Ammonium release by juveniles and adult females of the subtropical marine copepod Eucalanus pileatus

Release rates of ammonium by nauplii, copepodid stages (CIIand CIV) and adult females of the marine copepod Eucalanus pileatusat 0.1 and 3.0 mm³ 1^–1 of the diatom Thalassiosira weissflogiiwere determined at 20°C. When food was abundant, animalsof all stages released ammonium at similar rates per unit ash-freedry weight [24–35 nmol NH₄(mg AFDW)^–1 h^–1on average]. At low food levels, CIVs and adult females releasedammonium significantly more slowly than did the naupliior CIIs[28 and 24 versus 51 and 50 nmoles(mg AFDW)^–1h^–1].Because they weighed less (50%), low-food nauplii and CIIs hadhigher calculated weight-specific excretion rates, than high-foodones of the same stage but release rates per copepod were similarin the two food regimens. In contrast to the early life stages,the CIVs and adult females released less ammonium per copepodin the low-food than in the high-food environment. ¹GLERL - Contribution No. 380     

Omnivorousness in marine planktonic copepods

Adult females of Centropages furcatus and Temora stylifera hadsignificantly different feeding behaviours. At equal averagefood concentrations, C. furcatus ingested more nauplii of thecalanoid copepod Pseudodiaptomus coronatus and fewer diatomsthan T. stylifera. At 20°C and 2.0 µg of nitrogen.1.1.^–1of each food source, C. furcatus ingested 17.2% and 4.4% ofits body nitrogen daily as nauplii and diatoms, respectively.Ingestion rates of the same food sources by T. stylifera were5.1% and 14.5%, respectively. At the environmental concentrationssimulated in these experiments, the presence of diatoms didnot affect the ingestion of nauplii nor did the presence ofnauplii affect diatom ingestion. The behaviour of predator andanimal prey largely dictates the types and quantities of preyingested.     

Population structure and swarm formation of the cyclopoid copepod Dioithona oculata near mangrove cays

The cyclopoid copepod Dioithona oculata forms swarms in water>30 on deep among prop roots of red mangroves (Rhizophoramangle) which fringe protected areas of two lagoonal cays, TwinCays, Belize. During 7 of 8 months surveyed by in situ observation,swarms were present but differed in size from small cylindricalswarms (5–10 cm diameter) to bands extending up to 1200m Swarms were never observed at night Swarms formed at dawnwhen light intensities reached an average value of 13.82 (logioquanta cm^–Abstract. s^–1) and dispersed at dusk atsimilar intensities Swarms observed in June formed earlier anddispersed later in the day than swarms observed in January,their swarming behavior followed seasonal changes in light intensityMean dioithonan density in swarms (10 ml^–1) was much higherthan the mean density (0 15 ml^–1) of non-swarming dioithonansaround mangrove prop roots. In open water 3–5 m away fromthe mangroves, mean dioithonan density was 7 9 x 10^–5ml¹ during the day, and 2 68 x10^–3 ml^–1 at nightSwarms were composed predominantly of adults and copepodid stagesIV and V, although younger copepodid stages could be presentNauplii were never present. The ‘average copepodid stage’for all 95 swarms sampled was 5 3, where 6 0 represents a swarmwith only adults In open water 3–5 m away from the mangroves,the youngest copepodids (stage one) dominated the dioithonanpopulation during the day. At night when swarms dispersed toopen waters, average copepodid stage was higher (3 5) comparedwith the day value (1.2) in open waters. Although densitiesin swarms were higher in June than January, average copepodidstage in June was higher (5 6) than that in January (4.9). Ahigher percentage of adults were females during June than January.Therefore higher densities did not result from increases ofsmaller stages in swarms, but perhaps changes in behavior orpopulation structure.     

Changes in carotenoid composition in different developmental stages of copepods: Pseudocalanus acuspes Giesbrecht and Acartia spp.

