 1991) suggest that they could be very effective at transmitting specific mechanical information about their surroundings to the body of the worm, not unlike the vibrissae of cats. There has been little discussion of this potential function for capillary chaetae, although Woodin and others (2003) provide data that suggest a sensory role for capillary chaetae in the construction and sizing of maldanid tubes.

Hooked chaetae

The terminology to describe hooks is not used consistently among all authors writing about polychaetes. For the purposes of this comparison, therefore, we use the inclusive definition that a hook is a chaeta with a distally curved tip (Fig. 3). We realize that this describes many kinds of chaetae, some of which are not normally treated as hooks (for example, curved spines or falcigers), however, it may be the least biased way to survey what taxa have chaetae that function as anchors. Using this definition, hooks are actually found in at least some members of all major clades of the Polychaeta (Rouse and Fauchald 1997; Fig. 1, Table 2).

Fig. 3

Examples of hooked chaetae. (A) Clymenella torquata, dentate hooks emerging from neuropodium, scale 15 μm. (B) Owenia fusiformis, detail within a patch of tiny dentate hooks on the torus of one setiger, scale 10 μm. (C) Schizobranchia insignis, row of uncini, note broken chaetal tip, scale 10 μm. (D) Pectinaria gouldii, multi-dentate uncini, scale 5 μm.

Fig. 3

Examples of hooked chaetae. (A) Clymenella torquata, dentate hooks emerging from neuropodium, scale 15 μm. (B) Owenia fusiformis, detail within a patch of tiny dentate hooks on the torus of one setiger, scale 10 μm. (C) Schizobranchia insignis, row of uncini, note broken chaetal tip, scale 10 μm. (D) Pectinaria gouldii, multi-dentate uncini, scale 5 μm.

In terms of morphological variation, a hook may have a single plain tip (“falcate”) or few or many teeth (“dentate”). The proximal part of the hooked chaeta is a shaft that connects the hook head to the body. The shaft can vary in length; it may be a continuous, uninterrupted section (in which case the hook is considered to be “simple”), it may be separated by an articulation or joint (“compound”), or there may be a fold or incomplete articulation in the shaft (“pseudocompound”). Uncini are multidentate hooked chaetae in which only the hook head extends beyond the body wall; all uncini are simple, and in some the shaft is virtually absent or replaced with ligaments, while in others the shaft may be “long-handled” and deeply embedded in the body of the worm. Hooks may be found associated with the parapodium in either the dorsal notopodium or ventral neuropodium. They may occur singly or in bunches or fasciles, or they may be arrayed in single (or less commonly double) rows. When hooks occur in rows they are often associated with a neuropodium configured as a low mound of tissue that in some cases may nearly encircle the segment (“torus”). In the Oweniidae, the tiny hooks on the tori form velcro-like fields or patches rather than rows (Fig. 3B); in the siboglinid Riftia they also occur in dense patches.

Hooked chaetae are generally found on the bodies of worms that have an obvious need to anchor—those that live in tubes or are symbiotic or interstitial (Table 2; Woodin and Merz 1987). Of the 54 families in this comparison (Table 2), 39 are described as having some sort of hook. In 38 of those families there is at least one species that has an “anchored” lifestyle. The exception is the little-known Acrocirridae, which is especially interesting because the neuropodial hooks in the anterior and posterior segments face in different directions (Banse 1969). Of the 15 families without hooks (Table 2), we find three that have lifestyles in which the need for anchoring is predicted. The first is the Acoetidae, which live in unusual tubes made, at least in part, of their own woven notosetae (Pflugfelder 1934; Pettibone 1989). The tubes are reported to lack any membranous lining, and the carnivorous worms are described as partially leaving the tube during prey capture (Pettibone 1989). It would be very interesting to see if this unusual tube has attributes that make it difficult for the worm to be extracted from it and to understand how acoetids anchor their bodies as they extend from and retract back into their tubes. Eisig (1887, in Pettibone 1989) describes fishing for Polyodontes with bait and collecting only the anterior portions of the worms; the posterior sections had presumably pulled back into their tubes! The second exception is that some members of the Glyceridae construct semipermanent galleries (Glycera alba: Ockelmann and Vahl 1970) yet do not have hooked chaetae (Table 2), and it would be informative to learn how they maintain position within their galleries. Third, the Hesionidae are for the most part free-living, actively mobile worms. Some hesionid species (for example, Ophiodromus), however, are known as facultative commensals (or perhaps parasites) on the bodies of various hosts, including echinoderms (Hickok and Davenport 1957). Hesionids are not typically described as having hooked chaeta, although Ophiodromus does have small hooked ends at the tips of their compound chaetae; how they maintain their positions on their hosts is unknown.

