Mechanics and energetics of swimming in the small copepodAcanthocyclops robustus (Cyclopoida)

High-speed films of swimmingAcanthocyclops robustus were used to test a crustacean swimming-model based on numerical analysis of thrust production. Predicted body velocities and jump distances were usually within 75% of those observed. Most of the thrust which propels.A. robustus is produced by movements of the 2nd, 3rd, and 4th thoracic swimming legs, with only small contributions from the first thoracic swimming legs. A model analyzed without the first antennae suggested that the antennae do not produce significant thrust. The leg and antennal movements could be described with trigonometric equations (cosine curves), but were best described by polynomial fits of position vs time data from the films. Patterns of swimming velocity varied among four episodes that were modeled, and followed differences in swimming-leg motions. Model results for the small (cephalothorax length = 0.6 mm) cyclopoidA. robustus and those which have been reported for the large calanoid copepodPleuromamma xiphias and other swimmers indicate that mechanical efficiency (30%) does not scale with body size, whereas jump distance (one body length), proportion of thrust generated by hydrodynamic added mass (70%), and net cost of transport,C _p (40 to 109 cal g^–1 km^–1) do.Please address all correspondence and requests for reprints to G. Gust at the University of South Florida, St. Petersburg     

Metachronal swimming in Antarctic krill: gait kinematics and system design

Metachronal swimming, in which adjacent appendages stroke in sequence, is widespread among crustaceans inhabiting the transitional flow realm in which both viscosity and inertia effects are important. However, the design and operation of this propulsion system in response to various hydrodynamic, energetic, and behavioral needs have not been well investigated. We examine free-swimming Antarctic krill (Euphausia superba) as a model species and identify three distinct behavioral swimming gaits. The pleopod kinematics of these gaits, hovering, fast-forward swimming, and upside-down swimming, are quantified via image analysis of high-speed video. Pleopod stroke amplitude and frequency were found to vary significantly among these swimming modes. In order to increase swimming speed, krill were found first to increase stroke amplitude and secondarily to increase beat frequency. The kinematics of these distinct swimming modes provide insight as we consider multi-appendage metachronal swimming from a design standpoint. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key parameter in metachrony, the value of which is constrained to a narrow range for a wide variety of species.     

Locomotor activity patterns and feeding habits in the prawn Palaemon xiphias (Crustacea: Decapoda: Palaemonidae) in Alfacs Bay,Ebro Delta (northwest Mediterranean)

Diel activity rhythms of the prawn Palaemon xiphias Risso, 1818 from Alfacs Bay, Ebro Delta, were studied by time-lapse videorecordings. Activity displayed an endogenous circadian rhythm, with maximum activity at night. Feeding habits were studied by frequency of occurrence and by the points method. The food of P. xiphias consisted mainly of crustaceans; the remains of amphipods, isopods, mysids, copepods, decapods and ostracods were identified. The remaining items consisted of molluscs, polychaetes, ophiuroids, plant material, sand, and unidentified organic debris. The results indicate that P. xiphias is a predator of benthic invertebrates rather than a scavenger or detritus feeder. Diet composition changes with increasing size of the prawn.     

Effects of nano carbon black and single-layer graphene oxide on settlement,survival and swimming behaviour of Amphibalanus amphitrite larvae

The effects of two carbon-based nanomaterials, nano-sized carbon black (nCB), and single-layer graphene oxide (GO) on settlement of Amphibalanus amphitrite (Cirripedia, Crustacea) cypris larvae (cyprids) were assessed after 24, 48, and 72 h of exposure. Additionally, the effects of these nanomaterials on the mortality and swimming behaviour of the nauplius larvae (nauplii) of the same organism were determined after 24 and 48 h. The data indicate that nCB is more effective as a potential antisettlement agent than single-layer GO; moreover, nCB did not show any adverse effects on the larvae. The swimming behaviour of II stage nauplii of A. amphitrite exposed to a suspension of nCB was inhibited only at very high nCB concentrations (≥0.5 mg/mL). Single-layer GO, on the contrary, showed lower antisettlement effects and was more active in altering the survival and inhibiting the swimming behaviour of the nauplii. An indication of the toxic or non-toxic mechanisms of the antisettlement properties of both of these nanomaterials is provided by the reversibility of the antisettlement activity. In conclusion, we propose nCB as an innovative antifouling nanomaterial that shows low toxicity towards the model organism (crustaceans) used in this study.     