Carotenoid contents and composition were studied in three successivedevelopmental groups of copepod populations from the southernBaltic Sea. Samples were collected with >80% prevalence ofany one of three developmental groups: (i) nauplii, (ii) copepodidsI–III and (iii) copepodids IV–V and also adultsof Pseudocalanus acuspes Giesbrecht and Acartia spp. (Copepoda:Calanoida). Samples were collected over the years 1991–1999from May to September at six sampling sites in the southernBaltic Sea. The carotenoids were analyzed by means of reverse-phasehigh-performance liquid chromatography (RP-HPLC). The mean totalcarotenoid concentrations (astaxanthin, astaxanthin esters,canthaxanthin and other carotenoid unidentified and derivedfrom the diet) found in three groups of developmental stagesfor P. acuspes were 556 µg g^-1 dry weight (d.w.) in naupliarstages, 791 µg g^-1 in copepodids I–III, and 868µg g^-1 in copepodids IV–V and adults. The averagecarotenoid concentrations in Acartia spp. were 619, 764 and872 µg g^-1 d.w., respectively. Significant changes wereobserved in the proportion of carotenoids of separate developmentalgroups. Astaxanthin and canthaxanthin occurred in all developmentalgroups, with the evident dominance of astaxanthin. However,copepodid groups I–III, and especially IV–V includingadults, showed an increased proportion of astaxanthin esters.This suggests that astaxanthin is the main active carotenoidin copepod metabolism. It acts most probably as an efficientfree radical quencher and may be involved in rapid metabolismof stored lipids in mostly unfeeding young nauplii. The feedingstages (late nauplii and copepodids I–III and IV–V)are evidently able to metabolize this pigment by esterificationand further degradation.     

Post-embryonic developmental rates as a function of food type in the cyclopoid copepod, Mesocyclops thermocyclopoides Harada

We studied under laboratory conditions the post-embryonic developmentalrates (from nauplii to adult stage) of Mesocyclops thermocyclopoides,commonly found in the eutrophic and hypertrophic local ponds,in relation to different food types. Nauplii hatched from theeggs collected from laboratory-maintained mass cultures werereared at 25l.5C, using the following test diets: bacteria(Klebsiella aerogenes), cyanobacteria (Microcystis aeruginosa,as single cells and in the form of colonies), green algae (Chlorella,Scenedesmus and Chlorogonium), ciliates (Tetrahymena) and rotifers(Brachionus angularis). The developmental rates were fastest,and the proportion of nauplii reaching the copepodid stage andthat of copepodids reaching the adult stage the highest, withChlorogonium, followed by ciliates and rotifers. Developmentwas incomplete with bacteria and Microcys-tis.Ttie algae Chlorellaand Scenedesmus supported complete post-embryonic developmentonly when cultured in media enriched with beef extract, bactotryptoneand yeast extract. With rotifers as the exclusive food, >70%of the nauplii reached the copepodid stage, and >85% of thecopepodids the adult stage. The ratio (D_c/D_n) of copepodid duration(D_c) and naupliar duration (D_n) was significantly differentfrom 1.0, indicating the absence of isochronal development inM.thermocyclopoides. Our results show that post-embryonic developmentin this species is not possible with certain algal diets, probablybecause of nutritional deficiencies, and that the later stagenauplii are capable of capturing rotifers and utilizing themas food to complete their development to the adult stage.     

The seasonal abundance and production of Oithona nana (Copepoda:Cyclopoida) in Southampton Water

Recent studies indicate that Oithona spp. contribute significantlyto total copepod biomass. Little is known, however, about theirecological significance, particularly in the case of the estuarineOithona nana. A study comprising three sites within SouthamptonWater was conducted to evaluate the late-stage copepodite/adult(stages IV–VI) O. nana community, using 120-µm meshnets. Although present throughout the estuary, there was a strikingspatial gradient with O. nana most common in the upper estuary.A clear seasonal pattern was observed with O. nana as the mostabundant copepod species from late summer until early winter.It comprised 61% of all copepods recorded, with a biomass of757.22 mg C m^–3. Production estimates of O. nana werederived from the ‘instantaneous-growth’ approach,using appropriate growth equations. The estimated productionof O. nana ranged from 1.50 mg C m^–3 year^–1 withinthe lower estuary to 146.77 mg C m^–3 year^–1 in theupper estuary. In the upper estuary, this compares with productionrates of 187.47 mg C m^–3 year^–1 for all Acartiacongeners (excluding nauplii), the most common calanoid genus.Throughout the estuary, O. nana annual production represented18% of total copepod production clearly indicating that, atleast in the upper estuary, O. nana production may be directlycomparable with calanoid production.     

