Despite the acknowledged importance of small copepods of the genus Oithona in marine pelagic ecosystems, there is little information about their ecological role, potential food resources and egg production rates (EPR) in tropical environments. In the present study, feeding and EPR of adult females of two species of Oithona were determined in two different tropical marine food webs in North Queensland, Australia, during the 2011 austral autumn. Oithona attenuata was studied in the waters of the Great Barrier Reef lagoon, and Oithona dissimilis was studied in a mangrove area. Oithona spp. ingested dinoflagellates and ciliates preferentially to other prey items of the nano- and microplankton assemblage. Oithona spp. clearance rates on dinoflagellates and ciliates ranged from 3.7 to 10.4 mL female−1 day−1, and from 4.3 to 18.1 mL female−1 day−1, respectively. The daily body carbon ingested per female was <1% when feeding on dinoflagellates, and varied from 1 to 10% when feeding on ciliates. Our results suggest that Oithona spp. feed on small flagellates (5–20 µm), although the contribution of carbon to the diet was low (2.5–3.2% body carbon). Egg production and weight-specific EPR ranged from 0.22 to 3.34 eggs female−1 day−1, and 0.2–4.5% day−1 respectively. The ingestion rates measured in all the feeding experiments were too low to sustain metabolic and egg production costs, indicating that other food resources, not considered in this study, might contribute significantly to the diet of Oithona spp. in tropical environments.

## INTRODUCTION

Although the importance of small species of copepods (<1 mm) in marine ecosystems is now widely accepted (Turner, 2004), their function in plankton communities is not completely understood. Historically, marine zooplankton studies have focused on large organisms due to the use of plankton nets with relatively coarse mesh sizes (>200 µm; Gallienne and Robins, 2001); thus, knowledge about small copepods is still less than for larger species, particularly in terms of feeding and reproductive ecology. In tropical systems, small copepods are often the dominant zooplankton (Hopcroft et al., 1998) and have an important role in regenerating and exporting nutrients (McKinnon and Ayukai, 1996).

The small cyclopoid copepod genus Oithona is one of the most widespread and abundant in temperate and polar seas (Gallienne and Robins, 2001; Hopcroft et al., 2005; Castellani et al., 2007), as well as in tropical waters (Hopcroft et al., 1998; Satapoomin et al., 2004). In the tropics, Oithona spp. are extremely abundant in neritic areas (Rezai et al., 2004; Chew and Chong, 2011); and in Australian waters, Oithona is one of the most abundant copepod genera in coastal waters of the Great Barrier Reef (McKinnon and Thorrold, 1993; McKinnon et al., 2005) and is also dominant in mangrove habitats (Robertson et al., 1988; McKinnon and Klumpp, 1998; Duggan et al., 2008). Despite the abundance and important ecological role of Oithona spp. in the function of marine tropical ecosystems, there is limited information on their biology and ecology.

Feeding studies on Oithona spp. using cultured prey or natural assemblages have revealed an omnivorous diet (Nakamura and Turner, 1997). Oithona spp. have been reported to prefer motile to non-motile prey (Uchima and Hirano, 1986; Svensen and Kiørboe, 2000; Henriksen et al., 2007), feeding primarily on ciliates and dinoflagellates (Atkinson, 1995; Castellani et al., 2005a; Zamora-Terol et al., 2013) and able to feed carnivorously on copepod nauplii (Marshall and Orr, 1966; Lampitt, 1978). Phytoplankton, particularly diatoms and small flagellates, have been occasionally reported to make up a large fraction of the natural diet of Oithona spp. (Atkinson, 1996; Calbet et al., 2000). Contradictory results between different investigations indicate that many aspects of the natural diet of Oithona spp. remain unclear. Furthermore, the role of Oithona spp. as a link between the microbial and classical food web, although accepted, is not well parameterized, which limits our understanding of their trophic role in pelagic food webs.

The number of studies of Oithona spp. in tropical waters is very scarce, and mainly limited to aspects of composition of the copepod community and abundance (Rezai et al., 2004; Chew and Chong, 2011). To our knowledge, only Calbet et al. (Calbet et al., 2000) and McKinnon and Klumpp (McKinnon and Klumpp, 1998) have investigated feeding aspects of species of Oithona in subtropical and tropical waters, respectively. A few investigations have addressed aspects of the fecundity of Oithona spp. (Hopcroft and Roff, 1996; McKinnon and Klumpp, 1998), but the relative importance of different food items within the natural diet of Oithona spp. and their direct contribution to egg production still remains uncertain. Previous investigations, mostly from cold areas, described year-round reproduction (Dvoretsky and Dvoretsky, 2009) and low metabolic requirements for reproduction (Zamora-Terol et al., 2013). The key to the success of Oithona stems from its ability to survive and reproduce in unfavourable conditions; but the extent and effect of food limitation on the egg production of Oithona spp. in oligotrophic areas is not yet clear.

In this study, we investigated the feeding and fecundity of the genus Oithona in tropical waters of Australia, and assessed if the feeding and egg production patterns from arctic and temperate areas are similar to tropical ones.

## METHOD

### Study site and sampling

This study was conducted in two different ecosystems on the North Queensland coast of Australia: (i) waters of the Great Barrier Reef (GBR) lagoon, where there is a transition between the oceanic oligotrophic clear waters of the Coral Sea and turbid waters near the coast; and (ii) a mangrove system situated in Townsville, North Queensland. The fieldwork in the GBR lagoon took place on board RV Cape Ferguson in the Whitsunday Islands during the period 24 March–6 April 2011; whereas the fieldwork in the mangroves took place in the estuarine inlet of Ross Creek (sampled at Lowth's Bridge, Townsville), during April–June 2011 (Fig. 1). In the GBR lagoon, 11 stations were sampled during the cruise, but feeding experiments were only conducted at four stations due to bad weather conditions (Table I); in the mangroves, a single station was sampled twice and two experiments were carried out (Table I).

Table I:

Sampling stations at the Great Barrier Reef lagoon and mangroves in Ross Creek, Townsville

Location Station Coordinates Date Max depth (m) T (°C) Salinity Chl a (µg L−1
Great Barrier Reef lagoon POM476a 20°48.73′S 150°22.67′E 29-March 13 27.2 34.52 2.48
POM480 20°44.28′S 150°48.32′E 30-March 51 27.3 34.90 1.89
POM481 20°37.38′S 150°40.22′E 31-March 53 27.3 34.85 2.04
POM482a 20°54.92′S 150°25.4′E 01-April 17 26.9 33.34 0.98
POM483 20°57.13′S 149°4.81′E 02-April 25.7 27.44 4.65
POM484a 20°9.61′S 148°50.64′E 03-April 13 26.3 30.95 2.28
POM485a 20°19.54′S 148°50.64′E 04-April 13 26.2 30.84 2.43
Ross Creek Lowth's bridgea 19°26.04′S 146°81.82′E 12-May 24.0 ≈ 35b –
Lowth's bridgea 19°26.04′S 146°81.82′E 08-June 24.0 ≈ 35b –
Location Station Coordinates Date Max depth (m) T (°C) Salinity Chl a (µg L−1
Great Barrier Reef lagoon POM476a 20°48.73′S 150°22.67′E 29-March 13 27.2 34.52 2.48
POM480 20°44.28′S 150°48.32′E 30-March 51 27.3 34.90 1.89
POM481 20°37.38′S 150°40.22′E 31-March 53 27.3 34.85 2.04
POM482a 20°54.92′S 150°25.4′E 01-April 17 26.9 33.34 0.98
POM483 20°57.13′S 149°4.81′E 02-April 25.7 27.44 4.65
POM484a 20°9.61′S 148°50.64′E 03-April 13 26.3 30.95 2.28
POM485a 20°19.54′S 148°50.64′E 04-April 13 26.2 30.84 2.43
Ross Creek Lowth's bridgea 19°26.04′S 146°81.82′E 12-May 24.0 ≈ 35b –
Lowth's bridgea 19°26.04′S 146°81.82′E 08-June 24.0 ≈ 35b –

Location, sampling date, station depth, temperature (T), salinity and average chlorophyll a (Chl a) in the water column are indicated.

aFeeding experiment.

bApproximate value from McKinnon and Klumpp, 1998.