Relationship of oxygen consumption to swimming speed in Euphausia pacifica

Swimming efficiency (the ratio of thrust power required to overcome hydrodynamic drag to net metabolic energy expenditure) was calculated for the vertically migrating euphausiid Euphausia pacifica swimming at speeds of 1–20 cm s^–1 and at temperatures of 8° and 12°C. Efficiencies ranged from 0.014 to 2.8% at 8°C and 0.009 to 1.69% at 12°C. A comparison with efficiency in fishes 2–3 orders of magnitude larger in weight (efficiency range 10–25%) indicates that locomotion in E. pacifica is far less efficient, a probable result of the organism's small size (x=33.5 mg WW) and multiple-paddle mode of propulsion. Net cost of transport of E. pacifica is three to six times the cost of a hypothetical value for sockeye salmon. Low swimming efficiencies in zooplankton such as E. pacifica are responsible for the underestimation of zooplankton swimming costs. Multiple-paddle propulsion is less efficient than the undulatory mode of fishes.     

Calanoid copepod escape behavior in response to a visual predator

Calanoid copepods typically exhibit escape reactions to hydrodynamic stimuli such as those generated by the approach of a predator. During the summers of 2000, 2001 and 2004, two small calanoid species, Temora turbinata Dana, 1849 and Paracalanus parvus Claus, 1863 were exposed to a visual predatory fish, the blenny Acanthemblemaria spinosa Metzelaar, 1919, and their predator–prey interactions were recorded using both high-speed and standard videographic techniques. Copepod escape reaction components, including swimming pattern, reactive distance, turning rate, and jump kinetics, were quantified from individual predation events using motion analysis techniques. Among the observed escape reaction components, differences were noted between the species’ swimming patterns prior to attack and their response latencies. Temora turbinata was a continuous cruiser and P. parvus exhibited a hop-and-sink swimming pattern. During periods of sinking, P. parvus stopped beating its appendages, which presumably reduced any self-generated hydrodynamic signals and increased perceptual abilities to detect an approaching predator. Response latency was determined for each copepod species using a hydrodynamic stimulus produced by a 1 ms acoustic signal. Response latencies of T. turbinata were significantly longer than those of P. parvus. Despite some apparent perceptual advantages of P. parvus, the blenny successfully captured both species by modifying its attack behavior for the targeted prey.     

Fine-scale observations of the predatory behaviour of the carnivorous copepod <Emphasis Type="Italic">Paraeuchaeta norvegica</Emphasis> and the escape responses of their ichthyoplankton prey,Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.     

Habitat-induced morphological variation influences photosynthesis and drag on the marine macroalga <Emphasis Type="Italic">Pachydictyon coriaceum</Emphasis>

Photosynthesis, growth, distribution, and persistence of macroalgae are determined in part by the physical environment in which they live. Therefore, discerning how macroalgae interact with their physical environment is necessary to better understand their physiological performance. The purpose of this study was to examine what photosynthetic and hydrodynamic costs and benefits the morphology of Pachydictyon coriaceum (Phaeophyta) confers on the thallus in a given environment. Principal components analysis of morphometric measurements of Pachydictyon coriaceum from different flow habitats and depths separated thalli into three distinct morphs: shallow wave-exposed, shallow wave-protected, and deep. To test the hypothesis that thallus morphology affects net photosynthesis (NP), thalli of three morphotypes of P. coriaceum were incubated in an enclosed recirculating flume under three simulated light/water flow environments representing conditions from which the three morphotypes were collected. The wave-protected and deep morphs had significantly higher rates of photosynthesis than the wave-exposed morph for all three simulated environments. The dense, compact shape of the wave-exposed morph readily streamlines with flow and in doing so, potentially shades many of its internal blades likely accounting for its lower biomass-specific NP. Drag coefficients (C _d) were estimated for the three morphotypes over a range of flow velocities between 0.08 and 0.47 m s⁻¹. At lower water flow velocities (0.08–0.21 m s⁻¹), wave-exposed morphs had the lowest C _d among the three morphotypes. But drag coefficients of the three morphotypes converged with increasing flow velocities, and at velocities >0.31 m s⁻¹ there were no differences in C _d among the three morphotypes. The results of this study indicate that the environmentally-shaped morphs influence photosynthesis and, to a lesser degree, hydrodynamic forces acting on P. coriaceum.     