Perceptive performance and feeding behavior of calanoid copepods

The goal of this study was to determine variables associatedwith calanoid feeding behavior, and thus, to improve our understandingof the basics of calanoid feeding rates. These variables includedperiods and frequency of appendage motion, rates of cell clearance,distance at which a copepod first reacts to a cell which iseventually captured, and rate of water flow through the areacovered by the motions of a copepod's feeding appendages. Theeffects of these variables on feeding rates were determinedfor copepodids and adult females of the calanoid copepod Eucalanuspileatus at phytoplankton concentrations covering the rangeencountered by this species on the south-eastern shelf of theUSA. Our results indicate that the distance at which E.pileatusperceives phytoplankton cells increases {small tilde}2-foldas food concentrations decrease from 1.0 to 0.1 mm³ l^–1.These results lead us to hypothesize that this is due to increasedsensitivity of chemosensors on the copepods' feeding appendages.This 2-fold increase in perceptive distance amounts to a near4-fold increase in perceived volume which is close to the 6-foldincrease in volume swept clear (VSC) from 1.0 to 0.1 mm³ l^–1of Thalassiosira weissflogii. We assume that the increases inVSC by planktonic copepods, when food levels are below satiation,are largely a function of the sensory performance of the individualcopepod.     

Direct evidence of tidally oriented behavior of the copepod <Emphasis Type="Italic">Eurytemora affinis</Emphasis> in the Seine estuary

The calanoid copepod Eurytemora affinis dominates the zooplankton communities in most northern hemisphere estuaries. A recurrent behavior noticed in several estuaries suggests that this species maintains its horizontal position through vertical migration. In order to investigate this behavioral strategy in detail, we sampled E. affinis nauplii, copepodids and adults at high frequency at a fixed point in the low salinity zone of the Seine estuary. We took water samples in sub-surface and near bottom during a 12- and 50-h tide cycle, in May 2002 and May 2004, respectively. Since nauplii have weak swimming capabilities compared to copepodid and adult stages, we proposed here to consider them as passive particles. The variation of the relative abundances of nauplii to older stages regarding the vertical position was therefore viewed as direct evidence of the behavior of later stages. We showed that copepodids and adults tried to stay close to the bottom during the ebb, were re-suspended in the water column 1 h after the low tide during the maximum current velocity, and then vertically migrated back to the bottom layer when current velocity decreased. We also showed that copepodids behaved as adults, with only a lower velocity, taking more time to go near the bottom.     

Egg production and hatching success in the peri-Antarctic copepod Calanus simillimus

In situ rates of egg production and hatching success are reportedfor Calanus simillimus, one of the most abundant calanoid speciesin peri-Antarctic regions, during the Italian ‘Italics’cruise in the Straits of Magellan in March-April 1995. Low fecundity(8.6 eggs female^–1 day^–1) and fecal pellet production(0.8 fecal pellets female^–1 day^–1) in this periodindicate that the species was feeding very poorly. Sixty-sixof the 126 females sampled did not produce eggs and 80 of thesedid not produce any fecal pellets during the 24 h period ofincubation. Striking abnormal naupliar and embryonic developmentwas recorded in 81.8% of the eggs spawned. Aberrant eggs didnot undergo normal cleavage, and failed to develop to hatching.Deformed nauplii were asymmetrical and presented strong anatomicalanomalies in the total body length and number of swimming appendages.These results are discussed in the light of recent findingson the causes of low hatching success in copepods.     

The complete postembryonic development of Tropodiaptomus informis Kiefer, 1936 (Copepoda:Calanoida) reared in the laboratory

The complete postembryonic development of Tropodiaptomus informiscomprises six naupliar and six copepodid stages of which thelast is the adult. Of all the characteristics, it is the shapeand number of setae on the terminal segments of the antennulesin the late naupliar stages, and the shape and setae of exo-and endopods of the fifth legs from copepodids III to V whichare most useful in constructing relationships among eight diaptomidsreared.     

Ontogeny of growth, respiration and feeding rate of the freshwater calanoid copepod Eudiaptomus graciloides

