Feeding by an omnivorous planktonic copepod aetideus divergens Bradford

Aetideus divergens Bradford is representative of a large group of marine planktonic calanoid copepods which are omnivores or mixed feeders. Because very little quantitative information is available on the feeding behavior of these copepods, laboratory feeding experiments have been carried out with adult female A. divergens presented with various sizes and concentrations of diatoms and freshly hatched nauplii of Artemia. The copepod fed most efficiently on the largest size of diatom and on Artemia nauplii, but was peculiarly inefficient at feeding on small diatoms, even when they were available at very high concentrations. In this respect, the copepod differs from filterfeeding copepods, such as Calanus pacificus Brodsky. A possible explanation of this difference depends upon Aetideus being less capable of handling very small food particles than Calanus. Aetideus divergens and its congeners usually occur at subsurface depths not far below the mixed layer and seem to be adapted for feeding on large particles, possibly large phytoplanktonic organisms and fecal pellets, which sink out of the mixed layer.     

Implications of the feeding current structure of Euchaeta rimana, a carnivorous pelagic copepod, on the spatial orientation of their prey

Many marine planktonic organisms create water currents to entrainand capture food items. Rheotactic prey entrained within thesefeeding currents often exibit escape reactions. If the directionof escape is away from the feeding current, the prey may successfullydeter predation. If the escape is towards the center of thefeeding current, the prey will be re-entrained towards its predatorand remain at risk of predation. The direction of escape isdependent on (i) the ability of the prey to escape in a directiondifferent than its pre-escape orientation and (ii) the orientationcaused by the interaction of the prey's body with the movingfluid. In this study, the change in orientation of Acartia hudsonicanauplii as a result of entrainment within the feeding currentof Euchaeta rimana, a planktonic predatory copepod, was examined,When escaping in still water, A.hudsonica nauplii were ableto vary their pre-escape direction by only 10. This allowsonly a limited ability to escape in a direction different thantheir pre-escape orientation. Analyses of the feeding currentof E.rimana show the flow speed to be most rapid in the centralregion with an exponential decrease in speed distally. In contrast,flow vorticity is minimal in the center of the feeding currentand maximal at 1.75 mm along the antennae. As a result, thedegree of rotation of the prey towards the center of the feedingcurrent shows a strong dependency on the prey's location withinthe feeding current. The feeding current of E.rimana rotatedthe prey 14 when near the center of the flow field and up to160 when located more distal in the feeding current Since theprey's escape abilities cannot compensate for the rotation dueto the flow, this mechanism will maintain the escaping preywithin the feeding current of their predator. Therefore, thefeeding current facilitates predatory copepods in capturingprey by (i) increasing the amount of water which passes overtheir sensors and through their feeding appendages and (ii)controlling the spatial orientation of their prey prior to escape.     

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.     

Swimming and feeding behaviour of the planktonic copepod Clausocalanus furcatus

The objective of this study was to quantify motion and feeding behaviour of the small calanoid Clausocalanus furcatus, which is a common and abundant species in oligotrophic environments. Adult females were video recorded at 60 fields s^-1 using video equipment which allowed us to follow each free-swimming individual continuously in 3 1 vessels. At 20C under dark conditions, C.furcatus moved continuously along convoluted small loops at a mean speed of 10 mm s^-1 (corresponding to 10 body lengths s^-1). This motion was occasionally interrupted by sudden somersaulting performed at very high speed (up to 17 mm s^-1). The copepods only occasionally sank. There was no evidence that C.furcatus created feeding currents. Dinoflagellate cells offered as food appeared to be perceived by direct encounter at high speed, being instantaneously captured and ingested or rejected. The capture rates were correlated with the frequency of somersaulting. By moving fast and by associating a high turning rate with a high frequency of crossing the previous tracks, C.furcatus searched 22-26% of the explored volumes. The motion and feeding behaviour of C.furcatus show that the foraging tactic of this species is to explore small volumes of water rapidly. This strategy appears, so far, to be unique among small planktonic copepods.      