This general examination of the potential match between specific lifestyles and the chaetae that are characteristic of specific families is likely to become much more informative as we examine finer-scale covariation between the morphology of species and their natural histories. There are hints that at least in some cases there is a tight coupling; for instance we know of two terebellids that do not build tubes: Telothelepus capensis, in which hooks are reduced in number and found only on the abdomen, and Enplobranchus sanguineus, which lack hooks entirely (Day 1967). Finer-scale examinations will likely provide intriguing and instructive examples in which morphology and function are uncoupled.

The function of rows of hooked chaetae (including uncini) as anchors for tube-dwelling worms has been implicated in a number of different taxa by a variety of techniques. Morphologically, the orientation of hooks on the bodies of tube-dwelling worms is predictable by knowing the shape of the tube, irrespective of whether the worm lives head-up (for example, sabellids, oweniids, or terebellids) or head-down (for example, maldanids or pectinarids) in the tube. The hooks of tube-dwellers are oriented such that the tip of the hook is facing the direction in which the worm could be extracted from the tube (Woodin and Merz 1987). In this orientation the hooks are positioned to engage the tube wall and prevent removal of the worm from the tube by forces such as those produced by water movements or predators (Lauder 1980; Denny 1988; Nemeth 1997). The mechanical effectiveness of this mechanism has been demonstrated in various species of sabellids, oweniids, and maldanids, in which removal of the worm from a tube takes on average three times more force in the direction in which the tips engage the tube wall than in the reverse direction (Woodin and Merz 1987; Merz and Woodin 2000). That worms are actively employing this function is demonstrated by the observations that (i) anaesthetized worms present very little resistance to extraction from their tubes (Woodin and Merz 1987; Merz and Woodin 2000); (ii) when worms are forced out of their tubes in the direction that engages their hooks, those hooks and hook rows are damaged on the widest portion of the body, where the hooks would be expected to most rigorously engage the tube wall (Merz and Woodin 2000) (Fig. 3C); and (iii) worms removed from their tubes by carefully cutting open the tubes show scars and worn chaetae in these same regions (Merz and Woodin 2000). If or how the hooks of other taxa perform an anchoring function has not been demonstrated; of particular interest is how solitary hooks might function.

Jointed or compound chaetae

The terms “jointed” or “compound” chaetae have not been used consistently in the literature. In general, they are applied when the chaetal shaft is interrupted by a morphological elaboration or thinning (Fig. 4). The joint consists of a socket in which the distal blade of the chaeta is anchored by both a ligament and a hinge (Fig. 4). The joint is external to the body and is not directly connected to either muscles or nerves, and the distal blade is free to move, governed by the shape of the socket and the position and length of the ligament. This type of jointed chaeta (identified as ones with single ligaments by Rouse and Fauchald 1997) is known only from families within the Phyllodocida (Table 1). The development of this classic jointed chaeta was beautifully described by Gustus and Cloney (1973) and O'Clair and Cloney (1974) for a nereidid polychaete.

Fig. 4

Examples of compound or jointed chaetae. These joints function outside the body of the worm without direct connection to muscles or nerves. (A) Neanthes brandti, classic blade and socket jointed chaetae from the neuropodium. (B) N. brandti, chaetae, view into the joint of notopodial chaetae showing the single ligament that anchors the blade in the socket. (C) Ophiodromus pugettensis with a much shallower socket. (D) Phyllodoce groenlandia with dentate sculpturing associated with the socket. All scales 10 μm.

