Feeding interactions of the copepods Eurytemora affinis and Acartia bifilosa with the cyanobacteria Nodularia sp.

We measured ingestion and clearance rates of two Baltic Seacalanoid copepods, Eurytemora affinis and Acartia bifilosa,on toxic and non-toxic cyanobacteria Nodularia sp. using theisotope technique. Eurytemora affinis fed actively on the non-toxicstrain and moderately actively on the toxic strain, whereasA.bifilosa totally avoided feeding on both strains. This suggeststhat A.bifilosa rejected cyanobacterial filaments due to theirnutritional inadequacy or difficult manageability. The differentresponse of E.affinis to the non-toxic and toxic strains, inturn, shows that this copepod species was able to sense thepresence of the toxin in cyanobacterial filaments and thereforefed less on the toxic strain. The interaction between A.bifilosaand Nodularia sp. was further examined (with the particle countingmethod) by measuring the clearance rates of A.bifilosa on ediblegreen flagellates in the presence of cyanobacteria. The presenceor concentration of toxic Nodularia sp. did not affect grazingrates of A.bifilosa on Brachiomonas submarina. Since earlierstudies have shown that ingestion of Nodularia sp. decreasesegg production and increases mortality in E.affinis, we suggestthat the occurrence of Nodularia sp. blooms in the Baltic Seamay favour individuals of copepod species capable of selectivefeeding, such as A.bifilosa.     

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.     

Escape reaction performance of myelinated and non-myelinated calanoid copepods

Calanoid copepods from seven families in three superfamilies were exposed to a controlled near-field hydrodynamic stimulus and their escape reactions were recorded using high-speed videographic techniques. Copepod species have two distinct mechanisms for increasing conduction speed of neural signals: larger diameter nerve axons and insulated axons, i.e., myelination. Myelinated axons have been found in certain species of the more recently-evolved calanoid superfamilies. Copepod representatives from these superfamilies were expected to have shorter response latencies than species from more ancestral superfamilies due to the increased conduction speed of nerve impulses in myelinated neurons. Using frame-by-frame playback and computerized motion analysis techniques, response latency, jump speed, and acceleration were measured. Kinetic performance of copepods was highly variable, with mean escape speeds ranging between 100-250 mm s^− 1 and accelerations of 9-230 m s^− 2. Minimum behavioral response latencies of 2 ms were recorded for both myelinated and non-myelinated calanoids. There was no significant difference between the response latencies of copepods from the myelinated and non-myelinated superfamilies. Furthermore, no relationships were found between copepod latency and size for either myelinated or non-myelinated species. Previous research may suggest that myelin may shorten the response latencies of certain calanoid species. However, our results show that non-myelinated copepods are also capable of responding rapidly, within as few as 2 ms, to hydrodynamic stimuli and produce similar kinetic performance to myelinated species. The main advantage of myelination over giant nerve axons is their more efficient transfer of nerve impulses resulting in a metabolic energy savings. Although this energetic reward would be important for copepods in food-limited environments, for coastal copepods, in food-rich habitats, either mechanism is a viable solution.     

Effect of prey size on vulnerability of copepods to predation by the scyphomedusae Aurelia aurita and Cyanea sp.

A growing number of studies correlate changes in zooplanktonpopulations with abundance of medusae, but we cannot yet explainor predict the specific factors driving these interactions.This study demonstrates that the size of copepods has a significantinfluence on their vulnerability to predation by scyphomedusae.This finding is important because prey size, independent ofbehavior, has been neglected in theoretical models of predationby medusae. In experiments in a planktonkreisel, we used liveand heat-killed prey (Acartia hudsonica adults and copepodites)to separate the effects of copepod size and behavior on feedingrates by two medusae (Aurelia aurita and Cyanea sp.). Resultsrevealed that: differences in copepod size had a significantimpact on feeding rates, and thus small size can provide a refugefrom predation; behavior of adults diminished the liabilityassociated with larger size; and medusae with different morphologiesingested A.hudsonica at similar rates. Other experiments demonstratedthat medusae digested copepods at different rates based on preysize and predator species, findings that have implications forall future laboratory and field studies that assess feedingby scyphomedusae. Finally, this study illustrates how laboratorystudies serve as critical supplements to field observations.The effect of prey size on feeding rates can be confounded bydifferences in prey behavior, yet explains why small copepodswere typically ingested at relatively low rates by medusae.Size was clearly a dominant factor influencing copepod vulnerabilityacross scyphomedusan species, even those with very differentmorphologies. Future work should focus on the mechanisms ofsize selection, or the factors influencing contact and retentionrates.     