Table I:

Sampling stations at the Great Barrier Reef lagoon and mangroves in Ross Creek, Townsville

Location Station Coordinates Date Max depth (m) T (°C) Salinity Chl a (µg L−1
Great Barrier Reef lagoon POM476a 20°48.73′S 150°22.67′E 29-March 13 27.2 34.52 2.48
POM480 20°44.28′S 150°48.32′E 30-March 51 27.3 34.90 1.89
POM481 20°37.38′S 150°40.22′E 31-March 53 27.3 34.85 2.04
POM482a 20°54.92′S 150°25.4′E 01-April 17 26.9 33.34 0.98
POM483 20°57.13′S 149°4.81′E 02-April 25.7 27.44 4.65
POM484a 20°9.61′S 148°50.64′E 03-April 13 26.3 30.95 2.28
POM485a 20°19.54′S 148°50.64′E 04-April 13 26.2 30.84 2.43
Ross Creek Lowth's bridgea 19°26.04′S 146°81.82′E 12-May 24.0 ≈ 35b –
Lowth's bridgea 19°26.04′S 146°81.82′E 08-June 24.0 ≈ 35b –
Location Station Coordinates Date Max depth (m) T (°C) Salinity Chl a (µg L−1
Great Barrier Reef lagoon POM476a 20°48.73′S 150°22.67′E 29-March 13 27.2 34.52 2.48
POM480 20°44.28′S 150°48.32′E 30-March 51 27.3 34.90 1.89
POM481 20°37.38′S 150°40.22′E 31-March 53 27.3 34.85 2.04
POM482a 20°54.92′S 150°25.4′E 01-April 17 26.9 33.34 0.98
POM483 20°57.13′S 149°4.81′E 02-April 25.7 27.44 4.65
POM484a 20°9.61′S 148°50.64′E 03-April 13 26.3 30.95 2.28
POM485a 20°19.54′S 148°50.64′E 04-April 13 26.2 30.84 2.43
Ross Creek Lowth's bridgea 19°26.04′S 146°81.82′E 12-May 24.0 ≈ 35b –
Lowth's bridgea 19°26.04′S 146°81.82′E 08-June 24.0 ≈ 35b –

Location, sampling date, station depth, temperature (T), salinity and average chlorophyll a (Chl a) in the water column are indicated.

aFeeding experiment.

bApproximate value from McKinnon and Klumpp, 1998.

Fig. 1.

Maps showing the areas of sampling in the northeastern coast of Australia. In the right panel, the upper map shows the sampling station in the mangrove area in Townsville; and the lower map shows the stations located in the Whitsunday Islands area in the Great Barrier Reef lagoon.

Fig. 1.

Maps showing the areas of sampling in the northeastern coast of Australia. In the right panel, the upper map shows the sampling station in the mangrove area in Townsville; and the lower map shows the stations located in the Whitsunday Islands area in the Great Barrier Reef lagoon.

In the GBR lagoon, CTD casts at each station were made using a Seabird SBE19+ CTD (Sea-Bird Electronics) fitted with a Western Environmental Testing Laboratory (WET Lab) fluorometer from which water column mean chlorophyll concentrations were estimated. Potential prey of Oithona spp. in the GBR lagoon were collected by sampling subsurface water using a 10 L Niskin bottle, and copepods for experimental work were collected by gentle oblique tows using a 50-µm plankton net in the upper 20 m of the water column (Table I). In the mangroves, seawater containing microplankton was collected with a bucket and gently transferred to a carboy, whereas copepods were collected in short subsurface tows using a 100-µm plankton net. A coarser net was used in the mangroves to avoid high abundance of early stages of Oithona spp. in the samples collected. The copepods were collected in the morning, when the tide was high, to ensure the presence of Oithona species in the samples. Temperature in the mangroves was measured directly with a thermometer; and we referred to the values of salinity from the investigation conducted by McKinnon and Klumpp (McKinnon and Klumpp, 1998) in the same area.

### Feeding experiments in the Great Barrier Reef lagoon

Feeding experiments were conducted with natural particle assemblages. The water collected in the Niskin bottle was gently transferred to a carboy and amended with a nutrient mixture (15 µM NH4Cl and 1 µM Na2HPO4) to compensate for nutrient enrichment due to copepod excretion. This incubation water was then used to fill two replicates for time zero bottles, and three to four replicates for control (without copepods) and experimental (with copepods) bottles.

After the plankton tow, the copepods were immediately transferred into a bucket and transported to the laboratory on board. Adult females of Oithona attenuata were then sorted, washed in filtered seawater, and 30–35 individuals were placed in 500 mL polycarbonate bottles. All bottles were completely filled, and plastic film was placed over the mouth of the bottle to avoid bubbles. Oithona attenuata was chosen for the experiments in the GBR lagoon because it was the dominant Oithona species in the samples. After placing the copepods in the experimental bottles, both experimental and control bottles were incubated on deck for 24 h, and mixed by repeatedly turning upside down every 2 h (daytime) and 4–6 h (night) to avoid sedimentation of cells.

From the time zero bottles, 20 mL was removed for nanoplankton identification using DAPI staining, and 300 mL was fixed with 2% acid Lugol's solution for microplankton identification. Samples for estimation of grazing on the nanoplankton community (i.e. flagellates <10 µm) were fixed with 1% glutaraldehyde and kept at 4°C for at least 6 h. After that time, the samples were filtered onto 2-µm black polycarbonate membrane filters, and stained with the nucleic acid stain DAPI (5 µg mL−1). The filters were mounted on slides with immersion oil, and frozen for later analysis under epifluorescence microscopy. At least 300 nanoplankton cells were counted per sample, and classified by size (<5 and >5 µm).

After incubation, the control and experimental bottles were processed in the same way as the time zero bottles. The adult female Oithona attenuata were gently removed from the experimental bottles (using a 50-µm sieve), and examined under a stereomicroscope to check their viability during the experiment. Then the females were preserved in formaldehyde (4% final concentration) for later sizing and dissection of the egg sacs from egg-carrying females (see below Egg production measurements).

For the estimation of grazing on the microplankton community, 100 mL aliquots of the samples preserved in acid Lugol's solution were settled, and later processed using an inverted microscope. All microplankton cells present in the samples were counted (100–400 cells depending on in situ concentration). Diatoms, dinoflagellates and ciliates were identified (to genus where possible, or to broad groups), counted and classified by 10-µm sizes. Microplankton biovolumes were determined from their linear dimensions and volume equations for appropriate geometric shapes (Hillebrand et al., 1999; Olenina et al., 2006), and finally converted into carbon biomass according to the equations provided by Menden-Deuer and Lessard (Menden-Deuer and Lessard, 2000).

Feeding rates were calculated according to Frost's equations (Frost, 1972) after checking that prey growth rates in experimental bottles were significantly different than those in the control bottles (t-test P < 0.05). Weight-specific ingestion rates were calculated by using the length–weight regressions given by McKinnon and Klumpp (McKinnon and Klumpp, 1998) for adult female Oithona attenuata.