Fig. 4

Examples of compound or jointed chaetae. These joints function outside the body of the worm without direct connection to muscles or nerves. (A) Neanthes brandti, classic blade and socket jointed chaetae from the neuropodium. (B) N. brandti, chaetae, view into the joint of notopodial chaetae showing the single ligament that anchors the blade in the socket. (C) Ophiodromus pugettensis with a much shallower socket. (D) Phyllodoce groenlandia with dentate sculpturing associated with the socket. All scales 10 μm.

Members of the Eunicida, Flabelligeridae, and Acrocirridae are sometimes described as having compound or pseudocompound chaetae. In the case of the Eunicida, the distal blade is not free to move, either because it is limited by double ligaments (Rouse and Fauchald 1997; Fig. 5) or because there is no socket and the folds of the chaetal shaft that have been assumed to be a joint do not in fact bend (personal observation; Diopatra: Fig. 5C). We agree with Rouse and Fauchald (1997) that the details of morphology of compound chaetae have not been thoroughly explored and that examination of them will be important for both systematics and functional morphology. They would also be good subjects for developmental study. Bartolomaeus (1995, 1998) and Bartolomaeus and Meyer (1997) have demonstrated that hooks (including uncini) from a variety of families develop in basically the same way and that different hook morphologies are the result of variations in timing of the actin-filament system of the chaetoblast and surrounding follicle cells. We suspect that compound and pseudocompound chaetae may similarly be the result of slight modifications of the developmental program of chaetal formation. Knowing the developmental basis for these morphologies would help confirm or reject the proposed relationship among the Eunicida, Phyllodocida, and Terrebellida (Fig. 1), the three clades that have families with compound or pseudocompound chaetae.

Fig. 5

Examples of pseudocompound chaetae: (A) Lumbrinereis inflata, (B) Nothria elegans, (C) Diopatra ornata. All scales 10 μm.

Fig. 5

Examples of pseudocompound chaetae: (A) Lumbrinereis inflata, (B) Nothria elegans, (C) Diopatra ornata. All scales 10 μm.

There has been only one experimental study of the function of compound chaetae (Ophiodromus pugettensis: Hesionidae: Phyllodocida: Merz and Edwards 1998); however, several elegant descriptions do discuss chaetal extension, parapodial position, and gaits in the Phyllodocida (for example, Gray 1939; Marsden 1966; Mettam 1967, 1971). Many of the members of the Phyllodocida are active worms (Table 1) that display a variety of gaits from simple walking to swimming. Such movements involve the coordination of the parapodia, chaetae, and body wall musculature and produce changes in parapodial flexure and chaetal protrusion depending upon the position of the parapodium (Gray 1939; Marsden 1966; Mettam 1967, 1971). It is interesting that jointed chaetae are primarily associated with the ventral neuropodium in biramous taxa or the ventral side of the parapodium in uniramous taxa (Table 1), where they would be the attachment points to the substrate and bear the weight of the animal. The exception to this ventral orientation of jointed chaetae is found in forms that are holopelagic (for example, Lopadrorhynchidae, Pontodoridae, and Iospilidae) and in the Nereididae, which have swimming epitokes (the Syllidae may also develop jointed chaetae in the notopodium during epitoky, although different authors vary in their description of the swimming chaetae; see footnotes to Table 1). There is very little known about how chaetae specifically contribute to the mechanics of swimming in polychaetes. The arrangement of chaetae in the parapodium might suggest that they need to be distributed laterally (rather than ventrally) and that in the absence of interaction with a solid substratum, there is less selection for dorsal-ventral regionalization of chaetal types within the parapodium.

The ventral position of jointed chaetae in crawling forms may allow the tips of the chaetae to independently orient to the rugosities of the substratum, thus decreasing slippage and increasing stepping efficiency. This hypothesis was tested in O. pugettensis (Hesionidae), where jointed chaetae were trimmed either just distal to or immediately proximal to the joint. As expected, loss of the joint significantly reduced locomotory performance (measured as the maximum speed within a gait or stride distance) (Merz and Edwards 1998). Thus for compound chaetae that flex at the joint, the joint may have a clear function. The function, if any, of compound or pseudocompound chaetae that do not appear to bend at the joint is unknown. The compound (or pseudocompound) chaetae of Diopatra magna (Onuphidae: Eunicida), for example, do not bend at the joint (R. A. Merz and S. A. Woodin, unpublished data) (Fig. 5C). Without flexure they would appear to be the equivalent of simple, though thickened, capillaries or hooks.