A cholesterol-enriched diet enhances egg production and egg viability without altering cholesterol Content of biological membranes in the copepod Acartia hudsonica

Copepods may lack the capacity for de novo synthesis of cholesterol, while at the same time their dietary levels of sterol vary. We tested the hypothesis that copepods maintain the cholesterol contents of their biological membranes despite varying dietary levels of cholesterol. Acartia hudsonica were acclimated for 5 d to phytoplankton alone or phytoplankton supplemented with cholesterol, at a level sufficient to induce a maximal response on egg production rates. Biological membranes were prepared from the copepods and cholesterol contents assayed. Egg production and hatch rates were measured (the former to confirm that supplemented cholesterol was being assimilated). Analyses of marker enzymes indicate that the majority of membrane-associated cholesterol in the copepod resides in the plasma membrane. In membranes fractions, cholesterol normalized to protein or activity of Na+/K+-ATPase is not significantly different for supplemented and unsupplemented groups (29 and 33 mu g cholesterol mg(-1) protein, respectively; 0.24 and 0.25 mg cholesterol U(-1) Na+/K+-ATPase, respectively). At the same time, acclimating animals to a diet enriched with cholesterol enhances egg production by up to 1.8-fold and egg viability by 1.5-fold. We conclude that a cholesterol-enriched diet stimulates both egg production and hatching rates without altering cholesterol contents of plasma membranes in the copepod A. hudsonica.     

Autotrophic and heterotrophic nanoplankton in the diet of the estuarine copepods Eurytemora affinis and Acartia bifilosa

The ingestion of autotrophic and heterotrophic nanoplanktonby two estuarine copepods, Eurytenora affinis and Acarith bifilosa,was measured in various environmental conditions using the incubationmethod and epifluorescence microscopy. Egg production of thespecies was also deter mined in order to estimate their carbonrequirements. Assuming a gross efficiency of egg productionof 0.3, nanoplanktonic carbon ingested always met the carbonrequirements suggesting that, most of the time, other carbonsources could be unnecessaly. Nanoplankton ingestion by A.bifilosa(from 128 to 1693 cells ind.^–1 h^–1) was dominatedby autotrophic forms (60–97%) and was seriously affectedby high (>100 mg l^–1 suspended particulate matter (SPM)concentrations. Nanoplankton ingestion by E.affinis (from 300to 1049 cells ind^–1 h^–1) was relatively stable incomparison, but this latter species seemed to switch its grazingpressure from autotrophic to heterotrophic forms when SPM concentrationsincreased. Thus, two copepod species, living in the same estuary,presented two different feeding behaviours, probably to maximizeenergy input per unit of energy expenditure. Such differencescould contribute to the spatial and seasonal segregation ofthese species which is usually observed.      

Egg production of individual copepods of Acartia grani Sars from coastal waters: seasonal and diel variability

Seasonal and diel fecundity patterns of adult Acartia granifemales from a coastal area of Malaga Bay (south of Spain) werestudied between spring and autumn 1990. Copepod egg productionwas measured in situ by short-term (     

Sensory ecology of scyphomedusae

The sensory ecology of scyphomedusae is a subject ripe for investigation. There are sensory structures for which there is no experimental evidence of function, there are behavioral performances for which there are no demonstrated sense organs, and there are behaviors for which there are no adaptive explanations. Scyphomedusae are diploblastic animals with a primitive level of neurological complexity, yet they exhibit a wide range of complex and sophisticated behaviors, such as sun compass navigation and daily horizontal migrations. This paper reviews the sensory ecology of orientation and migratory behavior in scyphomedusae, with an emphasis on data from Aurelia aurita. Adaptive explanations for complex behavior of individuals must be evaluated at the population level in order to assess their ecological importance.     

