The flow field around a freely swimming copepod in steady motion. Part II: Numerical simulation

Three-dimensional, numerical simulations of the flow field arounda freely swimming model-copepod were performed using a finite-volumecode. The model copepod had a realistic body shape representedby a curvilinear body-fitted coordinate system. The beatingmovement of the cephalic appendages was replaced by a distributedforce field acting on the water ventrally adjacent to the copepod'sbody. In the simulations, we took into account that freely swimmingcopepods are self-propelled bodies through properly couplingthe Navier–Stokes equations with the dynamic equationfor the copepod's body. Flow fields were calculated for fivesteady motions: (1) hovering, (2) sinking, (3) upwards swimming,(4) backwards swimming and (5) forwards swimming. The numericalresults confirm the conclusions drawn from the theoretical analysisusing Stokes flow models by Jiang et al. [in a companion paper(Jiang et al., 2002a)] for a spherical copepod shape and showthat the geometry of the flow field around a freely swimmingcopepod varies significantly with the different swimming behaviours.When a copepod hovers in the water, or swims very slowly, itgenerates a cone-shaped and wide flow field. In contrast, whena copepod sinks, or swims fast, the flow geometry is not cone-shaped,but cylindrical, narrow and long. The relationships betweencopepods' swimming behaviour and body orientation, hydrodynamicconspicuousness, energetics as well as feeding efficiency werediscussed, based on the simulation data. It is shown that thebehaviour of hovering or swimming slowly is more energeticallyefficient in terms of relative capture volume per energy expendedthan the behaviour of swimming fast, i.e. for a same amountof energy expended a hovering or slow-swimming copepod is ableto scan more water than a fast-swimming one. The numerical resultsalso suggest that the flow field generated by a fast-swimmingcopepod enables the copepod to use mechanoreception to perceivethe food/prey and therefore increases the food concentrationin the swept volume and that the flow field around a free-sinkingcopepod favours the copepod's mechanoreception while minimizingthe energy expense, so that the energy budget can still be maintainedfor both cases.     

Copepod flow modes and modulation: a modelling study of the water currents produced by an unsteadily swimming copepod

Video observation has shown that feeding-current-producing calanoid copepods modulate their feeding currents by displaying a sequence of different swimming behaviours during a time period of up to tens of seconds. In order to understand the feeding-current modulation process, we numerically modelled the steady feeding currents for different modes of observed copepod motion behaviours (i.e. free sinking, partial sinking, hovering, vertical swimming upward and horizontal swimming backward or forward). Based on observational data, we also reproduced numerically a modulated feeding current associated with an unsteadily swimming copepod. We found that: (i) by changing its propulsive force, a copepod can switch between different swimming behaviours, leading to completely different flow-field patterns in self-generated surrounding flow; (ii) by exerting a time-varying propulsive force, a copepod can modulate temporally the basic flow modes to create an unsteady feeding current which manipulates precisely the trajectories of entrained food particles over a long time period; (iii) the modulation process may be energetically more efficient than exerting a constant propulsive force onto water to create a constant feeding current of a wider entrainment range. A probable reason is that the modulated unsteady flow entrains those water parcels containing food particles and leaves behind those without valuable food in them.     

Hydrodynamic interaction between two copepods: a numerical study

Numerical simulations were carried out to compute the flow fieldaround two tethered, stationary or swimming model-copepods withvaried separation distances between them and for different relativebody positions and orientations. Based on each simulated flowfield, the power expended by each copepod in generating theflow field and volumetric flux through the capture area of eachcopepod were calculated. The geometry of the flow field aroundeach copepod was visualized by tracking fluid particles to constructstream tubes. The hydrodynamic force on each copepod was calculated.Also, velocity magnitudes and deformation rates were calculatedalong a line just above the antennules of each copepod. Allthe results were compared to the counterpart results for a solitarycopepod (stationary or swimming) to evaluate the hydrodynamicinteraction between the two copepods. The calculations of thepower and volumetric flux show that no energetic benefits areavailable for two copepods in close proximity. The results ofthe stream tube and force calculations show that when two copepodsare in close proximity, the hydrodynamic interaction betweenthem distorts the geometry of the flow field around each copepodand changes the hydrodynamic force on each copepod. Two beneficialroles of the hydrodynamic interactions are suggested for copepodswarms: (1) to maintain the integrity of the swarms and (2)to separate the swarming members with large nearest neighbourdistances (usually more than five body lengths). To preventstrong hydrodynamic interactions, copepods in swarms have toavoid positions of strong interactions, such as those directlyabove or below their neighbours. The results of the velocitymagnitudes and deformation rates demonstrate that the hydrodynamicinteraction between two copepods generates the hydrodynamicsignals detectable by the setae on each copepod's antennules.Based on the threshold of Yen et al. (1992), the results showthat the detection distance between two copepods of comparablesize is about two to five body lengths. Copepods may employa simple form of pattern recognition to detect the distance,speed and direction of an approaching copepod of comparablesize.     