### Feeding experiments in the mangroves

The feeding experiments from Ross Creek were carried out at the Australian Institute of Marine Science in Townsville. Natural nano- and microplankton assemblages were also used in these experiments, and collected using a bucket and then gently transferred to a carboy. The copepods collected were transferred into a bucket filled with surface water, and immediately transported to the laboratory. Once in the laboratory, the nutrient mixture was added (see above), and the incubation water was gently siphoned through a 132-µm mesh to remove juveniles of Oithona spp. (very abundant in the samples) and other potential grazers. The water was then transferred into 250 mL polycarbonate bottles (two replicates for time zero, and four replicates for both control and experimental treatments). Although different species of Oithona were found in the samples (e.g. O. nishidai, O. aruensis, O. attenuata, O. dissimilis), we chose Oithona dissimilis on the basis of its abundance. Batches of 15–19 adult females were sorted and placed in the experimental bottles. The time zero bottles were fixed with 2% acid Lugol's solution, and the control and experimental bottles were placed in a plankton wheel rotating at 0.5 rpm in a constant temperature (25 ± 1°C) room for 24 h, with a natural light cycle. After the incubation time, the samples were fixed, settled and identified in the same way as for the GBR lagoon samples; feeding rates were also calculated in the same way (see above).

### Feeding selection index

Selective feeding by adult female Oithona spp. was assessed by using the electivity index (E*) of Vanderploeg and Scavia (Vanderploeg and Scavia, 1979). The electivity index was calculated as follows:
$Ei∗=Wi−(1/n)Wi+(1/n)$
where n is the total number of prey type in a given experiment, and the coefficient Wi is defined as,
$Wi=Fi∑Fi$

where $Fi$ is the clearance rate of the i food type, and $∑Fi$ is the sum of clearance rates of all food types. The electivity index (E) ranges between −1 and +1, where zero values represent no selective grazing, negative values correspond to avoidance and positive values represent selection. This index was chosen because it is specifically recommended for cases where food types are not equally abundant (Lechowicz, 1982). Selectivity indexes were computed within each feeding experiment based on average clearance values for each prey type.

### Egg production measurements

Zooplankton samples for the estimation of egg production rates (EPR) were taken using a 73-µm plankton net in the GBR lagoon, and a 50-µm plankton net in the mangroves, and preserved in 4% (final concentration) formalin for later analysis. Once in the laboratory, the samples were first split using a Folsom plankton splitter, and then 5 mL aliquots were taken using a Stempel pipette. Adult female Oithona spp. were counted in a mini-Bogorov chamber, and classified into those carrying egg sacs and those not carrying egg sacs.

The percentage of ovigerous females on each sampling day was calculated as the quotient of the estimated abundance of ovigerous females with respect to the total number of females. The abundance of eggs was calculated by multiplying the number of egg sacs by the average number of eggs per sac. The egg sacs found were dissected (10–40 eggs sacs dissected per sample, depending on the abundance) to estimate the mean clutch size.

The average population EPR (eggs female−1 day−1), computed for the ovigerous and non-ovigerous females, were calculated using the egg-ratio method according to the following equation, modified from Uye and Sano (Uye and Sano, 1995):
$EPR=CS×OFTF×D$
where CS is the clutch size (eggs female−1), OF is the number of ovigerous females present in the sample, TF is the total number of females, and D is the development time of the eggs. The egg development time (D) was calculated by using the equation from McKinnon and Klumpp (McKinnon and Klumpp, 1998) for Oithona nishidai (described as Oithona sp. 1) from nearby mangrove creeks in North Queensland as follows:
Weight-specific EPR were calculated using the length–weight regressions given by McKinnon and Klumpp (McKinnon and Klumpp, 1998) for adult female Oithona attenuata, and the egg carbon content estimated from the equation given by Uye and Sano (Uye and Sano, 1995). For this estimation, egg diameter and female prosome length were measured under an inverted microscope, using ×40 magnification. For each experiment, at least 10 egg sacs were dissected, and where possible, all females were sized; mean values were calculated for each experiment. The egg production efficiency, i.e. the fraction of ingested food converted into egg production, was calculated as the quotient of weight-specific egg production and ingestion rate and expressed as a percentage.

## RESULTS

### Environment and plankton community

During the cruise conducted in the GBR lagoon, the temperature was ∼26–27°C, the salinity varied between 28 and 35, and chlorophyll a concentration ranged from 0.98 to 4.65 µg L−1 (Table I). The lowest salinities and highest chlorophyll a concentrations were found in stations located closer to the coast (i.e. POM483, 484 and 485), and the highest chlorophyll a concentration recorded (4.65 µg L−1) occurred at the station closest to Mackay, in the proximity of the mouth of the Pioneer River. In the sampling conducted in Ross Creek, the temperature was 24°C. We did not measure salinity in the mangrove area, but we have provided a reference value from McKinnon and Klumpp (McKinnon and Klumpp, 1998) for the same area (Table I).

In the GBR lagoon, organisms between 2 and 10 µm in size numerically dominated the micro- and nanoplankton community (>500 cells mL−1), whereas dinoflagellates and ciliates were found in low abundances, never reaching more than 1 cell mL−1 (Table II). The mangrove microplankton community was dominated by ciliates, which reached a maximum of 17 cells mL−1. Diatoms were present in both ecosystems, with concentrations of 2–8 cells mL−1 in the GBR lagoon, and 11–20 cells mL−1 in the mangroves.

Table II:

Feeding experiments in the Great Barrier Reef lagoon (GBR lagoon) and mangrove

Location Date Oithona species n Prey Initial concentration

Clearance rate Ingestion rate Daily ration Electivity index
µg C L−1 cells mL−1 mL female−1 day−1 cells female−1 day−1 % body carbon day−1
GBR lagoon 01-April O. attenuata Dinoflagellates 0.30 0.7 ± 0.07 10.4 ± 2.93a 6.3 ± 1.45 0.7 ± 0.13 −0.07
Ciliates 0.66 0.6 ± 0.06 18.1 ± 2.90b 11.1 ± 0.87 1.7 ± 0.13 0.49
Nanoflag. < 5 µm 1.05 352.12 5.0 ± 2.44 1930.4 ± 718.53 1.18 –
Nanoflag. > 5 µm 2.07 92.50 6.1 ± 2.01a 530.2 ± 125.42 2.64 –
03-April O. attenuata Dinoflagellates 0.18 1.0 ± 0.06 6.7 ± 1.98a 6.3 ± 1.40 0.3 ± 0.06 −0.15
Ciliates 0.58 1.0 ± 0.04 10.8 ± 1.25b 10.3 ± 0.87 1.4 ± 0.12 0.41
Nanoflag. < 5 µm 1.82 603.12 negative values – – –
Nanoflag. > 5 µm 1.14 68.25 13.7 ± 6.46a 478.7 ± 195.26 3.18 –
04-April O. attenuata Dinoflagellates 0.29 0.9 ± 0.05 7.3 ± 1.90a 6.6 ± 1.31 0.3 ± 0.06 −0.12
Ciliates 1.20 0.9 ± 0.07 8.5 ± 2.70a 7.3 ± 2.00 1.0 ± 0.29 0.11
Mangrove 12-May O. dissimilis Dinoflagellates 0.18 1.0 ± 0.18 6.3 ± 4.97 – – 0.25
Ciliates 2.69 3.9 ± 0.35 4.3 ± 2.29 – – −0.33
08-June O. dissimilis Dinoflagellates 0.72 10.3 ± 0.27 3.7 ± 1.4 – – −0.57
Ciliates 2.83 15.7 ± 0.73 6.8 ± 1.09b 104.6 ± 11.88 10.1 ± 1.14 0.38
Location Date Oithona species n Prey Initial concentration