Morphology,fluid motion and predation by the scyphomedusa Aurelia aurita

Although medusan predators play demonstrably important roles in a variety of marine ecosystems, the mechanics of prey capture and, hence, prey selection, have remained poorly defined. A review of the literature describing the commonly studied medusa Aurelia aurita (Linnaeus 1758) reveals no distinct patterns of prey selectivity and suggests that A. aurita is a generalist and feeds unselectively upon available zooplankton. We examined the mechanics of prey capture by A. aurita using video methods to record body and fluid motions. Medusae were collected between February and June in 1990 and 1991 from Woods Hole, Massachusetts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita create fluid motions during swimming which entrain prey and bring them into contact with tentacles. We suggest that this mechanism dominates prey selection by A. aurita. In this case, we predict that medusae of a specific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected prey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusac because flow field velocity is a function of bell diameter. On the basis of prey escape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton such as barnacle nauplii and some slow swimming hydromedusae, while faster copepods will be negatively selected.     

Evidence for behavioral sensitivity to near-UV light in the deep-sea crustacean Systellaspis debilis

The role of UV light in the deep-sea environment has been discounted in the past, due to the assumptions that (1) there is insufficient UV light available for vision and, therefore (2) deep-sea organisms would not be sensitive to these wavelengths. A recent study that employed electrophysiological techniques on dark-captured deep-sea crustaceans demonstrated that several species of deep-sea crustaceans possess very high sensitivity to near-UV light. The current study was undertaken to determine if near-UV light would also elicit a behavioral response from these species. The species studied was Systellaspis debilis, an oplophorid shrimp whose daytime depth ranges from 600 to 700 m. A method for tethering shrimp was developed which allowed them to freely orient in response to changes in the ambient light field. Behavioral responses to changes in ambient light included changes in body tilt with respect to the horizontal plane, changes in swimming speed, and movement of the feeding appendages. These experiments, the first of their kind on a deep-sea organism, demonstrate that behaviorally, S. debilis is equally sensitive to very low intensities of blue-green and near-UV light.     

Oxygen consumption in the mullet Liza macrolepis with special reference to swimming velocity

Oxygen consumption of tagged (plastic opercular tag) and untagged mullet Liza macrolepis (Smith), forced to swim up to 22 cm/sec (fish size: 10 cm), increased proportionately to increase in swimming velocity above 5 cm/sec. The tag did not appear to cause any marked metabolic disturbance. The mean routine metabolic rate and the rate at 5 cm/sec were higher than the rates obtained for several higher levels of forced activity, possibly due to excitement and inefficient swimming at lower swimming speeds, as suggested by earlier workers. The interpretation of the elevation of a line drawn through the mean rates of metabolism at various swimming speeds, in relation to the standard metabolic rate, may be of value in quantitative expression of excitability of individual species.     

Comparative ultrastructure of lipid storage sites in femaleEuchaeta marina andPleuromamma xiphias (Copepoda: Calanoida)