The fossil record and chaetae

Another way to view the evolution of chaetae is to examine the fossil record. Even though chaetae and jaws are the polychaete features most likely to be fossilized (Colbath 1986, 1988; Briggs and Kear 1993), only in cases of exceptional preservation (lagerstätten) is it possible to see detailed structure of individual chaetae and their exact position on the body. There are relatively few such lagerstätten that provide whole-body polychaete fossils from which we can draw much information (see Briggs and Kear 1993 for a summary of whole-body polychaete fossil collections).

The earliest undisputed examples of polychaete body fossils are from the Burgess Shale of the Cambrian (543–490 mya). Dickinsonia, Spriggina, and Marywadea from the Ediacaran fauna of the Vendian (600–543 mya) have been discussed as polychaetes (Glaessner 1976; Conway Morris 1979), but this view has been rejected, in part because of their lack of chaetae (Conway Morris 1979). The remarkable preservation of the Burgess Shale polychaetes reveals a diversity of parapodia and chaetae. There are six named species: the biramous Canadia spinosa, Burgessochaeta setigera, Insolicoryphya pygma (described in Conway Morris 1979), and Wiwaxia corrugata (Butterfield 1990, but see Eibye-Jacobsen 2004 for an opposing view) and the uniramous Perochaeta dubia and Stephenoscolex argutus (Conway Morris 1979). These species display a variety of chaetal types, including capillaries, spines, broad flattened paleae, and stout hooks (P. dubia) (Conway Morris 1979). There is also variation in sculpturing including serrations and bifid tips (Conway Morris 1979; Eibye-Jacobsen 2004). There are no reported compound chaetae and nothing that resembles uncini.

The Ordovian (490–443 mya), Silurian (443–417 mya), and Devonian (417–354 mya) offer relatively few polychaete body fossils, and although Cameron (1967) and Sutton and others (2001) each describe three-dimensionally preserved worms, in neither case is it possible to evaluate the details of chaetal morphology. It is not until the Mazon Creek lagerstätten of the Carboniferous (354–290 mya) that there is another opportunity to see a variety of polychaetes and their chaetae. Thompson (1979) and Thompson and Johnson (1977) discuss 17 species that are recognizable as belonging to 10 modern families (most of these are confirmed by Fitzhugh and others 1997). The descriptions of the chaetae from the different authors do not entirely agree, although both confirm compound chaetae in the case of the hesionid Rutellifrons wolfforum (which also has internal asiculae that together with the compound chaetae represent a parapodial morphology that is characteristic of modern Phyllodocida).

The key current phylogenetic issue regarding fossil polychaetes is whether the Phyllodocida, the most derived crown clade according to the Rouse and Fauchald (1997) analysis (Fig. 1), is represented in the Cambrian (by Canadia or perhaps Wiwaxia; Conway Morris and Peel 1995; Rouse and Pleijel 2001) or is not found till approximately 140 million years later in the Carboniferous Mazon Creek formation (by at least Rutellifrons). Eibye-Jacobsen (2004) has done a cladistic analysis on the Burgess Shale polychaete fauna and concludes that neither Canadia nor Wiwaxia have the characteristics that should cause them to be considered members of the Phyllodocida (“the specific characters of the Aciculata and Phyllodocida are entirely absent,” according to Eibye-Jacobsen 2004, p 319) despite the remarkable similarity (convergence?) between the sclerites of Wiwaxia, the dorsal palae of Canadia, and the notochaetae of modern chrysopetalid polychaetes (Butterfield 1990). This leaves open the question of when compound chaetae first arose and in what way, if any, they were associated with what appears to be a radiation of the Phyllodocida in the Carboniferous (that is, in addition to the species associated with the Mazon Creek, Carboniferous Phyllodocida are reported from Bear Gultch, Montana, USA [Bottjer and others 2002] and Granton, Scotland [Briggs and Clarkson 1987]).