Chemoreception and the deformationof the active space in freely swimming copepods: a numerical study

A three-dimensional alga-tracking, chemical advection–diffusionmodel was used to calculate the deformation of the active spacesurrounding an alga entrained within the flow field around afreely swimming copepod. From the model, the advance warningtime resulting from the copepod's chemo-reception of the entrainedalga was quantified, and copepod chemoreception capability comparedfor several different swimming behaviors: hovering in the water,swimming slowly (swimming upward, swimming backward and swimmingforward), swimming fast (swimming upward, swimming backwardand swimming forward) and sinking (with the anterior pointingupward or downward). The results show that when it hovers orswims slowly, a copepod can use chemoreception to remotely detectindividual algae entrained by the flow field around itself.In contrast, a fast-swimming copepod is not able to rely onchemoreception to remotely detect individual algae. The possibilityof a free-sinking copepod using chemoreception to detect algalparticles is also indicated. It is shown that advection by thefluid motion dominates over diffusion in transporting the chemicalsignals inside the active space to the location of a copepod'schemoreceptors. The feeding current structure for a hoveringcopepod is described. It is suggested that the feeding currentstructure and re-routing or re-orienting response by a copepodin response to its antennule or other cephalic appendage inputsallow the copepod to capture the food particles that would otherwisepass outside its capture area and increase the amount of foodcaptured.     

Danger of zooplankton feeding: the fluid signal generated by ambush-feeding copepods

Zooplankton feed in any of three ways: they generate a feeding current while hovering, cruise through the water or are ambush feeders. Each mode generates different hydrodynamic disturbances and hence exposes the grazers differently to mechanosensory predators. Ambush feeders sink slowly and therefore perform occasional upward repositioning jumps. We quantified the fluid disturbance generated by repositioning jumps in a millimetre-sized copepod (Re ∼ 40). The kick of the swimming legs generates a viscous vortex ring in the wake; another ring of similar intensity but opposite rotation is formed around the decelerating copepod. A simple analytical model, that of an impulsive point force, properly describes the observed flow field as a function of the momentum of the copepod, including the translation of the vortex and its spatial extension and temporal decay. We show that the time-averaged fluid signal and the consequent predation risk is much less for an ambush-feeding than a cruising or hovering copepod for small individuals, while the reverse is true for individuals larger than about 1 mm. This makes inefficient ambush feeding feasible in small copepods, and is consistent with the observation that ambush-feeding copepods in the ocean are all small, while larger species invariably use hovering or cruising feeding strategies.     

Effects of animal density,volume, and the use of 2D/3D recording on behavioral studies of copepods

Studies on the behavior of copepods require both an appropriate experimental design and the means to perform objectively verifiable numerical analysis. Despite the growing number of publications on copepod behavior, it has been difficult to compare these studies. In this study, we studied two species of copepods, Eurytemora affinis and Pseudodiaptomus annandalei, and employed recently developed scaling and non-scaling methodology to investigate the effects of density and volume on the swimming behavior of individual organisms in still water. We also compared the results of two- and three-dimensional projections of the swimming tracks. A combination of scale-dependent and scale-independent analysis was found to characterize a number of behavioral observations very effectively. We discovered that (i) density has no effect except to increase the time spent in the swimming state of “breaking”, (ii) smaller volumes resulted in more complex trajectories, and larger volumes, like density, increased the time spent in the swimming state “breaking”, and (iii) three-dimensional projections gave a more accurate estimation of speed and the time spent cruising. When only a vertical 2D projection was used, “cruising” could be confused with “sinking”. These results indicate that both experimental conditions and the selection of 2D or 3D projection have important implications regarding the study of copepod behavior. The development of standardized procedures with which to compare the observations made in different studies is an issue of particular urgency.     

