Effects of the red tide dinoflagellate, Karenia brevis, on grazing and fecundity in the copepod Acartia tonsa

Among studies of copepod grazers fed harmful algae, decreasedgrazing and fecundity are the most common results. The causesof decreased grazing (physiological incapacitation, behavioralavoidance or lack of stimulation) and decreased fecundity (toxicversus nutritional effect) vary among studies. This study useda series of controlled laboratory experiments to investigatethe cause of decreased grazing and fecundity in the copepodAcartia tonsa fed sole and mixed diets of the harmful alga,Karenia brevis. Copepods fed K. brevis mixed with the nutritionallyviable dinoflagellate Peridinium foliaceum had higher ingestionrates and offspring production than copepods fed a sole dietof K. brevis (even when K. brevis was virtually nontoxic). Copepodsfed mixtures did not discriminate between P. foliaceum and K.brevis while feeding. The results of this study suggest thatK. brevis is not toxic to A. tonsa but lacks some chemical componentresponsible for stimulating a grazing response in A. tonsa aswell as the nutritional requirements for normal offspring production.     

The escape behavior of marine copepods in response to a quantifiable fluid mechanical disturbance

The threshold shear values needed to elicit the escape reactionto a quantifiable fluid mechanical disturbance were comparedbetween five free-swimming oceanic copepod species. The resultsindicate a significant difference in the threshold for differentspecies of copepods and between different age groups withina single species. In general, animals captured from more energeticregimes required a higher threshold than those captured frommore pacific locations. Labidocera madurae required the highestshear values with 51.5 s^–1 for 50% of the animals testedto elicit an escape reaction (S₅₀). Acartia tonsa and Euchaetarimana, in contrast, were behaviorally the most sensitive requiringan S₅₀ of only 1.5 and 4.1 s^–1, respectively, to initiatean escape reaction. Pleuromamma xiphias and Oithona requiredintermediate shear values with an S₅₀ of 7.2 and 8.1 s^–1.When compared to literature values, the threshold needed toelicit an escape reaction was consistently higher than averageenvironmental shear values. Threshold shear values also variedsignificantly with developmental stage. Naupliar stages of A.tonsarequired greater than six times the S₅₀ value required by adultsof the same species. This suggests that the higher vulnerabilityto predation of naupliar stages of copepods may not only reflectinferior escape strength, but may also result from the higherthreshold needed to elicit an escape reaction. This study supportsthe hypothesis that selective feeding patterns exhibited bypredators of copepods may be the result of the differentialbehavioral sensitivities of different species and developmentalstages of copepods.     

Predation, production and the organization of an estuarine copepod community

The copepod community of the estuaries near Beaufort, NC underwenta consistent seasonal succession from a spring assemblage dominatedby the medium-sized copepod Acartia tonsa (1 mm) to a summer—fallassemblage dominated by the small-bodied copepods Parvocalanuscrizsrirostris and Oithona colcarva (both 0.5 mm). However,in enclosure experiments during this period, A. tonsa dominatedthe community, due to higher growth rates and its predationon the nauplii of other species. Nutrient additions enhancedthe dominance by A. tonsa. The decline in abundance of A. tonsain the estuary was associated with increased abundance of planktivorousanchovies and silversides. In other enclosure experiments, planktivorousfish eliminated A. tonsa and other large copepods, althoughthey persisted in enclosures lacking fish. I conclude that predationby size-selective planktivorous fish prevents dominance by A.tonsa during summer—fall. ¹Present address: Institute of Marine Sciences, University ofNorth Carolina, 3407 Arendell Street, Morehead City, NC 28557,USA     

Capture mechanisms used by the lobate ctenophore, Mnemiopsis leidyi, preying on the copepod Acartia tonsa