Predation by a carnivorous marine copepod, Euchaeta elongata Esterly, on eggs and larvae of the Pacific hake, Merluccius productus

The predatory behavior of the carnivorous marine copepod, adultfemale Euchaeta elongata Esterly, feeding on eggs and larvaeof the Pacific hake, Merluccius productus, was examined in thelaboratory and in a natural setting. E. elongata did not feedon eggs. Predation on larvae is believed to depend on larvalswimming behavior: (1) predation rates were low on early stageyolk-sac larvae which are inactive swimmers and are relativelyundetected by the predator; (2) rates were high on middle stageyolk-sac larvae which are more active swimmers yet have a poorlydeveloped escape response; and (3) rates were low on largerlarvae which are able to escape the predator effectively. Starvedhake larvae were more vulnerable to predation due to a poorescape ability although they were less active and not easilydetected. The presence of naturally occurring alternative prey,Pseudocalanus sp., depressed the rate of E. elongata predationon hake larvae. In an analysis of field data, hake larvae andE. elongata were found to occupy the same depths in Dabob Bay.A high percentage of E. elongata collected had apparently beenfeeding on hake larvae, as indicated by the presence of pigmentsin their guts. Survival of hake larvae in late spring appearsto be relatively poor compared with early spring; poorer survivalin late spring may be due partly to an increase in the abundanceof invertebrate predators, such as E. elongata. ^*Current address: Northwest and Alaska Fisheries Center, 2725Montlake Blvd, Seattle, WA 98112, USA.     

Egg production of the marine,planktonic copepod,Calanus pacificus Brodsky: Laboratory observations

When acclimated to a continuous, superabundant food supply and constant temperature, Calanus pacificus Brodsky females produce eggs at a weight-specific rate ranging from 0.13 · day^?1 at 8°C to 0.21 · day^?1 at 15°C. Maximum weight-specific egg production rates do not change with seasonal changes in female body size. The relationship between egg production rate and food concentration is hyperbolic, with threshold and critical concentrations that are high relative to other species for which data are available. Food concentration and temperature influence spawning frequency (i.e., the time required for oocytes to mature) much more than the number of eggs in a single spawning event (i.e., clutch size). Clutch size is significantly related to female body size.     

Rapid firing rates from mechanosensory neurons in copepod antennules

Small detection distances coupled with rapid movements require copepods to respond to stimuli with behavioral latencies on the order of milliseconds. Receiving adequate sensory information in such a short time necessitates extremely rapid firing rates of the efferent receptors. Here we show that copepod mechanoreceptors can fire at frequencies up to 5 kHz in response to fluid mechanical stimuli. Neural activity at these frequencies enables these animals to code for a range of fluid velocities thus providing important information regarding the nature of different fluid disturbances.     

Seasonal population dynamics and production of Microsetella norvegica, a widely distributed but little-studied marine planktonic harpacticoid copepod

Microsetella norvegica is a widely distributed marine planktonicharpacticoid copepod, which is poorly known from the biologicalpoint of view. We investigated the seasonal population dynamicsandproduction of M. norvegica in the central part of the InlandSea of Japan. It occurred throughout the year, whilst its reproductionwas confined to the warm season between May and November. Theproportion of ovigerous females, which carry a single egg sac,was low (mean: 23.1%) in August and September, and high (53.6%)in October. Their brood size attained a maximum (mean: 15.8eggs per sac) in July and August and declined gradually to 6.2eggs in November. Duration time from egg laying to moultingto adulthood was temperature-dependent; it was 31.9 and 14.3days at 20 and 27°C, respectively, under excess food conditionsin the laboratory. An enormously high population abundance (7.32x 10⁴ individuals m^-3), which accounted for 86.5% of the totalcopepods, and biomass (69.6 mg C m^-3) gave an annual maximumproduction rate of 4.90 mg C m^-3 day^-1) in October. Naupliiand copepodites disappeared in December, and the overwinteringpopulation was represented by adults, mainly large females.Associations of M. norvegica with marine snow aggregates, whichhave often been found in oligotrophic waters, were not observedin the food-rich environment of the Inland Sea of Japan.     

Cryptic ecological diversification of a planktonic estuarine copepod, Acartia tonsa