Remote prey detection in Oithona similis: hydromechanical versus chemical cues

We quantified prey encounter rates and prey reaction distancesin the ambush-feeding cyclopoid copepod Oithona similis by videorecording freely swimming copepods at different concentrationsof prey, the dinoflagellate Gymnodinium dominans. Prey encounterrate increased with prey concentration, and a maximal clearancerate of 0.42 ± 0.10 ml h^–1 was estimated. The averagedistance (from the antennules) at which O.similis reacts toprey is 0.014 ± 0.007 cm. A simple prey encounter modelwas used to combine observed predator and prey velocities andprey reaction distance, and yielded a clearance rate similarto that estimated directly from prey encounter rates. The observedprey reaction distance was consistent with that estimated froma published model of hydromechanical prey perception. The possibilityof remote chemodetection was examined by modeling the distributionof solutes leaking out of a swimming cell. The cell leaves along slender chemical trail in its wake. However, since theambush-feeding O.similis is essentially stationary when perceivingprey, it is the width rather than the length of the trail thatmatters. Owing to advection, the chemical signal vanishes almostinstantaneously off the sides of the swimming flagellate, andsolute concentrations are below any likely detection thresholdwithin 40–50 µm from the flagellate. Our observationsare thus inconsistent with remote chemodetection in O.similis.The considerations are generalized, and it is concluded thatambush-feeding copepods, unlike cruisers and suspension feeders,cannot utilize chemical signals for the detection of individualprey, but rely on either hydromechanical detection or directinterception of prey.     

Detection and capture of inert particles by calanoid copepods: the role of the feeding current

Although there is a scarcity of supporting empirical evidence,it has long been suspected that calanoid copepods use mechanoreceptionto detect the presence and location of potential prey itemsentrained in the feeding current. In this study, we documentthe first observations showing a freely swimming calanoid copepod,Skistodiaptomus oregonensis, attacking prey-sized, non-motile,inert particles entrained in the feeding current before theparticles contact the copepod's sensory appendages. Feedingcurrent geometry, fluid velocities and associated behavioursthat characterize these interactions are described. The resultsof this study show how copepod swimming behaviour, coupled witha low-velocity feeding current, not only increases copepod encounterrates with inert prey by increasing direct contact rates, butalso increases the probability of detecting and capturing remotelylocated prey that have well-developed escape responses. In turbulentregimes, a far-reaching, low-velocity feeding current shouldincrease encounter rates, but only if coupled with behavioursthat quickly minimize separation distances once prey is detected.     

Resisting flow–laboratory study of rheotaxis of the estuarine copepod Pseudodiaptomus annandalei

Rheotaxis is a ubiquitous phenomenon among aquatic animals and thought to be an adaptation to maintain populations in flowing waters. While many estuarine copepods can retain their populations in estuaries with net seaward flow, rheotaxis of individual copepods has not been reported before. In this study, the behavior of a calanoid copepod Pseudodiaptomus annandalei in flow was examined in a recirculating laboratory flume. This estuarine copepod displayed different responses to ambient flow fields while swimming in the water column or attaching to the flume bed (walls). Copepods in the water column showed vigorous countercurrent swimming by occasional bounding when flow velocity was increased up to 2.1 cm s^?1, but none of the individuals in the water column were retained in the flume when flow speeds were higher than 4 cm s^?1. This indicates P. annandalei profits little from rheotaxis to withstand flow when they were swimming in the water column. Instead, more individuals attempted sinking downwards to the slow flow region near the flume bed (walls) and showed active substrate attachment to avoid being flushed out by the high-velocity channel flow. The results suggest that P. annandalei benefits from rheotaxis and association with the substrate which allows them to hold position well at ambient flow velocities up to 3 cm s^?1. These adaptive responses might be important for population maintenance.     