Persistence of Enterococcus faecalis in aquatic environments via surface interactions with copepods

Several human pathogens and fecal-pollution indicators may persist as viable organisms in natural environments, owing to their ability to activate different types of survival strategies. These strategies include adhesion on both abiotic and biotic surfaces and the entrance to the so-called viable but nonculturable (VBNC) state. In an 18-month survey for the detection of enterococci in both lake water and seawater, C. Signoretto et al. (Appl. Environ. Microbiol. 70:6892-6896, 2004) have shown that Enterococcus faecalis was detected mostly bound to plankton and in the VBNC state. In the present study, we show that in vitro adhesion of E. faecalis to copepods accelerated the entry of cells into the VBNC state relative to that of planktonic bacteria. VBNC E. faecalis cells maintained their adhesive properties to copepods and chitin (the main component of the copepod carapace), though to a reduced extent in comparison with growing cells. Sugar competition experiments showed interference with adhesion to both copepods and chitin by GlcNAc and only to copepods by D-mannose. Four enterococcal cell wall proteins present in both growing and VBNC cells and lipoteichoic acid were shown to be capable of binding chitin. The results indicate that copepods may represent an additional environmental reservoir of enterococci, thus suggesting the advisability of redesigning the protocols currently used for microbial detection during the evaluation of the microbiological quality of environmental samples.     

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.     

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.     

Determining the Advantages,Costs, and Trade-Offs of a Novel Sodium Channel Mutation in the Copepod Acartia hudsonica to Paralytic Shellfish Toxins (PST)

The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium fundyense, which produce paralytic shellfish toxins (PST). Adaptation to PSTs in other organisms is caused by a mutation in the sodium channel. Recently, a mutation in the sodium channel in A. hudsonica was found. In this study, we rigorously tested for advantages, costs, and trade-offs associated with the mutant isoform of A. hudsonica under toxic and non-toxic conditions. We combined fitness with wild-type: mutant isoform ratio measurements on the same individual copepod to test our hypotheses. All A. hudsonica copepods express both the wild-type and mutant sodium channel isoforms, but in different proportions; some individuals express predominantly mutant (PMI) or wild-type isoforms (PWI), while most individuals express relatively equal amounts of each (EI). There was no consistent pattern of improved performance as a function of toxin dose for egg production rate (EPR), ingestion rate (I), and gross growth efficiency (GGE) for individuals in the PMI group relative to individuals in the PWI expression group. Neither was there any evidence to indicate a fitness benefit to the mutant isoform at intermediate toxin doses. No clear advantage under toxic conditions was associated with the mutation. Using a mixed-diet approach, there was also no observed relationship between individual wild-type: mutant isoform ratios and among expression groups, on both toxic and non-toxic diets, for eggs produced over three days. Lastly, expression of the mutant isoform did not mitigate the negative effects of the toxin. That is, the reductions in EPR from a toxic to non-toxic diet for copepods were independent of expression groups. Overall, the results did not support our hypotheses; the mutant sodium channel isoform does not appear to be related to adaptation to PST in A. hudsonica. Other potential mechanisms responsible for the adaptation are discussed.     

Contagious fear: Escape behavior increases with flock size in European gregarious birds

Flight initiation distance (FID), the distance at which individuals take flight when approached by a potential (human) predator, is a tool for understanding predator–prey interactions. Among the factors affecting FID, tests of effects of group size (i.e., number of potential prey) on FID have yielded contrasting results. Group size or flock size could either affect FID negatively (i.e., the dilution effect caused by the presence of many individuals) or positively (i.e., increased vigilance due to more eyes scanning for predators). These effects may be associated with gregarious species, because such species should be better adapted to exploiting information from other individuals in the group than nongregarious species. Sociality may explain why earlier findings on group size versus FID have yielded different conclusions. Here, we analyzed how flock size affected bird FID in eight European countries. A phylogenetic generalized least square regression model was used to investigate changes in escape behavior of bird species in relation to number of individuals in the flock, starting distance, diet, latitude, and type of habitat. Flock size of different bird species influenced how species responded to perceived threats. We found that gregarious birds reacted to a potential predator earlier (longer FID) when aggregated in large flocks. These results support a higher vigilance arising from many eyes scanning in birds, suggesting that sociality may be a key factor in the evolution of antipredator behavior both in urban and rural areas. Finally, future studies comparing FID must pay explicit attention to the number of individuals in flocks of gregarious species.     