FemaleEuchaeta marina (Prestandrea, 1833) have one large, thin-walled lipid sac, whereas femalePleuromamma xiphias (Giesbrecht, 1889) have two separate and morphologically distinct lipid storage sites. One lipid site inP. xiphias corresponds to the mesenteric tissue that surrounds the anterior region of the midgut. The morphology of these cells resembles that of mammalian brown adipocytes. The cytoplasm is filled with extensive smooth endoplasmic reticulum, numerous mitochondria and several deposits of intracellular lipid. The second lipid site ofP. xiphias lies in the posterior region of the metasome and resembles the thin-walled lipid sac ofE. marina. Both lie adjacent to, but are not contiguous with, the narrow mesenteric tissue surrounding the last region of the midgut. Both sacs contain a single, large deposit of intracellular lipid enclosed by a very thin rim of cytoplasm and resemble mammalian white adipocytes. The different habitats and reproductive processes of these two copepod species may relate to the observed variations in lipid cell morphology. The reserve lipid inE. marina plays a primary role in reproduction and is linked closely with the continuous cycle of oocytic maturation. The lipids synthesized and stored byP. xiphias, a strong vertical migrator, may be influenced by food availability, a function of their mesopelagic habitat. The primary role of the reserve lipids in this copepod may be to provide energy during migrations and between feeding periods, with relatively less lipid being allocated to reproduction.     

Wind- and tide-induced currents in the Stagnone lagoon (Sicily)

The hydrodynamic circulation is analyzed in the coastal lagoon of Stagnone di Marsala, a natural reserve located in the north-western part of Sicily, using both experimental measurements and numerical simulations. Field measurements of velocities and water levels, carried out using an ultrasound sensor (3D), are used to validate the numerical model. A 3D finite-volume model is used to solve the Reynolds-averaged momentum and mass balance differential equations on a curvilinear structured grid, employing the k–e{\varepsilon} turbulence model for the Reynolds stresses. The numerical analysis allows to identify the relative contribution of the forces affecting the hydrodynamic circulation inside the lagoon. In the simulations only wind and tide forces are considered, neglecting the effects of water density changes. Two different conditions are considered. In the first both the wind stress over the free-surface and the tidal motion are imposed. In the second the wind action is neglected, to separately analyze the tide-induced circulation. The comparison between the two test cases highlights the fundamental role of the wind on the hydrodynamics of the Stagnone lagoon, producing a strong vertical recirculation pattern that is not observed when the flow is driven by tides only.     

Locomotory behaviour and swimming performance of the Norway lobster,Nephrops norvegicus,in the presence of an acoustic tag

Observations have been made on the locomotory behaviour and swimming performances of the Norway lobster, Nephrops norvegicus (L.), fitted with an acoustic transmitter ventrally under the cephalothorax. The walking behaviour of adult males (44 mm carapace length) appeared to be unaffected, but the tag caused significant reductions in certain measures of tail-flip swimming performance such as swimming speed and endurance. Flume-tank experiments in low water currents suggested that the transmitter would increase hydrodynamic drag during swimming by 9 to 32%, depending on lobster size. Given the weight and dimensions of the acoustic transmitters currently available, it is considered advisable to confine acoustic tracking studies to relatively large N. norvegicus.Correspondence to: C. J. Chapman     

The effects of zooplankton swimming behavior on prey-capture kinematics of red drum larvae, <Emphasis Type="Italic">Sciaenops ocellatus</Emphasis>

Most marine fishes undergo a pelagic larval phase, the early life history stage that is often associated with a high rate of mortality due to starvation and predation. We present the first study that examines the effects of prey swimming behavior on prey-capture kinematics in marine fish larvae. Using a digital high-speed video camera, we recorded the swimming velocity of zooplankton prey (Artemia franciscana, Brachionus rotundiformis, a ciliate species, and two species of copepods) and the feeding behavior of red drum (Sciaenops ocellatus) larvae. From the video recordings we measured: (1) zooplankton swimming velocity in the absence of a red drum larva; (2) zooplankton swimming velocity in the presence of a red drum larva; and (3) the excursion and timing of key kinematic events during prey capture in red drum larvae. Two-way ANOVA revealed that: (1) swimming velocity varied among zooplankton prey; and (2) all zooplankton prey, except rotifers and ciliates, increased their swimming velocity in the presence of a red drum larva. The kinematics of prey capture differed between two developmental stages in S. ocellatus larvae. Hyoid-stage larvae (3–14 days old) fed on slow swimming B. rotundiformis (rotifers) while hyoid-opercular stage larvae (15 days and older) ate fast moving A. franciscana. Hyoid-opercular stage red drum larvae had a larger gape, hyoid depression and lower jaw angle, and a longer gape cycle duration relative to their hyoid-stage conspecifics. Interestingly, the feeding repertoire within either stage of red drum development was not affected by prey type. Knowledge of the direct relationship between fish larvae and their prey aids in our understanding of optimal foraging strategies and of the sources of mortality in marine fish larvae.     