Perception of inert particles by calanoid copepods: behavioral observations and a numerical model

High-resolution video showed freely swimming Diaptomus sicilisattacking and capturing inert 5O µm polystyrene beadsthat were outside the influence of the copepod feeding current.The beads were frequently more than half a body length awayand were attacked after the ‘bow wake’ of the movingcopepod displaced the bead away from the copepod. To investigatethe hypothesis that deformation of streamlines around the copepodand its first antennae stimulated the attack response, a finiteelement numerical model was constructed. The model describedthe fluid interactions between a large object approaching asmaller object in a laminar flow at Reynolds number 5, whichis characteristic of the fluid regime experienced by foragingcopepods. The model revealed that fluid velocity fluctuationsand streamline deformations arose in the region between thetwo objects as separation distance between the objects decreased.The video observations and the model results support the hypothesesthat chemoreception is not required for the detection and captureof large phytoplankton cells [Vanderploeg et ai, in Hughes,R.N. (ed.), Behavioral Mechanisms of Food Selection. NATO ASISeries G20, 1990; DeMott and Watson, /. Plankton Res., 13, 1205-1222,1991], and that swimming behavior plays an integral role inprey detection. ⁴Present address: Academy of Natural Sciences Estuarine ResearchCenter, 10545 Mackall Road, St Leonard, MD 20685, USA     

The importance of protozooplankton as prey for copepods in the coastal areas of the central Irish Sea

This study evaluates food supply for copepods, highlighting the trophic relationship between copepods and protozooplankton. To test the hypotheses that protozooplankton prey are capable of sustaining the copepod standing stock in the western Irish Sea, the taxonomic and size composition of these two groups and the size-specific predation of copepods on protozooplankton were investigated. Protozooplankton and copepod samples were collected off the southwest coast of the Isle of Man using 1.7 l Niskin water bottles and two nets (64 and 280 μm meshes), respectively. Copepod predation on protozooplankton was calculated using weight-specific clearance rates from the literature, considering the availability of prey that was accessible to a given size of copepod. Low protozooplankton biomass was dominated by small cells (<60 μm), and high copepod biomass was dominated by small species, which were more efficiently collected by a 64-μm mesh net. However, large copepods were only collected by a 280-μm mesh net, suggesting that the combination of the two nets provided a better estimate of copepod biomass. Predation by the copepod assemblage in the Irish Sea removed 1–47% and 0.5–22% of ciliates and dinoflagellates standing stock, respectively, resulting in 1–40% of the copepod feeding requirement per day. Contrary to our hypothesis, copepods could not meet their feeding requirements by grazing only on the microzooplankton prey (15–200 μm), and other food sources (i.e. nanoplankton) must be important additional dietary components to copepods in the Irish Sea. Handling editor: S. M. Thomaz     

Mass density contrast in relation to the feeding currents in calanoid copepods

Theoretical analyses show that positively buoyant copepods areable to generate feeding currents by adopting upside-down bodypositions and pushing water upward. Thus, the excess buoyancyacting on the copepods will be balanced and cone-shaped feedingcurrents generated to transport water to the capture areas.The intensities of the feeding currents, which can be measuredin the present modeling study by calculating the volumetricflux going through the capture areas, are proportional to themass density contrasts between the copepods and the ambientseawater. The mass density contrasts may vary spatially andtemporally depending on copepod body contents and on the propertiesof the seawater immediately surrounding them. We focus on thecase where the mass density contrast between a wax ester-richcopepod and its ambient seawater can vary strongly with depthbecause wax esters are more compressible and 6–10 timesmore thermally expansible than seawater. These theoretical analysesshow that the intensities of the feeding currents generatedby wax ester-rich copepods vary strongly with depth. Our conclusionsfrom these theoretical analyses need to be tested by directobservations. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

Copepod feeding in the ocean: scaling patterns,composition of their diet and the bias of estimates due to microzooplankton grazing during incubations

Here, we report insights from the compilation and analysis of data on marine calanoid copepod feeding rates in the ocean. Our study shows that food availability and body weight are major factors shaping copepod feeding rates in the field, with a relatively minor role of temperature. Although the maximal feeding rates of copepods that are observed in the field agree with the well-known 3/4 of body size scaling rule for animals, copepod feeding in the oceans is typically limited and departs from this rule. Ciliates and dinoflagellates appear to be highly relevant in the composition of copepod diets, and this represents an indirect increase in the flux of primary production that is likely to reach the upper trophic levels; this contribution is higher in the less productive systems and may help to explain accounts of proportionally higher standing stocks of copepods supported per unit of primary producer biomass in oligotrophic environments. Contrary to common belief, diatoms emerge from our dataset as small contributors to the diet of copepods, except in some very productive ecosystems. We have also evaluated the bias in the estimation of copepod grazing rates due to within-bottle trophic cascade effects caused by the removal of microheterotrophs by copepods. This release of microzooplankton grazing pressure accounts for a relevant, but moderate, increase in copepod grazing estimates (ca. 20–30%); this bias has an effect on both the carbon flux budgets through copepods and on our view of their diet composition. However, caution is recommended against the indiscriminate use of corrections because they may turn out to be overestimates of the bias. We advise that both uncorrected and corrected grazing rates should be provided in future studies, as they probably correspond to the lower and upper boundaries of the true grazing rates.     