Although the lobate ctenophore Mnemiopsis leidyi is an influentialplanktonic predator, the mechanisms enabling it to capture itscharacteristically wide range of prey have not been systematicallyexamined. We recorded interactions between free-swimming M.leidyiand two stages (nauplii, adults) of the calanoid copepod Acartiatonsa in order to determine a mechanistic explanation of thisfeeding process. Prey encounter with Mnemiopsis involved twodifferent processes. The first depended on fluid motions createdby the nearly continuous beating of cilia lining the four auricles.These cilia created a low-velocity flow in which A.tonsa naupliiwere entrained (94% of naupliar encounters) and transportedpast the oral lobes onto the tentillae (oral tentacles). Thenauplii, although capable of rapid escape responses, generallyappeared to be insensitive to the current in which they werecarried. The second process relied upon the collision of swimmingprey with the inner surfaces of the oral lobes and was not obviouslyinfluenced by the auricular feeding currents. Adult A.tonsawere rarely entrained in the auricular flow, but, instead, propelledthemselves into contact with the oral lobes (97% of adult encounters).Both prey capture processes functioned simultaneously. The synergisticfunctioning of these processes probably explains the broad patternsof prey ingestion found by in situ studies of Mnemiopsis feeding.     

Effects of prey motility and concentration on feeding in Acartia tonsa and Temora longicornis: the importance of feeding modes

Feeding experiments were conducted with the ambush-feeding copepodAcartia tonsa and the feeding-current-generating copepod Temoralongicornis. The copepods were offered a mixed diet of the dinoflagellateHeterocapsa triquetra and the ciliate Balanion comatum of similarcell size. The dinoflagellate was offered at a constant concentrationof 10–15 cells mL^–1, whereas the ciliate was offeredat a variety of concentrations, ranging from 7 to 57 cells mL^–1.Copepods with different feeding modes possess different mechanismsfor prey detection, suggesting that the two copepods would responddifferently to the two prey types. Both copepods had significantlyhigher clearance rates on the highly motile ciliate than onthe less motile dinoflagellate. In encounters between A. tonsaand its prey, we argue that this is due to the higher hydromechanicalsignal generated by the ciliate. The advection feeding copepodT. longicornis fed on the two prey according to their relativeconcentrations; in this case, we suggest that although B. comatumis capable of detecting feeding-current-generating predators,the feeding current velocity generated by T. longicornis isgreater than the escape velocity of this ciliate.     

A model for Acartia tonsa: effect of turbulence and consequences for the related physiological processes

A mathematical model of the energy budget of the coped Acartiatonsa is proposed. It takes into account the processes of ingestion(encounter, capture, ingestion sensu stricto), digestion (assimilation,gut transit and egestion), catabolic expenses and biomass production.In order to represent the potential effects of small-scale turbulenceon the whole physiological processes of a copepod, some processsubmodels already published are combined. A major assumptionof the model is a satiety effect resulting from midgut filling,which leads to a decrease in the feeding activity. The modelpermits the simulation of the short-term dynamics of ingestionunder different food and turbulence conditions, as well as anintegrated physiological balance over 24 h. The model is validatedthrough comparison with data at both scales Simulations showthat turbulence increases ingestion rates and gut contents,and causes a decrease in gut passage time and assimilation efficiency.As a consequence, the dependent physiological processes areaffected differently by turbulence, which preferentially increasesegestion and egg-production rates Simulated daily ingestionrates of A.tonsa, for suspension feeding on. Thalassiosira weissflogiiand for ambush predation on Strombidium sulcatum, are in goodagreement wth the available experimental observations. The concurrentdirect effect of turbulence on the copepod's metabolism, dueto increased escape reactions, is also simulated. Results ofthe model show that a switch from suspension feeding on diatomsin calm condition to ambush predation on ciliates in turbulentconditions, might allow A.tonsa to maintain its gross growthefficiency at the same level. It is suggested that a dynamicrepresentation of processes occurring over a time scale of afew seconds is necessary to obtain, once integrated over 24h, the correct simulation of the effect of microscale turbulenceon ingestion and the related physiological processes.     

Characteristics of the high frequency escape reactions of Oithona SP.