If Cambrian worms were like modern worms in the association between lifestyles that require an anchoring function and the presence of curved spines or hooks, then we would predict that P. dubia (reported as having hooks by Conway Morris 1979) was perhaps commensal or symbiotic or lived in a burrow. Conway Morris (1979) has suggested that it may have scavenged or been a burrower. The new Cambrian fossils coming out of China may enlighten our views of both hooks and compound chaetae.

In addition to whole-body fossils some information about early polychaetes can be derived from feeding and movement trace fossils, scolecodonts (fossil jaws), and tubes with identifiable construction types (such as the use of calcium carbonate or interior patterns of compaction or decoration (Aller and Yingst 1978; see Butterfield [2003] for a discussion of preservation). Given the difficulty of assigning taxonomic identity uniquely to feeding or movement paths (Jensen 2003; see however, Seilacher 1977), trace fossils have been largely ignored, although in some instances very identifiable patterns may be revealed (for example, the spiral feeding patterns associated with Paranoidae in the Cretaceous [146–65 mya]; Hantzschel 1975).

Fossil jaws and tubes, however, have been uniquely identified with particular taxa, and the pattern produced by jaws indicates that Eunicida were extant in the Ordovician (Kielan-Jaworowska, 1966; Edgar 1984; Colbath 1986) and the Phyllodocida only at the end of the Paleozoic (543–248 mya) (Nakrem and others 2001; Eriksson and Bergman 2003). The pattern is not altogether clear, in part because scolecodont systematics are in disarray (Eriksson 1999).

Tube- or burrow-dwellers appear to have existed in the Cambrian; Terebellid burrows are reported from the lower Palaeozoic but with more certainty from the Silurian (Thomas and Smith 1998). Serpulid tubes occur in the Orodvician (Knight-Jones 1981), terebellid and siboglinid tubes are known from the Silurian (Little and others 1997; Thomas and Smith 1998), spionid tubes are identified from the Devonian (Blake and Evans 1973), sabellarid tubes from the Carboniferous (Howell 1962), and pectinarid tubes from the Permian (290–248 mya) (Howell 1962), and from the Eocene (55–34 mya) there are some remarkable tubes in which the internal membrane has been preserved (Schweitzer and others 2005). The modern-day association of tube dwelling and the occurrence of hooks lets us predict that any body fossils found with these tubes should bear some sort of hook. To our knowledge, however, there are no such body fossils. This may be due to the tubes themselves actually precluding the fossilization processes necessary for unusually good preservation. A combination of lack of disturbance and unique geochemical conditions (commonly mediated by adjacent sediment and microbial activity that preserves organic tissues and/or causes a rapid precipitation of minerals) is thought to be critical in the preservation processes that produce lagerstätten (Allison and Briggs 1991a,b; Briggs 2003). A pulse of sedimentation that coats the body surfaces and then results in rapid burial is one scenario that would set up the right circumstances for unusual preservation; however, obligate tube dwellers stuck in their tubes are unlikely to be coated and deposited in this way.