Observations of neutral buoyancy in diapausing copepods Calanoides acutus during Antarctic winter

The herbivorous Antarctic copepod Calanoides acutus overwinters inactively in a resting stage (diapause) at depths below 500 m. It is assumed that during diapause C. acutus is neutrally buoyant in order to retain energy reserves otherwise depleted by swimming activities. However, so far, no experimental observations on its buoyancy have been reported and our knowledge of buoyancy regulation mechanisms is incomplete. In the present study, species-specific differences in buoyancy were assessed visually. Observations were made of specimens from the diapausing cohort of C. acutus and compared to another herbivorous copepod Calanus propinquus, which overwinters actively feeding in the upper water layers. Freshly caught copepods were anaesthetized in a 3-amino-benzoic acid ethyl ester (MS222) in seawater solution in order to exclude the influence of swimming movements on buoyancy control. It was shown that C. propinquus was negatively buoyant, whereas diapausing C. acutus remained neutrally buoyant. This is the first record that neutral buoyancy in diapausing copepods is maintained by the biochemical body composition without the additional need of swimming movements.     

Evolutionary implications of swimming behaviour in meiobenthic copepods

Body morphology is said to be the all important factor in determining swimming prowess in copepods. Fusion and differentiation of the body (tagmosis) is coupled with advance into the pelagic realm of the Gymnoplea and is thought, by the provision of a rigid thoracic tagma, to promote swimming efficiency. Thus pelagic copepods are believed to be secondarily derived from bottom dwelling predecessors. Experimental evidence is presented to show that the majority of bottom dwelling harpacticoid families, including the most primitive and the most advanced, have representatives that undergo active sustained swimming movements. Such a widespread occurrence is indicative of a conservative evolutionary trait. This primitive behaviour is linked to precopulatory association which takes place necessarily in the water column; it is a feature retained by representatives of all copepod orders. The implication of cephalic appendage vibration (feeding currents) is the essential feature in the swimming success of the Gymnoplea; planktonic efficiency in these is suggested to have evolved coincident with, rather than because of increased tagmosis.     

Hydrocarbon contamination decreases mating success in a marine planktonic copepod

The mating behavior and the mating success of copepods rely on chemoreception to locate and track a sexual partner. However, the potential impact of the water-soluble fraction of hydrocarbons on these aspects of copepod reproduction has never been tested despite the widely acknowledged acute chemosensory abilities of copepods. I examined whether three concentrations of the water-soluble fraction of diesel oil (0.01%, 0.1% and 1%) impacts (i) the swimming behavior of both adult males and females of the widespread calanoid copepod Temora longcornis, and (ii) the ability of males to locate, track and mate with females. The three concentrations of the water-soluble fraction of diesel oil (WSF) significantly and non-significantly affect female and male swimming velocities, respectively. In contrast, both the complexity of male and female swimming paths significantly decreased with increasing WSF concentrations, hence suggesting a sex-specific sensitivity to WSF contaminated seawater. In addition, the three WSF concentrations impacted both T. longicornis mating behavior and mating success. Specifically, the ability of males to detect female pheromone trails, to accurately follow trails and to successfully track a female significantly decreased with increasing WSF concentrations. This led to a significant decrease in contact and capture rates from control to WSF contaminated seawater. These results indicate that hydrocarbon contamination of seawater decreases the ability of male copepods to detect and track a female, hence suggest an overall impact on population fitness and dynamics.     