Metabolic rates of midwater crustaceans as a function of depth of occurrence off the Hawaiian Islands: Food availability as a selective factor?

During July of 1983, 1986, and 1987, we measured rates of oxygen consumption of 234 individuals of 17 species of midwater crustaceans (orders Decapoda, Mysidacea, and Euphausiacea) off the Hawaiian islands at depths from the surface to greater than 1200 m. The routine metabolic rates declined with increasing depths of the species' occurrence to an extent greater than could be accounted for by depth-related changes in body size or water temperature. Most species appeared able to regulate their oxygen consumption down to the lowest oxygen partial pressures found in their depth range (20 mm Hg O₂), but did not regulate to such low oxygen partial pressures as did similar midwater crustaceans off California, where oxygen levels reach as low as 6 mm Hg. Metabolic rates of the shallower-living, but not the deepest-living Hawaiian crustaceans were significantly higher than those of Californian crustaceans. This is interpreted as indicating that the metabolic rates of midwater crustaceans are not adapted specifically to differing levels of primary production and that the decline with depth of metabolic rates in these species is not the result of food limitation at depth. The data are, however, consistent with the hypothesis that lower metabolic rates at depth are due to the relaxation of selection pressures relating to visual predation near the surface.     

Comparison of sinking velocity,swimming velocity,rotation and path characteristics among six marine dinoflagellate species

Six marine dinoflagellate species representing a range of equivalent spherical diameters between 12 and 36 m were examined for several characteristics that influence their translation velocity. Sinking velocities estimated by three independent techniques and applied to swimming and narcotized cells generally agreed, and followed the cell-size relationships previously reported for diatoms. Dinokont sinking and swimming velocities both decreased with increasing surface area: volume ratio, but a small desmokont deviated from the dinokont relationships. Sinking velocities influenced the relative ascent/descent capabilities of a species. The swim:sink ratio decreased as equivalent spherical diameter increased to 25 m and then remained constant at 7.6, despite further increases in cell size. This relationship suggests a minimum required swimming capability relative to cell size. The swim:sink ratio increased with increasing surface area:volume ratio for all the surveyed species. Out observations of decreasing cell rotation:translation ratio and increasing cell drag with increasing cell size supported the hypothesis that the dinoflagellate flagellar apparatus generates maximum swimming velocity at intermediate cell sizes. However, an alternate analysis supported the hypothesis that swimming velocity increases with cell size and that variations among genera are due to subtle differences in the basic dinoflagellate propulsion system. A three-dimensional helical path index provided a more realistic estimate of the actual translation velocity (along the helix axis) during diel vertical migration when applied as a correction factor to the more typically measured helix velocity (along the helix) of a given dinoflagellate.     

Availability of dissolved organic matter offsets metabolic costs of a protracted larval period for <Emphasis Type="Italic">Bugula neritina</Emphasis> (Bryozoa)