The recent discovery of cryptic species in marine holoplankton, organisms that 'drift' in oceanic currents throughout their life cycle, contrasts with their potential for long-distance passive dispersal and presumably high gene flow. These observations suggest that holoplankton species are adapting to surprisingly small-scale oceanographic features and imply either limited dispersal or strong selection gradients. Acartia tonsa is a widespread and numerically dominant estuarine copepod containing deep mitochondrial lineages within and among populations along the northwestern Atlantic coast. In this study, we intensively investigated A. tonsa populations in Chesapeake Bay with the goals of testing species status for the deep lineages and testing for their association with environmental features over space and time. Phylogenetic analyses of DNA sequences from mitochondrial cytochrome c oxidase I (mtCOI) and the nuclear ribosomal internal transcribed spacer (nITS) resolved two concordant monophyletic clades. Deep divergence between the two clades (13.7% uncorrected sequence divergence for mtCOI and 32.2% for nITS) and genealogical concordance within sympatric populations strongly suggest that the two clades represent reproductively isolated cryptic species. Based on restriction fragment length polymorphisms of mtCOI, representatives from the two clades were found consistently associated with contrasting salinity regimes (oligohaline vs. meso-polyhaline) with an overlap between 2 and 12 PSU in samples from 1995 to 2005. Finding these patterns in one of the best-known estuarine copepods reinforces the conclusion that marine biodiversity is underestimated, not only in terms of species numbers, but also with respect to niche partitioning and the potential importance of ecological divergence in marine holoplankton.     

Predator-induced diet vertical migration in a planktonic copepod

Predator evasion is the most commonly hypothesized reason fordiel vertical migrations undertaken by a wide variety of planktonicorganisms in lakes and seas, yet direct evidence remains elusive.We tested the predation hypothesis by exposing enclosed populationsof a marine copepod Acartia hudsonica to caged or free-rangingindividuals of their natural predator, the planktivorous fishGasterosteus aculeatus. After little more than a week, adultcopepods changed their vertical distribution and migration behaviordepending on the presence or absence of predation. Only free-rangingfish induced vertical migration in the copepod population. Cagedfish had no effect, indicating that vertical migration was nota simple chemically mediated response of copepods to the predator.Rather, copepods seemed to react to the presence of predatorsby other means, perhaps visual or mechanical stimuli, and toexhibit a downward escape response which, because encounterswith visually orienting fish occur chiefly in the daytime, effectivelylimited the copepods' occurrence in the upper water column tothe night-time hours. Alternatively, because fish imposed heavymortality on copepods, it is possible that selective predationaltered the proportions of individuals with fixed, geneticallydetermined migration behaviors. We suggest experiments to distinguishthese alternatives.     

Chemical, mechanical and visual cues in the vertical migration behavior of the marine planktonic copepod Acartia hudsonica

Recent experimental evidence in both marine and freshwater systemsindicates that predators can induce vertical migration behaviorin individual zooplankters, yet the specific cues by which zooplankterssense their predators appear to vary. In situ manipulation experimentswere carried out with enclosed populations of the marine planktoniccopepod Acartia hudsonica to re-examine the potential role ofchemical cues in the behavior of A.hudsonica, and to test explicitlyfor the role of mechanical or visual stimuli in triggering verticalmigration behavior in this species. Adult female copepods wereinduced to vertically migrate (descend) when exposed to fishmimics during the day, but no such response occurred when thecopepods were exposed to Fish mimics during the night. Moreover,copepods exhibited no changes in vertical distribution whenexposed to water which, having recently held a natural predator(the threespine stickleback, Gasterosteus aculeatus), was presumedto be laden with predator-produced chemical exudates. Predator-mediatedmechanical or visual cues, or a hierarchy of both, are responsiblefor eliciting vertical migration behavior in adult female A.hudsonica.These results, together with those of other investigations demonstratingthe inducing role of chemical exudates, indicate that the stimulieliciting vertical migration in zooplankton can be expectedto vary between species.     

Photoreactivation of UV-induced damage to embryos of a planktonic copepod

UV irradiation damaged the embryos of the planktonic copepodCalanus sinicus. The hatching rate was reduced, and the hatchedlarvae were deformed by the UV irradiation at unweighted intensitiessimilar to those occurring naturally. However, the UV-induceddamage was reduced by simultaneous irradiation with the visiblelight from a fluorescent lamp. This photoreactivation may playan important role in reducing the mortality of this speciesin the marine environment.     

Extraordinary prolongation of the life cycle in a freshwater planktonic copepod

The limnetic copepod Cyclops scutifer Sars shows an extraordinarily prolonged life cycle in a temperate lake near Oslo, Norway. Developmental sequences become one, two, and three years old, and there are circumstantial evidence of a four year old fraction. Prolongation of the cycle is mainly caused by two consecutive periods of diapause, the first as small copepodids during the second winter, the other as large copepodids during the third winter. There is a marked concentration of diapausing individuals in a small area of the bottom at the deepest part of the lake. There is evidence that this concentration is caused by a horizontal migration to areas with optimal oxygen conditions. The adaptive value of a prolongation of the life cycle is assumed to be an increase in the reproductive potential caused by a spreading out of the reproductive period by three year classes succeeding each other throughout the summer.     