The role of photoreception in the swarming behavior of the copepod Dioithona oculata

The copepod Dioithona oculata forms dense swarms near mangrove prop roots that are centered around shafts of light penetrating the mangrove canopy. Swarms can be created in the laboratory within light shafts created with a fiber optic light pipe. Laboratory observations of swarming behavior were recorded using video cameras, and the swimming behavior of the copepods and density of the swarms were quantified using video‐computer motion and image analysis techniques. Swarm formation results from a combination of phototactic and klino‐kinetic behavior. Dark adapted copepods initially exhibit a photophobic response to a light shaft, but become positively phototactic within 3–5 min after exposure to the light. Copepod aggregation rates under the light fit a saturation model, suggesting that copepods are attracted independently to the swarm marker. Copepods reverse their swimming direction when they encounter light intensity gradients near the edge of a light shaft, which aids in maintaining the swarm. Swarm formation can occur in the laboratory at light intensities as slow as 0.1 μM photons m^‐2 s^‐1, which is similar to light intensities at dawn when they are first observed to form in nature. Swarm formation appears to have an endogenous rhythm, as copepods will not form swarms at night under a light shaft.     

Factors affecting feeding selectivity of visual predators on the copepod Acartia tonsa: locomotion,visibility and escape responses

Visual predation by fish on copepods involves prey encounter, attack and capture; during any of these processes prey selection can occur. Developmental changes in copepods, including increases in swimming speed, size and image contrast increase the encounter rate and distance at which they can be detected by predators. Copepods compensate for this increase vulnerability with age through diel vertical migration and improved escape capabilities. This study quantifies the changes in swimming speed and movement pattern with developmental stage of the copepod Acartia tonsa, using a video-computer system for motion analysis. Changes in visible size and image contrast with developmental stage were quantified under simulated natural illumination conditions using a video based image analysis system. The escape responses of the naupliar stages of the copepod Acartia tonsa were quantified in response to a stationary pipette sucking in water at a constant speed. Accurate quantification of the parameters that affect feeding selectivity of planktivorous fish will provide the basis for evaluation of their relative importance in future studies.     

Planktonic predators and copepod abundance near the Dutch coast

Hypotheses that planktonic predators are responsible for thespring-summer decrease in copepod abundance and that the dominantpredator, Pleurobrachia pileus, is associated with high concentrationsof copepods were investigated at a station near the Dutch coast.Neither hypothesis was supported. Predators and copepods weresampled together with a 156 L ‘water box’ from lateApril through early July, including the season of P.pileus abundance.Using predators and copepods from the same box samples, hencefrom the same water parcel, feeding rates on copepod naupliiand copepodites + adults were measured onboard ship. Less than6% of the copepods, the sensitivity of the method, were removedper day. In additional shipboard feeding experiments net-caughtP.pileus were added to ambient copepod densities. By combiningwater volume cleared of copepods with ambient P.pileus densityin the sea, the predicted impact was 0–1.6% of copepodseaten per day from late April to early July. The hypothesisthat P.pileus associates with copepod concentrations was testedby comparing abundances of both groups from the same box samples.The correlations were not significant for any sample series.The ways measurement methods have restricted progress in understandingthe predatory impact of Pleurobrachia sp. are also considered.     

Across-shelf predatory effect of Pleurobrachia bachei (Ctenophora) on the small-copepod community in the coastal upwelling zone off northern Chile (23{degrees} S)

To estimate the predation effect of the predominant ctenophorePleurobrachia bachei on the small-copepod community in the upwellingarea off Mejillones (23°S), northern Chile, a series ofoceanographic cruises and predation experiments were conductedin the austral springs 2000, 2001 and 2002. The daily consumptionrates and predatory effect of P. bachei on the small copepods(in terms of % of standing stock and biomass removed daily)were determined at three stations located in relation to theshelf-break (coastal, shelf-break and oceanic) reaching valuesup to 4.5% per day of the <1500 µm copepod standingstock. Our results indicate that the ctenophores were most abundantat the coastal station, that small copepods dominated the copepodcommunity (being more abundant nearshore), and that the relativefrequency of ctenophores with copepods in their guts was alsohigher near the coast. The predatory effect of P. bachei onthe small-copepod community was also higher in the coastal zone.However, the effect of this predation on the copepod biomassin terms of carbon did not decrease steadily seawards, whichmay be due to the larger sized copepods consumed at the offshorestations. Determinations of predatory effect on the secondaryproduction of the more abundant small-copepod populations (i.e26% daily in 2000) suggest that this single species of Pleurobrachiais modulating the population growth rate of the small copepods,the copepod community size structure, and maybe even the alternanceof key species in the Mejillones coastal upwelling zone.