For nearly a century researchers have investigated the uptake and utilization of dissolved organic matter (DOM) by marine invertebrates, but its contribution to their growth, reproduction, and survival remains unclear. Here, the benefit of DOM uptake was assessed for the marine bryozoan Bugula neritina (Linnaeus 1758) through performance comparisons of individuals in the presence and absence of DOM. The experiments were performed using B. neritina collected from floating docks in Beaufort, NC, USA from July to September 2004. Seawater was subjected to ultraviolet irradiation to reduce naturally occurring DOM, and then enriched with either 1 μM of palmitic acid or a mixture containing 1 μM each of glucose, alanine, aspartic acid and glycine. Larvae in DOM-enriched and DOM-reduced treatments were sampled and induced to metamorphose following 1, 6, 12, and 24 h of continuous swimming at 25°C. Sampled larvae were assessed for initiation of metamorphosis, completion of metamorphosis, and ancestrular lophophore size to determine the extent to which energy acquired from DOM uptake could offset the metabolic costs of prolonged larval swimming. DOM treatment had no significant effect on initiation of metamorphosis, but did have a significant effect on completion of metamorphosis and lophophore size. Larvae swimming in DOM-enriched treatments for 24 h experienced a 20% increase in metamorphic completion rate, compared to larvae swimming for 24 h in the DOM-reduced treatment. In addition, larvae in the amino acid and sugar mixture for 24 h had a significantly larger lophophore surface area and volume (23 and 31%, respectively), compared to larvae in DOM-depleted seawater. To ensure that the increases in performance found in larvae with access to DOM were not due to a decrease in metabolic activity, the respiration rates for these larvae were compared to those of larvae in DOM-depleted seawater. There were no significant differences between these treatments, indicating that the increases in performance were due to the energy acquired from DOM. These results clearly show that for B. neritina, DOM uptake results in increased metamorphic success and in the size of the feeding apparatus following an extended larval swimming duration.     

Characterization of bedload intermittency near the threshold of motion using a Lagrangian sediment transport model

At the smallest scales of sediment transport in rivers, the coherent structures of the turbulent boundary layer constitute the fundamental mechanisms of bedload transport, locally increasing the instantaneous hydrodynamic forces acting on sediment particles, and mobilizing them downstream. Near the critical threshold for initiating sediment motion, the interactions of the particles with these unsteady coherent structures and with other sediment grains, produce localized transport events with brief episodes of collective motion occurring due to the near-bed velocity fluctuations. Simulations of these flows pose a significant challenge for numerical models aimed at capturing the physical processes and complex non-linear interactions that generate highly intermittent and self-similar bedload transport fluxes. In this investigation we carry out direct numerical simulations of the flow in a rectangular flat-bed channel, at a Reynolds number equal to Re = 3632, coupled with the discrete element method to simulate the dynamics of spherical particles near the bed. We perform two-way coupled Lagrangian simulations of 48,510 sediment particles, with 4851 fixed particles to account for bed roughness. Our simulations consider a total of eight different values of the non-dimensional Shields parameter to study the evolution of transport statistics. From the trajectory and velocity of each sediment particle, we compute the changes in the probability distribution functions of velocities, bed activity, and jump lengths as the Shields number increases. For the lower shear stresses, the intermittency of the global bedload transport flux is described by computing the singularity or multifr actal spectrum of transport, which also characterizes the widespread range of transport event magnitudes. These findings can help to identify the mechanisms of sediment transport at the particle scale. The statistical analysis can also be used as an ingredient to develop larger, upscaled models for predicting mean transport rates, considering the variability of entrainment and deposition that characterizes the transport near the threshold of motion.     

Energetic costs of swarming behavior for the copepod Dioithona oculata

The cyclopoid copepod Dioithona oculata forms dense swarms within shafts of sunlight that penetrate the mangrove prop-root habitat of islands off the coast of Belize. Previous studies, based on in situ video recordings and laboratory studies, have shown that D. oculata is capable of maintaining fixed-position swarms in spite of currents of up to 2 cm s⁻¹. The purpose of this study was to examine the energetic costs of maintaining these swarms, in terms of increased metabolic costs of maintaining position in currents and in terms of reduced feeding rates in densely packed swarms during the day. Using a sealed, variable-speed flow-through chamber, the respiration rates of D. oculata were measured while swarms maintained position in different current speeds. The results indicate that active metabolism (swimming at maximum speed to maintain the swarm in a current) is approximately three times greater than routine metabolism (normal swimming speeds in the absence of currents), indicating a significant metabolic cost of maintaining swarms in the presence of currents. In addition, gut-pigment analysis indicated that feeding rates of these copepods were often reduced in swarms during the day compared to when the copepods were dispersed at night. Given the high “cost” of swarming, the adaptive value of swarming in terms of reduced predation, increased opportunities for mating, and reduced dispersal, must be substantial. Received: 4 June 1997 / Accepted: 18 September 1997