Clearance rate Ingestion rate Daily ration Electivity index
µg C L−1 cells mL−1 mL female−1 day−1 cells female−1 day−1 % body carbon day−1
GBR lagoon 01-April O. attenuata Dinoflagellates 0.30 0.7 ± 0.07 10.4 ± 2.93a 6.3 ± 1.45 0.7 ± 0.13 −0.07
Ciliates 0.66 0.6 ± 0.06 18.1 ± 2.90b 11.1 ± 0.87 1.7 ± 0.13 0.49
Nanoflag. < 5 µm 1.05 352.12 5.0 ± 2.44 1930.4 ± 718.53 1.18 –
Nanoflag. > 5 µm 2.07 92.50 6.1 ± 2.01a 530.2 ± 125.42 2.64 –
03-April O. attenuata Dinoflagellates 0.18 1.0 ± 0.06 6.7 ± 1.98a 6.3 ± 1.40 0.3 ± 0.06 −0.15
Ciliates 0.58 1.0 ± 0.04 10.8 ± 1.25b 10.3 ± 0.87 1.4 ± 0.12 0.41
Nanoflag. < 5 µm 1.82 603.12 negative values – – –
Nanoflag. > 5 µm 1.14 68.25 13.7 ± 6.46a 478.7 ± 195.26 3.18 –
04-April O. attenuata Dinoflagellates 0.29 0.9 ± 0.05 7.3 ± 1.90a 6.6 ± 1.31 0.3 ± 0.06 −0.12
Ciliates 1.20 0.9 ± 0.07 8.5 ± 2.70a 7.3 ± 2.00 1.0 ± 0.29 0.11
Mangrove 12-May O. dissimilis Dinoflagellates 0.18 1.0 ± 0.18 6.3 ± 4.97 – – 0.25
Ciliates 2.69 3.9 ± 0.35 4.3 ± 2.29 – – −0.33
08-June O. dissimilis Dinoflagellates 0.72 10.3 ± 0.27 3.7 ± 1.4 – – −0.57
Ciliates 2.83 15.7 ± 0.73 6.8 ± 1.09b 104.6 ± 11.88 10.1 ± 1.14 0.38

The Oithona species, number of replicates (n) and date of the experiments are indicated. The initial prey concentration (in cells and biomass), clearance rates (mL female−1 day−1) and ingestion rates (in cells and daily ration) are shown. The electivity index for ciliates and dinoflagellates is also shown. Asterisks (*) indicate significant feeding rates. Nanoflag.: nanoflagellates.

at-test < 0.05.

bt-test < 0.01.

Table II:

Feeding experiments in the Great Barrier Reef lagoon (GBR lagoon) and mangrove

Location Date Oithona species n Prey Initial concentration

Clearance rate Ingestion rate Daily ration Electivity index
µg C L−1 cells mL−1 mL female−1 day−1 cells female−1 day−1 % body carbon day−1
GBR lagoon 01-April O. attenuata Dinoflagellates 0.30 0.7 ± 0.07 10.4 ± 2.93a 6.3 ± 1.45 0.7 ± 0.13 −0.07
Ciliates 0.66 0.6 ± 0.06 18.1 ± 2.90b 11.1 ± 0.87 1.7 ± 0.13 0.49
Nanoflag. < 5 µm 1.05 352.12 5.0 ± 2.44 1930.4 ± 718.53 1.18 –
Nanoflag. > 5 µm 2.07 92.50 6.1 ± 2.01a 530.2 ± 125.42 2.64 –
03-April O. attenuata Dinoflagellates 0.18 1.0 ± 0.06 6.7 ± 1.98a 6.3 ± 1.40 0.3 ± 0.06 −0.15
Ciliates 0.58 1.0 ± 0.04 10.8 ± 1.25b 10.3 ± 0.87 1.4 ± 0.12 0.41
Nanoflag. < 5 µm 1.82 603.12 negative values – – –
Nanoflag. > 5 µm 1.14 68.25 13.7 ± 6.46a 478.7 ± 195.26 3.18 –
04-April O. attenuata Dinoflagellates 0.29 0.9 ± 0.05 7.3 ± 1.90a 6.6 ± 1.31 0.3 ± 0.06 −0.12
Ciliates 1.20 0.9 ± 0.07 8.5 ± 2.70a 7.3 ± 2.00 1.0 ± 0.29 0.11
Mangrove 12-May O. dissimilis Dinoflagellates 0.18 1.0 ± 0.18 6.3 ± 4.97 – – 0.25
Ciliates 2.69 3.9 ± 0.35 4.3 ± 2.29 – – −0.33
08-June O. dissimilis Dinoflagellates 0.72 10.3 ± 0.27 3.7 ± 1.4 – – −0.57
Ciliates 2.83 15.7 ± 0.73 6.8 ± 1.09b 104.6 ± 11.88 10.1 ± 1.14 0.38
Location Date Oithona species n Prey Initial concentration

Clearance rate Ingestion rate Daily ration Electivity index
µg C L−1 cells mL−1 mL female−1 day−1 cells female−1 day−1 % body carbon day−1
GBR lagoon 01-April O. attenuata Dinoflagellates 0.30 0.7 ± 0.07 10.4 ± 2.93a 6.3 ± 1.45 0.7 ± 0.13 −0.07
Ciliates 0.66 0.6 ± 0.06 18.1 ± 2.90b 11.1 ± 0.87 1.7 ± 0.13 0.49
Nanoflag. < 5 µm 1.05 352.12 5.0 ± 2.44 1930.4 ± 718.53 1.18 –
Nanoflag. > 5 µm 2.07 92.50 6.1 ± 2.01a 530.2 ± 125.42 2.64 –
03-April O. attenuata Dinoflagellates 0.18 1.0 ± 0.06 6.7 ± 1.98a 6.3 ± 1.40 0.3 ± 0.06 −0.15
Ciliates 0.58 1.0 ± 0.04 10.8 ± 1.25b 10.3 ± 0.87 1.4 ± 0.12 0.41
Nanoflag. < 5 µm 1.82 603.12 negative values – – –
Nanoflag. > 5 µm 1.14 68.25 13.7 ± 6.46a 478.7 ± 195.26 3.18 –
04-April O. attenuata Dinoflagellates 0.29 0.9 ± 0.05 7.3 ± 1.90a 6.6 ± 1.31 0.3 ± 0.06 −0.12
Ciliates 1.20 0.9 ± 0.07 8.5 ± 2.70a 7.3 ± 2.00 1.0 ± 0.29 0.11
Mangrove 12-May O. dissimilis Dinoflagellates 0.18 1.0 ± 0.18 6.3 ± 4.97 – – 0.25
Ciliates 2.69 3.9 ± 0.35 4.3 ± 2.29 – – −0.33
08-June O. dissimilis Dinoflagellates 0.72 10.3 ± 0.27 3.7 ± 1.4 – – −0.57
Ciliates 2.83 15.7 ± 0.73 6.8 ± 1.09b 104.6 ± 11.88 10.1 ± 1.14 0.38

The Oithona species, number of replicates (n) and date of the experiments are indicated. The initial prey concentration (in cells and biomass), clearance rates (mL female−1 day−1) and ingestion rates (in cells and daily ration) are shown. The electivity index for ciliates and dinoflagellates is also shown. Asterisks (*) indicate significant feeding rates. Nanoflag.: nanoflagellates.

at-test < 0.05.

bt-test < 0.01.

In the copepod community of the GBR lagoon, different species of Oithona were identified, such as O. attenuata, O. nana, O. oculata, O. setigera and O. simplex. Oncaea spp. and unidentified nauplii were also present in high abundances in the samples, together with Microsetella spp. and unidentified calanoid copepods. On the other hand, in the mangrove ecosystem, the plankton community was totally dominated by Oithona spp. (O. aruensis, O. attenuata, O. dissimilis and O. nishidai), with very high abundances of juveniles.