Gut evacuation rates of Antarctic copepods during austral spring

The results of gut evacuation experiments performed on Antarctic copepods during the austral spring are presented and discussed. Four species of large copepods commonly occurring in the Indian sector of the Antarctic Ocean were studied: Calanus propinquus, Calanoides acutus, Rhincalanus gigas and Pleuromamma robusta. For each species two experiments were carried out, one in daytime and one in night-time, except for Calanoides acutus, which was only studied at night. P. robusta showed pigment gut retention in all experiments. The results showed that all species studied had a longer gut passage time than that previously recorded and that gut evacuation rate appears to decrease during daytime. Accepted: 1 October 1998     

Feeding, excretion and egg production by individuals and populations of the marine, planktonic copepods, Acartia spp. and Centropages furcatus

Diel variations in vertical distribution, gut pigment content,ammonium excretion and egg production were investigated foradult females of Acartia erythraea and A.pacifica in the verticallymixed Inland Sea of Japan and Centropages furcatus in the stratified,neritic Gulf of Mexico. Gut pigment content and egg productionrate were maximal at night and ammonium excretion was maximalduring the daytime. Neither A.erythraea nor A.pacifica adultfemales showed an apparent diel migration, but the former werehighly concentrated in the surface layer during the afternoon.In contrast, C.furcatus adult females showed a clear diel migration,residing immediately above the bottom during the daytime andbeing concentrated between 10 and 25 m depth during the nighttime.Individual-based data on gut content and excretion and egg productionrates were combined with vertical-distribution data to calculatepopulation values. In the Inland Sea of Japan, the resultantpattern for Acartia spp. reflected the diel variation in physiologicalrates and even distribution of adult females, except for theafternoon, surface aggregation of A.erythraea. In the Gulf ofMexico, the pattern for C.furcatus reflected largely the dielvariation in each rate process and the heterogeneous distributionof adult females in the water column. Elevated nocturnal feedingactivity of these copepods may be due to an endogenous rhythm.The daytime maximum in ammonium excretion and night-time maximumin egg production rate indicated approximate half-day and daytime lags, respectively, after the intake of food until itsconversion into dissolved excreta and released eggs.     

The nature of interactions that contribute to postzygotic reproductive isolation in hybrid copepods

Deleterious interactions within the genome of hybrids can lower fitness and result in postzygotic reproductive isolation. Understanding the genetic basis of these deleterious interactions, known as Dobzhansky-Muller incompatibilities, is the subject of intense current study that seeks to elucidate the nature of these deleterious interactions. Hybrids from crosses of individuals from genetically divergent populations of the intertidal copepod Tigriopus californicus provide a useful model in which to study Dobzhansky-Muller incompatibilities. Studies of the basis of postzygotic reproductive isolation in this species have revealed a number of patterns. First, there is evidence for a breakdown in genomic coadaptation between mtDNA-encoded and nuclear-encoded proteins that can result in a reduction in hybrid fitness in some crosses. It appears from studies of the individual genes involved in these interactions that although this coadaptation could lead to asymmetries between crosses, patterns of genotypic viabilities are not often consistent with simple models of genomic coadaptation. Second, there is a large impact of environmental factors on these deleterious interactions suggesting that they are not strictly intrinsic in nature. Temperature in particular appears to play an important role in determining the nature of these interactions. Finally, deleterious interactions in these hybrid copepods appear to be complex in terms of the number of genetic factors that interact to lead to reductions in hybrid fitness. This complexity may stem from three or more factors that all interact to cause a single incompatibility or the same factor interacting with multiple other factors independently leading to multiple incompatibilities.     

Motion behavior of nauplii and early copepodid stages of marine planktonic copepods

The goal of this study was to quantify periods of activity andvelocities of late naupliar and early copepodid stages of planktoniccopepods occurring regularly on the southeastern continentalshelf of the USA. We obtained quantitative information on eightspecies, including adult females of Oithona plumifera. All studieswere conducted at food concentrations near or above satiationlevels. Activities ranged from 0.85% (adult females of O.plumifera)to 100% of time (nauplii and copepodids of various calanoidspecies). Motion velocities (excluding escape motion) coveredmore than one order of magnitude: from 0.39 mm s^–1 fornauplii of Temora stylifera to 5.24 mm s^–1 for naupliiof Oncaea mediterranea. Ranges of activities of species rangefrom occasional for early juveniles to adult females of O.plumiferato 100% for the same range of T.stylifera, the latter creatinga feeding current from N III onwards, the former not at all.Of notable interest is Centropages velificatus which moves intermittentlyas a late nauplius, continuously as an early copepodid and intermittentlyas an adult. All observed calanoid late nauplii and copepodidsmove in three dimensions, excluding copepodids of the shelfbreak/oceanicParacalanus aculeatus. The results indicate not only significantdifferences in motion behavior between cyclopoids and calanoids,but also between calanoid species. Yet, some calanoid speciesshow little ontogenetic changes at all.