The cyclopoid copepod Oithona was found to exhibit multiple escape reactions when entrained within quantified flow fields. The distance of the escape from the flow‐producing predator mimic (a siphon) decreased with a concurrent decrease in escape distance and speed. Although individuals became behaviorally more sensitive to fluid motion with each successive escape reaction, the lower thresholds were not manifested as more distant escape reactions. Overall, the escape distance and speed decreased significantly with decreased signal strength. The decrease may result from the high cost of the escape reaction, which was calculated to account for 36% of the normal metabolic rate and 500 times the cost of normal swimming.     

Nitrogen excretion by the calanoid copepod Acartia tonsa: results of mesocosm experiments

Rates of excretion of NH₄⁺ urea and dissolved primary amines(DPA) by Acartia tonsa were measured over two 24-h periods inestuarine mesocosms. Animals (     

Differential grazing by Acartia tonsa on a dinoflagellate and a tintinnid

The calanoid copepod, Acartia tonsa Dana, ingests both the dinoflagellate,Heterocapsa triquetra (Ehrenberg) Stein, and the tintinnid ciliate,Favella sp. In laboratory experiments its ingestion rate increaseswith increasing dinoflagellate density to a maximum at     

Feeding efficiency of the larval ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora, Lobata)

Larval stages of the ctenophore Mnemiopsis leidyi rely on metazoanprey, such as Acartia tonsa nauplii and copepodites, to supporthigh growth rates. However, M. leidyi larvae <0.5 mm (totallength) had low retention efficiencies (REs) (proportion ofencountered prey actually ingested), 5.78 ± 2.6% (mean± SE), of nauplii and were often damaged by their encounters.REs of nauplii rapidly increased, 38.94 ± 3.73%, as larvaegrew to a size of     

Effects of nitrogen stressed algae on different Acartia species

We studied the reproductive response of two copepod species,Acartia tonsa and Acartia clausii, fed algae with differentC:N ratios (4.5 and 9.1 molar ratios respectively) in orderto investigate the influence of nutritional imbalances on calanoidcopepods egg production. Adult females were incubated with thecryptophyte Rhodomonas sp. at saturating concentrations. Thealgae were cultured under nitrogen depleted and sufficient conditions.Ingestion rates of the animals fed with different algae andtheir response in terms of egg production and hatching successof the eggs were quantified. Both species produced more eggwhen fed with nitrogen-limited algae. Ingestion rates and egghatching differed between species, but were not significantlyaffected by the quality of the food. The only difference betweenthe two species in their reaction to food quality was that A.tonsa increased the number of resting eggs, whereas no restingegg production was observed in A. clausii when fed with nitrogenlimited algae. These results support the recent suggestion thata moderately high prey C:N ratio (10–15) supports a higheregg production than a C:N ratio substantially <10.     

Response of two zooptankton grazers to an ichthyotoxic estuarine dinoflagellate

The dinoflagellate Pfiesteria piscicida (gen. et sp. nov.).a toxic ‘ambush predator’, has been implicated asa causative agent of major fish kills in estuanne ecosystemsof the southeastern USA. Here we report the first experimentaltests of interactions between P.piscicida and estuarine zooplanktonpredators. specifically the rotifer Brachionus plicatilis andthe calanoid copepod Acartia tonsa. Short-term (10 day) exposureof adult B.plicatilis to P.piscicida as a food resource, aloneor in combination with the non-toxic green algae Nannochlorisand Tetraselmis. did not increase rotifer mortality relativeto animals that were given only non-toxic greens Similarly,short-term (3 day) feeding trials using adult A.tonsa indicatedthat the copepods survived equally well on either P.piscicidaor the non-toxic diatom Thalassiosira pseudonana. Copepods giventoxic dinoflagellates exhibited erratic behavior, however, relativeto animals given diatom prey. The fecundity of B.plicatiliswhen fed the toxic dinoflagellate was comparable to or higherthan that of rotifers fed only non-toxic greens We concludethat, on a short-term basis, toxic stages of P.piscicida canbe readily utilized as a nutritional resource by these commonestuarine zooplankters. More long-term effects of P.piscicidaon zooplankton, the potential for toxin bioaccumulation acrosstrophic levels, and the utility of zooplankton as biologicalcontrol agents for this toxic dinoflagellate. remain importantunanswered questions.     