### Feeding rates

In the GBR lagoon significant ingestion (t-test P < 0.05) of ciliates and dinoflagellates was found (Table II). Clearance rates of Oithona attenuata in the GBR lagoon ranged from approximately 8 to 18 mL female−1 day−1 on ciliates, and from 7 to 10 mL female−1 day−1 on dinoflagellates (Fig. 2; Table II). Ingestion rates in terms of number of cells ranged from 10 to 105 ciliates female−1 day−1, and from 5 to 36 dinoflagellates female−1 day−1 (Table II), whereas when expressed as a percentage of body carbon ingested, daily rations were generally low, ranging from 1 to 1.7% day−1 for ciliates, and 0.3 to 0.7% day−1 for dinoflagellates (Fig. 3; Table II). No feeding on diatoms was observed (data not shown). Feeding on small flagellates (<10 µm) was only significant on prey items larger than 5 µm, whereas no significant ingestion was found on nanoplankton of 2–5 µm (Table II). Clearance rates reported for nanoflagellates >5 µm varied from 6 to 14 mL female−1 day−1, accounting for 2.5–3.2% of body carbon ingested (Table II).

Fig. 2.

Oithona spp. clearance rate (F, cells female−1 day−1) on ciliates and dinoflagellates in the Great Barrier Reef (GBR) lagoon (black symbols) and mangrove (white symbols). Each point represents the mean value (±1 SE). Asterisks (*) indicate statistically significant grazing rates (t-test, P< 0.05). Power equation fits are shown. Cil-C, ciliate concentration; Dino-C, dinoflagellate concentration.

Fig. 2.

Oithona spp. clearance rate (F, cells female−1 day−1) on ciliates and dinoflagellates in the Great Barrier Reef (GBR) lagoon (black symbols) and mangrove (white symbols). Each point represents the mean value (±1 SE). Asterisks (*) indicate statistically significant grazing rates (t-test, P< 0.05). Power equation fits are shown. Cil-C, ciliate concentration; Dino-C, dinoflagellate concentration.

Fig. 3.

Weight-specific rates of Oithona spp. (A) Daily ration (DR, % body carbon ingested) on protozooplankton biomass (Proto-b, µg C L−1); (B) Relationship between weight-specific egg production (SEPR, % day−1) and weight-specific ingestion (WSIR, % day−1) on protozooplankton. Linear regression fit to the data is shown. Black and white symbols correspond to the GBR lagoon and mangrove experiments, respectively. Dashed lines show linear regression.

Fig. 3.

Weight-specific rates of Oithona spp. (A) Daily ration (DR, % body carbon ingested) on protozooplankton biomass (Proto-b, µg C L−1); (B) Relationship between weight-specific egg production (SEPR, % day−1) and weight-specific ingestion (WSIR, % day−1) on protozooplankton. Linear regression fit to the data is shown. Black and white symbols correspond to the GBR lagoon and mangrove experiments, respectively. Dashed lines show linear regression.

In the feeding experiments conducted in the mangroves, in most cases clearance rates were positive but not statistically significant; only in one case were ciliates significantly ingested (104.6 cells female−1 day−1, which accounted for a daily ration of 10.1% day−1; Table II). Clearance rates of Oithona dissimilis in the mangroves ranged from 4.3 to 6.8 mL female−1 day−1 for ciliates, and from 3.7 to 6.3 mL female−1 day−1 for dinoflagellates (Fig. 2; Table II). Ingestion rates varied between 22.6 and 104.6 cells female−1 day−1 for ciliates, and from 3.7 to 6.3 cells female−1 day−1 for dinoflagellates (taking into account the non-significant values; Fig. 2; Table II).

The results of the electivity index to check selective feeding on protozooplankton showed a positive selection for ciliates and a negative selection for dinoflagellates in all feeding experiments (Table II). There is one exception: in the first experiment in the mangroves, dinoflagellates were selected and not ciliates. However, in this experiment, it should be mentioned that we found non-significant feeding rates for all of the prey items.

### Egg production

Egg production of Oithona attenuata in the GBR lagoon ranged between 0.22 and 1.44 eggs female−1 day−1, corresponding to weight-specific EPR of 0.2–1.6% day−1 (Table III). Females carrying egg sacs were not present in the tows, so the percentage of ovigerous females in the population had to be estimated from the detached egg sacs found in the plankton net, and this estimation accounted for 1.5–10.3% ovigerous females (Table III). Mean clutch sizes ranged between 14 and 15 eggs female−1, reaching maximum values of 30 eggs female−1. The mean prosome length of the females varied from 423 to 435 µm, with corresponding carbon contents of 0.45 to 0.49 µg (Table III). EPR of O. dissimilis in the mangroves were higher than those of O. attenuata in the GBR lagoon, with a range of 1.69–3.34 eggs female−1 day−1, and weight-specific EPR in the range 2.2–4.5% day−1. The mean clutch size varied from 10 to 18 eggs female−1, and maximum clutch sizes were up to 24 eggs female−1. Ovigerous females were present in the tows, and the percentage in the population varied between 35.7 and 39%. The mean prosome length of the females was 389–394 µm, with female body carbon of 0.37–0.38 µg (Table III).

Table III:

Fecundity of Oithona spp. in the Great Barrier Reef lagoon (GBR lagoon) and mangrove

Location Date Oithona species PL (µm) Female BW (µg C) Ovigerous females (%) Avergage clutch size (eggs female−1 ± SE) Max. clutch size (eggs female−1EPR (eggs female−1day−1SEPR (% day−1GGE (%)
GBR lagoon 01-April O. attenuata 423 0.45 10.3a 14 ± 0.9 30 1.44 1.6 67
03-April O. attenuata 435 0.49 1.5a 15 ± 2.2 24 0.22 0.2 13
04-April O. attenuata 426 0.46 2.5a 14 ± 1.2 20 0.34 0.4 29
Mangrove 12-May O. dissimilis 394 0.38 39 10 ± 0.6 16 1.69 2.2 57
08-June O. dissimilis 389 0.37 35.7 18 ± 0.5 24 3.34 4.5 40
Location Date Oithona species PL (µm) Female BW (µg C) Ovigerous females (%) Avergage clutch size (eggs female−1 ± SE) Max. clutch size (eggs female−1EPR (eggs female−1day−1SEPR (% day−1GGE (%)
GBR lagoon 01-April O. attenuata 423 0.45 10.3a 14 ± 0.9 30 1.44 1.6 67
03-April O. attenuata 435 0.49 1.5a 15 ± 2.2 24 0.22 0.2 13
04-April O. attenuata 426 0.46 2.5a 14 ± 1.2 20 0.34 0.4 29
Mangrove 12-May O. dissimilis 394 0.38 39 10 ± 0.6 16 1.69 2.2 57
08-June O. dissimilis 389 0.37 35.7 18 ± 0.5 24 3.34 4.5 40

Prosome length (PL, µm) and female body weight (female BW, µg C) for each species are indicated. The percentage of ovigerous females, average (eggs per female ± SE) and maximum clutch size, egg production rate (EPR, eggs per female and day), and weight-specific egg production rate (SEPR, % day−1), egg production efficiency (i.e. GGE, %) are shown.

aOvigerous females estimated only from dettached eggs.