The calanoid copepod, Eudiaplomus graciloides, was reared fromegg to adult on uni-algal diets (0.1. 0.5 and 2.5 mg dry wt1^–1) using the green alga, Chlamydomonas reinhardtii,as food, or on a mixed diet consisting of Lake Esrom water filteredthrough a plankton net with pore size 45 µm and supplementedwith C. reinhardtii (2.5 mg dry wt 1^–1). On the mixeddiet at 21.0°C growth in body dry wt (W, µg dry wt)was exponential, and the growth constants were 0.21 day^–1in the early to mid juvenile stage (N1 - C4) and 0.11 day^–1in the late juvenile to early adult stage (C4-A). At 14.5°Cthe corresponding growth rate constants were 0.10 and 0.08 day^–1.Similar growth rates were found at uni-algal concentrationsof 0.5 and 2.5 mg dry wt I^–1, and it was argued that thethreshold concentration for growth in Eudiaptomus was closeto 0.1 mg dry wt I^–1. The clearance (C, ml h^–1)of copepodites was measured on the uni-algal diets. The constantsof the regression (C = aW^b) were: a = 0.125, b = 0.858 (2000C. reinhardtii ml^–1), a = 0.068, b = 0.849 (10 000), a= 0.028, b = 0.875 (50 000). Ingestion rates were calculatedfrom the clearances and the average algal concentrations. Atthe three food levels the average daily rations were 30, 67and 125% of body dry wt. The respiration rate (R, nl O₂ h^–1)was measured in individuals reared on the mixed diet. The constantsof the regression (R = aW^b) were: a = 4.82, b = 1.07 (nauplii,14.5°C), a = 4.17, b = 0.904 (copepodites and adults, 14.5°C),a = 6.87, b = 0.757 (copepodites and adults, 21.0°C). Nosignificant difference in the respiration rate of copepoditesreared on uni-algal diets and the mixed diet could be demonstrated.Energy budgets were calculated. The assimilation efficiencyand the gross growth efficiency of copepodites decreased markedlywith increasing food concentration, the net growth efficiencyvaried from an average of 0.44 at the lowest algal concentrationto 0.60 on the mixed diet. The results are discussed in relationto previous findings with both freshwater and marine copepods.     

The role of feeding behavior in sustaining copepod populations in the tropical ocean

A fundamental question regarding marine copepods is how themany species coexist and persist in the oligotrophic environment(i.e. Hutchinson’s paradox). This question is addressedwith a stochastic, object-oriented Lagrangian model that explicitlysimulates the distinct foraging behaviors of three prominenttropical species: Clausocalanus furcatus, Paracalanus aculeatusand Oithona plumifera. The model also individually tracks allprey cells. Each particle’s motion combines sinking, turbulentdiffusion and active swimming when applicable. The model successfullysimulates observed size-partitioned carbon uptake rates. Basedon the model results, the wide-ranging translational ambit employedby C. furcatus is best suited for the acquisition of passiveprey while the relatively stationary behavior of O. plumiferapromotes the capture of larger, quickly sinking cells. The modelresults further suggest that the slow velocities and feedingcurrent employed by P. aculeatus are best suited for acquiringthe smallest cells though it also has a slight advantage overC. furcatus in acquiring the largest prey. A resource threshold,at a prey concentration of 530 cells mL^–1, is consistentlyexhibited by all three modeled species. Overall, these resultsimply that the size-partition preferences due to their differentforaging behavior contribute to the coexistence of these threespecies. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

On assessment of prey ingestion by copepods

The consumption of photosynthetic and heterotrophic cells byan abundant calanoid copepod species feeding on natural planktoncommunities was quantified with a state-of-the-art image-analysissystem. Late copepodid stages of Eucalanus pileatus did notingest bactena, small photosynthetic and heterotrophic nanoplankton,or the abundant Ceratium spp. in quantifiable amounts Althoughdiatoms were by far the most abundant cells (in terms of POC1^–1), the copepods ingested a higher percentage of ciliatesin relation to their abundance than of diatoms and small heterotrophicdinoflagellates in the first experiment, and ingested a higherpercentage of dinoflagel lates and ciliates compared to diatomsin the second experiment Heterotrophic cells sufficiently largeto be captured were repeatedly preferred by E.pileatus overautotrophs of similar or larger size. More over, among the cellswhich could be individually perceived by this calanoid, largerones were not pre ferred over smaller cells, implying that someaspect of food quality can be as significant as prey size. Theseresults support the notion (e.g. Kleppel, Mar. Ecol Prog. Ser.,99, s183–195, 1993) that feeding by copepods will be underestimatedif ingestion of heterotrophic food organisms is not quantified.While the proposed microscope-based method is comparativelyslow (     

Detection of infrasonic water oscillations by copepodids of Lepeophtheirus salmonis (Copepoda Caligida)