Sublethal effects of the toxic dinoflagellate Karenia brevis on marine copepod behavior

Apart from grazing interactions, little is known regarding thesublethal effects of Karenia brevis cells on copepod behavior.We conducted grazing and mortality experiments with K. breviscells and brevetoxins (PbTx-2), establishing routes of toxicityfor the copepods Acartia tonsa, Temora turbinata and Centropagestypicus. Subsequent behavioral experiments determined whethercopepod swimming and photobehavior, both behaviors involvedin predator avoidance, were impaired at sublethal K. brevisand PbTx-2 levels. Copepods variably grazed toxic K. brevisand non-toxic Prorocentrum minimum at bloom concentrations.Although copepods accumulated brevetoxins, significant mortalitywas only observed in T. turbinata at the highest test concentration(1 x 10⁷ K. brevis cells L^–1). Acartia tonsa exhibitedminimal sublethal behavioral effects. However, there were significanteffects on the swimming and photobehavior of T. turbinata andC. typicus at the lowest sublethal concentrations tested (0.15µg PbTx-2 L^–1, 1 x 10⁵ K. brevis cells L^–1).Although physiological incapacitation may have altered copepodbehavior, starvation likely played a major role as well. Thesedata suggest that sublethal effects of K. brevis and brevetoxinon copepod behavior occur and predicting the role of zooplanktongrazers in trophic transfer of algal toxins requires knowledgeof species-specific sublethal effects.     

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.     

Le zooplancton de la baie de Sh?diac (Nouveau-Brunswick)

Analyses of boreal zooplankton of Shediac Bay demonstrate theabundance of copepods (81%) and meroplankters (18%). Whetherexclusively pelagic or not, 67 species are mentioned for thefirst time in this Northumberland Strait area, out of 76 recordedwithin 23 higher taxa. The fluctuations were observed from Mayto November and pointed out the dominance of such copepods asAcartia tonsa, A. clausi, Oithona similis and Centropages hamatusin relation with temperature, salinity and food distributions. ¹Adresse actuelle: Station marine de Tul?ar, B.P. 141, Universit?de Madagascar, (R?p. Malgache)     

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.     

THE ULTRASTRUCTURE OF CILIATED CELLS FROM THE SIPHON OF SOLEN CAPENSIS (MOLLUSCA, BIVALVIA)

The structure of ciliated cells from the siphon of Solen capensishas been studied by both scanning and transmission electronmicroscopy. Two types of ciliated cell, based on the numberand length of cilia have been described which resemble thosedescribed in Donax. Type I is characterized by having 26–57({macron}= 43, n = 50) cilia which are 2.5 µm in length;Type II has fewer cilia (5–10; {macron}= 7) which are5 µn long. Both are primary receptors. Estimations ofabundance show that receptors are most numerous on the tipsof the siphon tentacles (8.8 x 10³/mm²). (Received 15 January 1985;     

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     

The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs overa period of several days. Differences in the escape responseof fish larvae during the hatching period have not hithertobeen studied. In this study, the escape response of walleyepollock (Theragra chalcogramma) larvae over the hatching periodwas examined. Escape speed, response to multiple touches witha fine probe, response to water currents generated by a predatorand predation by euphausiids (Thysanoessa inermis) and amphipods(Pleusirus secorrus) were measured in the laboratory. Otolithmeasurements of field-collected larvae support a broad hatchingperiod for walleye pollock eggs in the sea similar to that observedin the laboratory. The escape response of walleye pollock larvaewas affected by rank in the order of hatching, thus with respectto predation, hatching order may affect the survival of larvaein the sea. Early hatching larvae were smaller, less sensitiveto tactile stimulation, had a slower, weaker escape responseand higher laboratory rates of predation mortality than thosethat hatched later.