Table III:

Fecundity of Oithona spp. in the Great Barrier Reef lagoon (GBR lagoon) and mangrove

Location Date Oithona species PL (µm) Female BW (µg C) Ovigerous females (%) Avergage clutch size (eggs female−1 ± SE) Max. clutch size (eggs female−1EPR (eggs female−1day−1SEPR (% day−1GGE (%)
GBR lagoon 01-April O. attenuata 423 0.45 10.3a 14 ± 0.9 30 1.44 1.6 67
03-April O. attenuata 435 0.49 1.5a 15 ± 2.2 24 0.22 0.2 13
04-April O. attenuata 426 0.46 2.5a 14 ± 1.2 20 0.34 0.4 29
Mangrove 12-May O. dissimilis 394 0.38 39 10 ± 0.6 16 1.69 2.2 57
08-June O. dissimilis 389 0.37 35.7 18 ± 0.5 24 3.34 4.5 40
Location Date Oithona species PL (µm) Female BW (µg C) Ovigerous females (%) Avergage clutch size (eggs female−1 ± SE) Max. clutch size (eggs female−1EPR (eggs female−1day−1SEPR (% day−1GGE (%)
GBR lagoon 01-April O. attenuata 423 0.45 10.3a 14 ± 0.9 30 1.44 1.6 67
03-April O. attenuata 435 0.49 1.5a 15 ± 2.2 24 0.22 0.2 13
04-April O. attenuata 426 0.46 2.5a 14 ± 1.2 20 0.34 0.4 29
Mangrove 12-May O. dissimilis 394 0.38 39 10 ± 0.6 16 1.69 2.2 57
08-June O. dissimilis 389 0.37 35.7 18 ± 0.5 24 3.34 4.5 40

Prosome length (PL, µm) and female body weight (female BW, µg C) for each species are indicated. The percentage of ovigerous females, average (eggs per female ± SE) and maximum clutch size, egg production rate (EPR, eggs per female and day), and weight-specific egg production rate (SEPR, % day−1), egg production efficiency (i.e. GGE, %) are shown.

aOvigerous females estimated only from dettached eggs.

There was a linear relationship between the weight-specific ingestion of protozooplankton and the weight-specific EPR of Oithona spp. (Fig. 3). An average egg production efficiency (i.e. gross growth efficiency, GGE) of 40% was estimated from the slope of the linear regression between the weight-specific ingestion of protozooplankton and the weight-specific EPR (Fig. 3).

## DISCUSSION

### Feeding rates of Oithona spp.

From our feeding experiments, we expected to clarify the natural diet of Oithona spp. in tropical areas. We have confirmed the preference of Oithona spp. for protozooplankton, but overall, ingestion rates were too low to cover basic metabolic and egg production costs.

When food resources were low, Oithona attenuata ingested dinoflagellates at similar rates to ciliates in the GBR lagoon (i.e. 35–45% of the protozooplankton diet was composed of dinoflagellates). Higher feeding rates of Oithona spp. on ciliates found in earlier studies (Nakamura and Turner, 1997; Castellani et al., 2005a; Zamora-Terol et al., 2013) have led to speculation about active selection for ciliates. This argument is supported by the selection index calculated in this study (Table II). In contrast, O. similis cleared heterotrophic dinoflagellates at rates 1.3 times higher than ciliates in the Southern Ocean (Atkinson, 1996); and Atienza et al. (Atienza et al., 2006) found that Mediterranean O. nana preferred ciliates under some conditions, and in others cleared both ciliates and dinoflagellates at similar rates. The potential selection for ciliates might be determined by the strict ambush feeding behaviour of Oithona spp. Ambush copepods hang motionless in the water and attack prey that swim through their perceptive sphere (Svensen and Kiørboe, 2000). The fast swimming of ciliates increases the encounter rate with copepods, furthermore, ciliates generate strong hydrodynamic signals making them easier to detect by mechanoreceptors on the antennule (Jonsson and Tiselius, 1990; Atkinson, 1995; Kiørboe and Visser, 1999).

Despite agreement on the importance of protozooplankton in the diet of Oithona spp. in most investigations (e.g. Nishibe et al., 2010; Zamora-Terol et al., 2013), calculations of daily rations based only on protozooplankton ingestion have sometimes resulted in carbon feeding rates too low to sustain basal metabolism. These results have led to suggestions about the potential contribution of other prey to the diet of Oithona spp., necessary to cover the metabolic expenses and growth (Drits and Semenova, 1984; McKinnon and Klumpp, 1998; Castellani et al., 2005a).

In this study, we expected to find positive ingestion rates of Oithona spp. on small cells (<10 µm) such as nanoflagellates because of the low availability of other potential prey (i.e. ciliates and dinoflagellates). The ability of Oithona to capture and ingest cells <5 μm has been demonstrated in laboratory studies (Eaton, 1971; Lampitt and Gamble, 1982), but our results found no ingestion of cells <5 µm in the GBR lagoon. This result is supported by the earlier study by Nakamura and Turner (Nakamura and Turner, 1997), who reported that Oithona similis did not feed on cells 2–8 μm. On the other hand, Calbet et al. (Calbet et al., 2000) and Vargas and González (Vargas and González, 2004) reported that nanoflagellates were the main prey ingested by a number of species of Oithona from subtropical environments. Our results are not yet conclusive, since the samples from the GBR lagoon were very “dirty”, with a lot of sediment accumulated on the filter, and it is possible that we may have underestimated clearance on smaller cells. Consequently, we cannot completely discard the possibility that Oithona spp. are able to feed on small cells (<10 µm), but this needs to be confirmed in future experiments.

Ingestion of nauplii by Oithona spp. has been widely reported (Marshall and Orr, 1966; Lampitt, 1978; Lampitt and Gamble, 1982; Drits and Semenova, 1984; Uchima and Hirano, 1986; Nakamura and Turner, 1997); thus sporadic feeding of Oithona spp. on nauplii might contribute to achieving daily rations necessary to cover metabolic expenses. In the laboratory, we have observed ingestion of nauplii by O. dissimilis and O. aruensis from the mangroves, although we did not quantify this ingestion. This observation is supported by the investigation of McKinnon and Klumpp (McKinnon and Klumpp, 1998), who reported cannibalistic feeding of Oithona spp. on paracalanid nauplii in different rivers of North Queensland. This carnivorous diet may be especially important in environments with high abundance of nauplii, such as neritic areas (Böttjer et al., 2010). Furthermore, feeding on nauplii would make components of the pico- and nanoplankton available for higher trophic levels (Roff et al., 1995), thus enhancing the role of Oithona linking the microbial and the classical food web.

The mechanosensory capabilities of Oithona spp., together with the results reported in our feeding experiments, allow us to hypothesize that diatoms are unlikely to be a major component in the natural diet of Oithona spp. in the environments investigated. The sinking behaviour of diatoms, and the signals they emit, are not in the hyrdromechanical threshold detection of Oithona spp. (Kiørboe and Visser, 1999; Svensen and Kiørboe, 2000). It should be mentioned that when conducting feeding experiments with the bottle incubation method, there is the possibility of bias in the estimation of copepod feeding rates due to trophic cascade effects caused by the removal of microheterotrophs by copepods (Nejstgaard et al., 1997). However, we consider it unlikely that this caused a high bias in our feeding rates because neither the concentration of ciliates nor the effect of copepod grazing on ciliates were sufficient to significantly affect our estimation of copepod grazing rate. An estimate for the release of microzooplankton grazing pressure has been considered to increase the copepod grazing estimates by ∼2030% (Saiz and Calbet, 2011).

### Egg production

Overall, clutch size and EPR reported for Oithona spp. in this study fall within the range of previous reported values from field studies of Oithona from tropical and subtropical areas (Table IV). However, the percentage of ovigerous females in our study was rather low (1.5–39%), and maximum EPR did not achieve values reported in previous investigations with similar-sized Oithona species (Uye and Sano, 1995; McKinnon and Klumpp, 1998), probably because of the low proportion of ovigerous females in the population. When egg sacs are in a late developmental stage, mechanical stress can cause detachment from the body of the females (S. Zamora-Terol, pers. obs.). Thus, the turbulence in the water due to stormy conditions during our cruise could have caused the loss of egg sacs from the adult females. When egg sacs of Oithona spp. are lost during collection, they are not immediately replaced (Hopcroft and Roff, 1996), further contributing to bias in our estimation of the number of ovigerous females.