The cues that trigger infection of fish by parasitic copepodsare largely unknown. We show that copepodids of the parasiticcopepod Lepeophtheirus salmonus respond to uniform, linear accelerations,which are similar to those found in front of a swimming fish.Copepodid responses to vibrations at 1, 3, 5 and 10 Hz frequencywere filmed and analysed. The animals were stimulated in a completelywater-filled, clear perspex chamber, which was suspended likea swing in four wires from a steel frame. The chamber was movedby a vibrator which was fed amplified signals from a sine waveoscillator. On stimulation, copepodids responded by executingswimming bursts of 1–3 s duration. There was no apparentpreferred swimming direction. Sensitivity was highest at 3 Hz,with a threshold value of 5 x 10^–3 m s^–2 (rms).At 1 Hz the threshold was <6 dB higher, and sensitivity wasmarkedly reduced at 10 Hz, where the threshold was 1.8 x 10^–1m s^–2 rms. These results indicate that the copepodidsmay react to the near-field accelerations produced within centimetresof a swimming fish. Acceleration sensitivity may therefore bea cue that triggers high-speed swimming and subsequent infestationof the host. If this ability is present in holoplanktonic copepods,it may facilitate detection and escape from predatory fish.     

Effect of temperature and food concentration on post-embryonic development, egg production and adult body size of calanoid copepod Eurytemora affinis

Post-embryonic development time, egg production rate and adultbody size of calanoid copepod Eurytemora affinis from Lake Ohnuma,Japan, were determined under six temperature-food conditions(10³,5 x 10³, 10⁴ and 5 x 10⁴ cells ml^–1 at 15°C,5 x 10⁴ cells ml^–1 at 10and20°C) in the laboratory.The measured parameters varied with both temperature and foodconcentration. Development time from hatching to adult femalewas 9.2, 11.4 and 22.8 days at 20, 15 and 10°C respectively,at the highest food concentration. The males developed to adultat one to two days earlier than the females. An effect of foodshortage on development time occurred at the lowest food concentration.This development time was 24.8 days even at 15°C, beingtwice as long as that at the highest food concentration. Prosomelength of these food-limited females was approximately 75% ofwell-fed ones, which reduced by only 10% with increasing temperaturefrom 10 to 20°C. Clutch size (C, eggs clutch^–1) ofwell-fed individuals depended on prosome length of the adultfemale (L, mm), and was expressed as an equation: C = 65.2 L³⁸³. Clutch size of individuals reared at less than 10⁴ cellsml^–1, however, mostly laid below the estimated curve,especially at the lowest food concentration being only 10% ofthat at the highest food concentration. These results suggestthat food availability is a more important factor affectingpopulation growth of E.affinis in Lake Ohnuma than variationof temperature.     

Seasonal patterns in gonad maturation, sex ratio and size in some small, high-latitude copepods: implications for overwintering tactics

Seasonal changes in gonad maturation, sex ratio and size aredescribed for field populations of the marine calanoid copepodsPseudocalanus acuspes (Giesbrecht, 1881) and Acartia longiremisLilljeborg in Balsfjorden, northern Norway. Reproductive populationsof both species were present from March to September. Beginningin August, gonads in P.acuspes late copepodites and A.longiremisadult females switched to an undeveloped resting state. Dormancywas terminated in late winter, several weeks before the vernalbloom. Small samples of Pseudocalanus minutus (Boeck) indicatedboth an earlier induction and termination of the overwinteringphase in this species. Overwintering populations of P.acuspescopepodite stage V were dominated by males, but sex ratios evenedout at the time of spring moulting and mating. Juvenile maleswere larger than the females, except after the vernal bloomand in early summer. The findings are discussed in relationto seasonal life history tactics. ¹Present address: Department of Aquatic Biology, The NorwegianCollege of Fishery Science, University of Tromsø, Dramsveien201B, N-9037 Tromsø, Norway     

Cannibalistic feeding behavior of the brackish-water copepod Sinocalanus tenellus

Cannibalistic feeding behavior of the brackish-water copepodSinocalanus tenellus was examined in the laboratory using CI-II,CIII-IV and CVI female as predators and NI-II, NIII-IV, NV-VIand CI-II as prey. In each prey-predator combination, the ingestionrate increased with increasing prey density to an asymptoticvalue. Cannibalism took place even when phytoplankton was availableas an alternative food supply. Based on a daily ration, theoptimal prey stages for CVI females, CIII-IV and CI-II are NI-VI,NI-IV and NI-II respectively. Under average, natural prey density(10 nauplii l^–1), S tenellus can achieve only a smallfraction (max 9%) of the daily minimum food requirement by cannibalisticfeeding. However, the impact of cannibalism on naupliar survivorshipcan be significant. When adult females occur at a density of10 l^–1, the mortality due to cannibalism attains 99.2%during the naupliar stages.     