A potential role for functional morphology in phylogenetic analysis

The complementary resources of cladistic analysis and new molecular data are changing our view, sometimes in dramatic ways, of the relationships among the polychaetes (for example, McHugh 1997; Halanych and others 2002). Currently there is not strong agreement among molecularly based analyses or between the analyses based on morphological data (for example, Rouse and Fauchald 1997) and those based on molecular data (see Hall and others 2004; Bleidorn and others 2003; and Rousse and others 2004 for recent examples). At this point a reasonable position is to continue to gather new data of both types and analyze, compare, and evaluate the resulting patterns. It also makes sense to review the morphological data used by Rouse and Fauchald (1997) because this is currently the basis of comparison in evaluating molecular data (for example, Hall and others 2004; Bleidorn and others 2003; Rousset and others 2004 and many others) and is included in analyses that use both morphological and molecular information (for example, Rousset and others 2004). At least three different issues need to be considered if these morphological data are to provide the strongest phylogenetic signal possible. We illustrate these issues using chaetal examples, but these problems are associated with many kinds of morphological data. The first issue is that different authors describe the same structures with different terms (or vice versa). As can be seen by the voluminous footnotes accompanying Tables 1 and 2, there is substantial variation in how a given set of structures can be described (even by a fairly small pool of authors working within the same historical framework). In the case of chaetae there is some casualness about chaetal definitions (for example, there is no use of numerical guidelines to distinguish the aspect ratio of capillaries from spines and no specified range of angles that distinguish curved chaetae from hooks). The result is lack of consistency in use of terminology. The second issue is that in some cases there have been careful descriptions and reviews of specific morphological details that vary within a group for cladistic analysis, but that detailed information has remained associated with only that group. For instance, Fitzhugh (1989) did an extensive description of the sculpture of chaetae as part of a cladistic analysis of the Sabellida. Unfortunately, the same sort of detailed analysis has not happened in many other groups, and the potentially rich cladistic implications of sabellid chaetal variation have not been drawn into broader analyses (for example, Rouse and Fauchald 1997). It may be that the morphologies described by Fitzhugh (1989) are restricted to the Sabellida and would not be useful on a wider scale, however, because the other groups have not been examined in the same way, we do not know. Third is the question of whether homoplasy results in misleading or homogenized categorization of characters. Some would argue that homoplasy is revealed only by cladistic analysis. While that may be true, it is also reasonable to recognize that when organisms live under the same physical conditions, the physical challenges of that lifestyle will have been met. One is not being an adaptationist to recognize, for instance, that organisms that fly or glide through the air must have some structure that provides lift. It does not mean that all such organisms will achieve that function in the same way (for example, bat, insect, and samara wings are very different, although all recognizable as wings) or that all organisms possessing such structures can fly (for example, ostriches have wings). The point is that by recognizing the common physical requirements of a habit it is possible to predict what specific needs must have been met by the organisms living in it. Examining how those physical challenges are solved across taxa gives us a richer sense of how organisms live their lives. It also alerts us to when we should pay special attention to the morphological details and development of particular structures (for example, those that have a role in the function in question) if they are to provide the strongest possible phylogenetic signal.

We think that the tube-dwelling habit is a circumstance where it would be useful to keep the potential for convergence (homoplasy) in mind. From the perspective of a worm, living in a tube requires the ability to move within that tube (which will have a far more uniform substratum and geometry than the world outside the tube), to resist removal from it, to feed from the tube, to eliminate wastes from it, and to access oxygen either by moving water or by positioning respiratory surfaces outside the tube. Unless all tube-dwelling worms are monophyletic (an outcome not suggested by any phylogenetic analysis we know), we expect that different lineages will have met these challenges with common morphological themes whose distinctive details could be overlooked if one were not considering the possibility of convergence.

At this point we are left with a large number of questions relevant to other members of this symposium. It would be very interesting to know what genes control chaetal form and position so that we could understand how they differ or are the same among the major clades of polychaetes. Is there a commonality in the gene sequences for different hook morphologies that parallel the observations made by Bartolomaeus (1995, 1998) and Bartolomaeus and Meyer (1997)? How similar are the corresponding sequences for the varieties of compound chaetae? Knowing what genes are important in the development and patterning of chaetae (and the mode of their control) will likely provide independent and powerful data that will help us understand the evolution of this important group.

Thanks to Ken Halanych for organizing the symposium and to SICB and the National Science Foundation for their support of the symposium and portions of this research. We appreciate the comments of two anonymous reviewers and the assistance of Meg Spencer in accessing references and Sarah Berke in processing the figures. The manuscript was improved by stimulating conversations with David Wethey, Brian Clark, and Doug Erwin.

Conflict of interest: none declared.

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Author notes

From the symposium “WormNet: Recent Advances in Annelid Systematics, Development, and Evolution” presented at the annual meeting of the Society for Integrative and Comparative Biology, January 4–8, 2005, at San Diego, California.