Table IV:

Comparison of fecundity parameters of different species of Oithona from tropical and subtropical areas

Species Location Area T (°C) Chl a (µg L−1PL (µm) Weight (µg C) Clutch size

EPR SEPR References
Eggs per sac Eggs per female Eggs female−1 day−1 day−1
O. aruensis Haughton River Tropical – – 280 0.30 3.1–9.3 – 0.8–12.3 0.01–0.12 McKinnon and Klumpp, 1998
Oithona sp. 1a – – 320 0.61 3.2–9.5 – 2.3–15.3 0.02–0.13
O. aruensis Cape York Rivers Tropical 22.2–29.5 – 280 0.30 3–6.7 – 0.87–5.25 0.015–0.088
Oithona sp. 1a 22.2–30.6 – 320 0.61 3.9–6 – 2.28–5.17 0.019–0.042
Oithona sp. 2 24.7–28.9 – 280 0.30 3.3–7.1 – 0.08–11.85 0.001–0.22
O. attenuata Exmouth Gulf Tropical 21.3–23.2 0.15–0.35 340 0.55 7–18 – 3.2 0.023 McKinnon and Ayukai, 1996
O. simplex 21.3–23.2 0.15–0.35 270 0.31 4–8 – 2.4 0.069
O. davisae Fukyama Harbour Subtropical 10–28 1.00–3.23 276–331 0.20–0.25 – 11–28.5 2.6–11.6 – Uye and Sano, 1995
O. brevicornis Grand-Lahou Lagoon Tropical 20–30 – – – – 10–16 0.11–3.33 0.001–0.046 Etilé et al., 2012
O. plumifera Andaman Sea Tropical 28–30b 0.19 559 ± 1.7 0.81 – – 0.2–11.2 (2.4 ± 0.2) 0.05 ± 0.01 Satapoomin et al., 2004
O. plumifera Jamaica Tropical 28 ± 1.5 0.11–2.6 – – – 5–21 (12.9 ± 3.7) – – Hopcroft and Roff, 1996
O. nana 28 ± 1.5 0.11–2.6 – – – 9–26 (17–20) – –
O. simplex 28 ± 1.5 0.11–2.6 – – – 4–10 (7) – –
O. nana Jamaica Tropical 28 – – 0.53c (0.21) – – – 0.31–0.37 Hopcroft and Roff, 1998
O. plumifera – – 1.9c (0.76) – – – 0.56
O. simplex – – 0.55c (0.22) – – – 0.21
O. attenuata GBR lagoon Tropical 27–28 0.98–2.5 423–435 0.47 3–15 5–30 0.22–1.44 0.002–0.016 This study
O. dissimilis Ross Creek 24–25 – 389–394 0.38 6–12 6–24 1.69–3.34 0.022–0.045
Species Location Area T (°C) Chl a (µg L−1PL (µm) Weight (µg C) Clutch size

EPR SEPR References
Eggs per sac Eggs per female Eggs female−1 day−1 day−1
O. aruensis Haughton River Tropical – – 280 0.30 3.1–9.3 – 0.8–12.3 0.01–0.12 McKinnon and Klumpp, 1998
Oithona sp. 1a – – 320 0.61 3.2–9.5 – 2.3–15.3 0.02–0.13
O. aruensis Cape York Rivers Tropical 22.2–29.5 – 280 0.30 3–6.7 – 0.87–5.25 0.015–0.088
Oithona sp. 1a 22.2–30.6 – 320 0.61 3.9–6 – 2.28–5.17 0.019–0.042
Oithona sp. 2 24.7–28.9 – 280 0.30 3.3–7.1 – 0.08–11.85 0.001–0.22
O. attenuata Exmouth Gulf Tropical 21.3–23.2 0.15–0.35 340 0.55 7–18 – 3.2 0.023 McKinnon and Ayukai, 1996
O. simplex 21.3–23.2 0.15–0.35 270 0.31 4–8 – 2.4 0.069
O. davisae Fukyama Harbour Subtropical 10–28 1.00–3.23 276–331 0.20–0.25 – 11–28.5 2.6–11.6 – Uye and Sano, 1995
O. brevicornis Grand-Lahou Lagoon Tropical 20–30 – – – – 10–16 0.11–3.33 0.001–0.046 Etilé et al., 2012
O. plumifera Andaman Sea Tropical 28–30b 0.19 559 ± 1.7 0.81 – – 0.2–11.2 (2.4 ± 0.2) 0.05 ± 0.01 Satapoomin et al., 2004
O. plumifera Jamaica Tropical 28 ± 1.5 0.11–2.6 – – – 5–21 (12.9 ± 3.7) – – Hopcroft and Roff, 1996
O. nana 28 ± 1.5 0.11–2.6 – – – 9–26 (17–20) – –
O. simplex 28 ± 1.5 0.11–2.6 – – – 4–10 (7) – –
O. nana Jamaica Tropical 28 – – 0.53c (0.21) – – – 0.31–0.37 Hopcroft and Roff, 1998
O. plumifera – – 1.9c (0.76) – – – 0.56
O. simplex – – 0.55c (0.22) – – – 0.21
O. attenuata GBR lagoon Tropical 27–28 0.98–2.5 423–435 0.47 3–15 5–30 0.22–1.44 0.002–0.016 This study
O. dissimilis Ross Creek 24–25 – 389–394 0.38 6–12 6–24 1.69–3.34 0.022–0.045

T, temperature; PL, prosome legth; EPR, egg production rate; SEPR, weight-specific egg production rate. Numbers in brackets indicate mean.

aOithona sp. 1, described later as O. nishidai (McKinnon, 2000).

bSurface temperature.

cAsh free dry weight (AFDW), in brackets recalculated assuming carbon as 40% of AFDW.

Table IV:

Comparison of fecundity parameters of different species of Oithona from tropical and subtropical areas

Species Location Area T (°C) Chl a (µg L−1PL (µm) Weight (µg C) Clutch size

EPR SEPR References
Eggs per sac Eggs per female Eggs female−1 day−1 day−1
O. aruensis Haughton River Tropical – – 280 0.30 3.1–9.3 – 0.8–12.3 0.01–0.12 McKinnon and Klumpp, 1998
Oithona sp. 1a – – 320 0.61 3.2–9.5 – 2.3–15.3 0.02–0.13
O. aruensis Cape York Rivers Tropical 22.2–29.5 – 280 0.30 3–6.7 – 0.87–5.25 0.015–0.088
Oithona sp. 1a 22.2–30.6 – 320 0.61 3.9–6 – 2.28–5.17 0.019–0.042
Oithona sp. 2 24.7–28.9 – 280 0.30 3.3–7.1 – 0.08–11.85 0.001–0.22
O. attenuata Exmouth Gulf Tropical 21.3–23.2 0.15–0.35 340 0.55 7–18 – 3.2 0.023 McKinnon and Ayukai, 1996
O. simplex 21.3–23.2 0.15–0.35 270 0.31 4–8 – 2.4 0.069
O. davisae Fukyama Harbour Subtropical 10–28 1.00–3.23 276–331 0.20–0.25 – 11–28.5 2.6–11.6 – Uye and Sano, 1995
O. brevicornis Grand-Lahou Lagoon Tropical 20–30 – – – – 10–16 0.11–3.33 0.001–0.046 Etilé et al., 2012
O. plumifera Andaman Sea Tropical 28–30b 0.19 559 ± 1.7 0.81 – – 0.2–11.2 (2.4 ± 0.2) 0.05 ± 0.01 Satapoomin et al., 2004
O. plumifera Jamaica Tropical 28 ± 1.5 0.11–2.6 – – – 5–21 (12.9 ± 3.7) – – Hopcroft and Roff, 1996
O. nana 28 ± 1.5 0.11–2.6 – – – 9–26 (17–20) – –
O. simplex 28 ± 1.5 0.11–2.6 – – – 4–10 (7) – –
O. nana Jamaica Tropical 28 – – 0.53c (0.21) – – – 0.31–0.37 Hopcroft and Roff, 1998
O. plumifera – – 1.9c (0.76) – – – 0.56
O. simplex – – 0.55c (0.22) – – – 0.21
O. attenuata GBR lagoon Tropical 27–28 0.98–2.5 423–435 0.47 3–15 5–30 0.22–1.44 0.002–0.016 This study
O. dissimilis Ross Creek 24–25 – 389–394 0.38 6–12 6–24 1.69–3.34 0.022–0.045
Species Location Area T (°C) Chl a (µg L−1PL (µm) Weight (µg C) Clutch size