Seasonal species composition and distribution of barnacle larvae in Avacha Inlet (Kamchatka)

The annual dynamics of the numbers and distribution of barnaclelarvae in Avacha Inlet were studied. Planktonic cirripede larvaewere found in the area studied from late March at a temperatureof –1–0°C until October. The highest densityof 18 000 m^–3 was observed in mid-April. Balanus crenatusand Semibalanus cariosus were the dominant species, Chthamalusdalli and Balanus balanus larvae being less abundant. Naupliardensity was maximal at 0–5 m and decreased with depth.Maximum densities of B.crenatus larvae were observed in RakovayaBay, where there are many ships and a seawater-cooled powerstation. Most nauplii of the intertidal barnacle S.cariosus,which prefers wave-exposed open coast, were taken in the AvachaInlet narrows Settling on artificial substrata was most intensivefrom late May to early June     

Population dynamics of Aurelia aurita (Scyphozoa) from an isolated brackish lake, with particular reference to sexual reproduction

The annual cycle of abundance, growth and sexual reproductionof the scyphomedusa Aurelia aurita was examined over a 12 monthperiod in an isolated, shallow, brackish, man-made body of water:Horsea Lake, England. Strobilation started in December and ephyraewere present through June in varying numbers, suggesting a 7month period of near-continuous recruitment. Horsea Lake hasa dense A.aurita population, with the peak abundance of 24.9m^–3 occurring in mid-May. Medusae (>10 mm) were presentin the water column from March through January. Maximum belldiameter of 105 mm was observed in late September. However,medusa growth was very poor (G ranged between –0.03 and0.07 day^–1 over a 12 month period), and monthly mean belldiameters were typically <50 mm. For most of the year, theabundant A.aurita population was most likely severely food limitedas a result of a numerically and species-poor mesozooplanktoncommunity dominated by the calanoid copepods Acartia margalefiand Acartia tonsa. Nevertheless, the very small A.aurita medusaewere still able to attain sexual maturity and reproduce. Ripefemale medusae carrying fertilized eggs and planula larvae werefirst observed in May and June, although the main period ofplanula larva production was September-December. In November,all females were ripe, the smallest being 19–20 mm belldiameter. At both Horsea Lake and a nearby productive estuary,Southampton Water, planula larva production increased linearlywith wet weight, although compared with the larger A.auritamedusae from Southampton Water, reproductive output at HorseaLake was low. Aurelia aurita in Horsea Lake showed plasticityin its population characteristics in response to environmentalconditions, with both fecundity and size to maturity varyingon a seasonal basis. Medusae appeared to partition the availablefood resources into either somatic growth when food was abundantor reproductive effort when food was scarce.     

Rates of ingestion and their variability between individual calanoid copepods: direct observations

The goals of this study were to determine rates of ingestionand fecal pellet release, and their variability, for individualplanktonic copepods over extended periods of time (>20 min).Ingestions and rejections of individual cells of the diatomThalassiosira eccentrica by adult females of the calanoid Paracalanusaculeatus were directly quantified by observing individual copepodscontinuously at cell concentrations ranging from 0.1 toli mm³l^–1.Average ingestion rates increased with increasing food concentration,but were not significantly different between 0.3 and 1.0 mm³l^–1(9.8 and 32.7 µg Cl^–1) of T.eccentrica. Rates ofcell rejections were low and similar at 0.1 and 0.3, but weresignificantly higher at 1.0 mm³ l^–1. The coefficientsof variation for average ingestion rates of individual copepodshardly differed between food concentrations, ranging from 17to 22%, and were close to those for average fecal pellet releaseintervals which ranged from 15 to 21%. A comparison betweenindividuals at each food concentration found no significantdifferences at 1.0; at 0.1 and 0.3 mm³l^–1, respectively,ingestion rates of four out of five females did not differ significantlyfrom each other. Average intervals between fecal pellet releaseswere similar at 0.3 and 1.0 mm³l^–1 of T.eccentrica, butsignificantly longer at 0.1 mm³ l^–1. Fecal pellet releaseintervals between individuals were significantly different ateach food concentration; these significant differences wereattributed to rather narrow ranges of pellet release intervalsof each individual female. Potential sources/causes of variabilityin the sizes and rates of copepods in the ocean are evaluated. ³Present address: Great Lakes Environmental Research Laboratory,2205 Commonwealth Boulevard, Ann Arbor, Ml 48105, USA