Bacteria associated with a marine planktonic copepod in culture. II. Degradation of fecal pellets produced on a diatom, a nanoflagellate or a dinoflagellate diet

Copepods (A cartia tonsa Dana) were allowed to feed on culturedalgal species simulating the yearly boreal phytoplankton succession,where the spring is characterized by diatoms, the summer bynanoflagellates and the autumn by dinoflagellates. Degradationof freshly produced fecal pellets in suspension was followedover time by volume recording and estimation of carbon loss.The fecal pellets produced on a diatom diet were very denseand showed a significantly slower degradation than those producedon nanoflagellate or dinoflagellate diets. This indicates thatthe algal texture, i.e. the mechanical robustness given by inertstructures, determines the degradation rate of the pellets.The degradation rate was also very dependent on the algal concentrationfor the grazer (565 µg Cl^–1 T₁     

Endogenous control of spawning in a marine copepod

Daily pattern of egg release has been studied in the marine calanoid copepod, Labidocera aestiva (Wheeler) under cycling light-dark (12L-12D, 12D-12L) conditions, and constant light (24L) or darkness (24D). Under cycling conditions maximum egg release occurred during the dark phase. Rhythmic spawning was also evident under 24D, but not under 24L. The results suggest that the pattern of nocturnal egg release is regulated by an endogenous rhythm entrained by light.     

Priority effects in a planktonic bloom-forming marine diatom

Priority effects occur when a species or genotype with earlier arrival has an advantage such that its relative abundance in the community or population is increased compared with later-arriving species. Few studies have dealt with this concept in the context of within-species competition. Skeletonema marinoi is a marine diatom that shows a high degree of genetic differentiation between populations over small geographical distances. To test whether historical events such as priority effects may have been important in inducing these patterns of population differentiation, we performed microcosm experiments with successive inoculation of different S. marinoi strains. Our results show that even in the absence of a numerical advantage, significant priority effects were evident. We propose that priority effects may be an important mechanism in initiating population genetic differentiation.     

Stoichiometry of some marine planktonic crustaceans

Atomic C:N ratios in calanoid copepods were generally belowthe Redfield ratio (6.6), except for the fifth copepodid stageof Calanus sp. The C:P ratios in copepods were generally closeto, or higher than the Redfield ratio of 106, but low C:P ratioswere found in Acartia clausi (63±7) and in the cladocerans(Podon sp.: 34±5 and Evadne sp.: 59±22).     

Metagenomic analysis of a complex marine planktonic thaumarchaeal community from the Gulf of Maine

Thaumarchaea, which represent as much as 20% of prokaryotic biomass in the open ocean, have been linked to environmentally relevant biogeochemical processes, such as ammonia oxidation (nitrification) and inorganic carbon fixation. We have used culture-independent methods to study this group because current cultivation limitations have proved a hindrance in studying these organisms. From a metagenomic data set obtained from surface waters from the Gulf of Maine, we have identified 36,111 sequence reads (containing 30 Mbp) likely derived from environmental planktonic Thaumarchaea. Metabolic analysis of the raw sequences and assemblies identified copies of the catalytic subunit required in aerobic ammonia oxidation. In addition, genes that comprise a nearly complete carbon assimilation pathway in the form of the 3-hyroxypropionate/4-hydroxybutyrate cycle were identified. Comparative genomics contrasting the putative environmental thaumarchaeal sequences and 'Candidatus Nitrosopumilus maritimus SCM1' revealed a number of genomic islands absent in the Gulf of Maine population. Analysis of these genomic islands revealed an integrase-associated island also found in distantly related microbial species, variations in the abundance of genes predicted to be important in thaumarchaeal respiratory chain, and the absence of a high-affinity phosphate uptake operon. Analysis of the underlying sequence diversity suggests the presence of at least two dominant environmental populations. Attempts to assemble complete environmental genomes were unsuccessful, but analysis of scaffolds revealed two diverging populations, including a thaumarchaeal-related scaffold with the full urease operon. Ultimately, the analysis revealed a number of insights into the metabolic potential of a predominantly uncultivated lineage of organisms. The predicted functions in the thaumarchaeal metagenomic sequences are directly supported by historic measurements of nutrient concentrations and provide new avenues of research in regards to understanding the role Thaumarchaea play in the environment.