EPR SEPR References
Eggs per sac Eggs per female Eggs female−1 day−1 day−1
O. aruensis Haughton River Tropical – – 280 0.30 3.1–9.3 – 0.8–12.3 0.01–0.12 McKinnon and Klumpp, 1998
Oithona sp. 1a – – 320 0.61 3.2–9.5 – 2.3–15.3 0.02–0.13
O. aruensis Cape York Rivers Tropical 22.2–29.5 – 280 0.30 3–6.7 – 0.87–5.25 0.015–0.088
Oithona sp. 1a 22.2–30.6 – 320 0.61 3.9–6 – 2.28–5.17 0.019–0.042
Oithona sp. 2 24.7–28.9 – 280 0.30 3.3–7.1 – 0.08–11.85 0.001–0.22
O. attenuata Exmouth Gulf Tropical 21.3–23.2 0.15–0.35 340 0.55 7–18 – 3.2 0.023 McKinnon and Ayukai, 1996
O. simplex 21.3–23.2 0.15–0.35 270 0.31 4–8 – 2.4 0.069
O. davisae Fukyama Harbour Subtropical 10–28 1.00–3.23 276–331 0.20–0.25 – 11–28.5 2.6–11.6 – Uye and Sano, 1995
O. brevicornis Grand-Lahou Lagoon Tropical 20–30 – – – – 10–16 0.11–3.33 0.001–0.046 Etilé et al., 2012
O. plumifera Andaman Sea Tropical 28–30b 0.19 559 ± 1.7 0.81 – – 0.2–11.2 (2.4 ± 0.2) 0.05 ± 0.01 Satapoomin et al., 2004
O. plumifera Jamaica Tropical 28 ± 1.5 0.11–2.6 – – – 5–21 (12.9 ± 3.7) – – Hopcroft and Roff, 1996
O. nana 28 ± 1.5 0.11–2.6 – – – 9–26 (17–20) – –
O. simplex 28 ± 1.5 0.11–2.6 – – – 4–10 (7) – –
O. nana Jamaica Tropical 28 – – 0.53c (0.21) – – – 0.31–0.37 Hopcroft and Roff, 1998
O. plumifera – – 1.9c (0.76) – – – 0.56
O. simplex – – 0.55c (0.22) – – – 0.21
O. attenuata GBR lagoon Tropical 27–28 0.98–2.5 423–435 0.47 3–15 5–30 0.22–1.44 0.002–0.016 This study
O. dissimilis Ross Creek 24–25 – 389–394 0.38 6–12 6–24 1.69–3.34 0.022–0.045

T, temperature; PL, prosome legth; EPR, egg production rate; SEPR, weight-specific egg production rate. Numbers in brackets indicate mean.

aOithona sp. 1, described later as O. nishidai (McKinnon, 2000).

bSurface temperature.

cAsh free dry weight (AFDW), in brackets recalculated assuming carbon as 40% of AFDW.

Maximum EPR in the current study were rather low in comparison with other field studies (Table IV). This may have occurred due to the effects of food limitation on the fecundity of the females. In tropical waters, high metabolic costs associated with high water temperatures can limit the growth of copepods under conditions of low food resources (McKinnon, 1996; McKinnon and Duggan, 2003). Thus we should consider the possibility that in the GBR lagoon, egg production was limited by the low protozooplankton concentration, assuming them to be the main prey for Oithona spp. This conclusion is supported by previous studies in tropical areas which have suggested that food availability limited the fecundity of Oithona species (Webber and Roff, 1995; Hopcroft and Roff, 1998; McKinnon and Klumpp, 1998).

Furthermore, it should be considered that we might have been sampling in a period of low productivity for Oithona. This could be confirmed by the seasonal abundance of Oithona attenuata described in the GBR lagoon where an abundance decrease was recorded in March–April (McKinnon and Thorrold, 1993). Alternatively, highly variable EPR, with no clear seasonal pattern, have been reported in mangrove systems of northeast Australia (McKinnon and Klumpp, 1998). In those mangroves, food limitation was suggested as the main factor regulating egg production; these authors observed how a rapid increase in egg production was subsequent to river flooding, suggesting that improved trophic conditions enhanced egg production. Seasonal phenomena in tropical and subtropical areas, such as changes in the wind fields (i.e. monsoons or trade winds), could enhance primary production by upwelling processes during some periods (Postel, 1990; Peterson, 1998). It has been shown how copepod egg production rapidly responds to the improved food availability arising from such events (McKinnon and Thorrold, 1993). Our sampling in the GBR lagoon started at the beginning of the dry season, thus it could be hypothesized that we were dealing with a period of low productivity in the area, which could explain low EPR. To confirm this, future investigations should study seasonal feeding and fecundity of Oithona spp. in tropical areas to determine the effects of dry and wet seasons on their productivity.

### Metabolic requirements

Metabolic requirements for Oithona spp. at temperatures similar to the ones observed in this study have been found to vary 18–26% of the body carbon per day (Hiromi et al., 1988; Atienza et al., 2006). Castellani et al. (Castellani et al., 2005b) estimated an energy demand of 32% for O. similis at 25°C. If we consider these values, daily rations found in this study are far from sufficient to sustain metabolic costs. Maximum daily ration estimated, assuming an egg production efficiency of either 16% (O. davisae: Zamora-Terol and Saiz, 2013), 30% (Straile, 1997) or 41% (this study, assuming GGE = SEPR/WSIR) (Fig. 3), are higher (respectively, ≈ 28, 15 and 11%) than our daily rations estimated from direct ingestion (Table II), but still too low to sustain the basic metabolism at the temperatures reported in this study.

In the GBR lagoon, the inclusion of ingestion of flagellates <5 µm would increase the carbon ingested by ∼1.2% (Table II). If we also incorporate potential ingestion of nauplii (1.2 nauplii per day; McKinnon and Klumpp, 1998), then this would increase the body carbon ingested by 7–21% and 8–24% in females from the GBR lagoon and the mangroves, respectively (considering a nauplii of 32 and 92 ng C; Almeda et al., 2010). Daily rations considering the above-mentioned prey would be 12–27% in the GBR lagoon (considering extra feeding on flagellates and nauplii), and 12–35% in the mangroves (considering extra feeding only on nauplii). Those “recalculated” daily rations might be sufficient to sustain basic metabolism at our temperatures (≈ 24–28°C).

The reported feeding rates on protozooplankton in this study, and the potential relevance of a carnivorous diet, confirm the key role of Oithona spp. linking the microbial and the classical food web in tropical waters. Furthermore, our study suggests that the feeding strategy of Oithona spp. in tropical, temperate (Castellani et al., 2005a; Atienza et al., 2006) and polar (Atkinson, 1996; Zamora-Terol et al., 2013) seas is similar, since protozooplankton seems to be the major contribution to the natural diet. We suggest that food availability plays an important role in the capability of Oithona to achieve maximum EPR in tropical waters, in contrast to polar areas where temperature seems to play a major role (Ward and Hirst, 2007; Zamora-Terol et al., 2013).

## FUNDING

This work was funded by the Spanish Ministry of Economy and Competitiveness through a Ph.D. fellowship to S.Z.-T. (BES-2008-004231), and the project CTM2007-60052 to E.S.

## ACKNOWLEDGEMENTS

We thank the captain and crew on board RV Cape Ferguson for assistance with field sampling in the GBR; and Sam Talbot for her help in the lab, and edition comments. We finally thank two anonymous reviewers and Marja Koski for their constructive comments on the manuscript.

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