No abstract

Copepods may remotely detect predators from the velocity gradients these generate in the ambient water. Each of the different components and characteristics of a velocity gradient (acceleration, vorticity, longitudinal and shear deformation) can cause a velocity difference between the copepod and the ambient water and may, therefore, be perceived by mechanoreceptory setae. We hypothesised that the threshold value for escape response to a particular component depends solely on the magnitude of the velocity difference (= signal strength) it generates. In experiments we isolated the different components and noted the minimum intensities to which the copepod Acartia tonsa responded. As hypothesised, threshold signal strengths due to longitudinal and shear deformation were similar, -0.015 cm S-', and were invariant with developmental stage. The latter implies that the threshold deformation rate for response scales inversely with size, i.e. that large stages respond to lower fluid deformation rates than small stages and, hence, may detect predators at longer distances. Signals due to vorticity and acceleration did not elicit escape responses, even though their magnitude exceeded threshold signal strength due to deformation. We suggest that A. tonsa cannot distinguish such signals from those due to their own behaviour (sinking, swimming, passive reorientation due to gravity) because they cause a similar spatial distributions of the signal across the body. Reinterpretation of data from the literature revealed that threshold signal strength due to deformation varies by ca 2 orders of magnitude between copepods and exceeds the neurophysiological response threshold by more than a factor of 10. In contrast, threshold deformation rates vary much less, -0.5 to 5 S-' Model calculations suggest that such threshold deformation rates are just sufficient to allow efficient predator detection while at the same time just below maximum turbulent deformation rates, thus preventing inordinate escapes.

ABSTFUCT Turbulence may enhance contact rates between planktonic predators and their prey We formulate slmple and general models of prey encounter rates, taklng into account the behaviours and motility patterns of both prey and predator as well as turbulent fluld motlon Uslng these models we determine the levels of turbulence (as clmipation rate) at which ambient fluid motlon is lmportant in enhancing prey encounter rates for vanous types of predators (e g ambush and cruise predators, suspension feeders) Generally, turbulence has the largest effect on prey encounters for predators with low motility and long reactlon distances Also, turbulence is most important for meso-sized (mm to cm) predators and insignificant for smaller and larger predators The effect of turbulence on copepods is specifically examined For copepods that establish feeding currents, turbulence is of minor importance, for ambush feeding copepods, such as Acartldae and many cyclopoids, turbulence has a dominant influence on prey encounter rates The effect on cruising predators is intermediate Application of the models to situations examined experimentally demonstrates a hlgh predictive performance Finally we explore and model the potentially negatlve effects of turbulence on copepod feedlng currents, prey perception and capture success At typical and even high turbulent lntenslties none of these IS slgnificantly affected

The copepod Acartia tonsa has 2 different prey encounter strateg~es. It can generate a feeding current to encounter and capture immobile prey (suspension feeding) or it can sink slowly and perceive motile prey by means of mechanoreceptors on the antennae (ambush feeding). We hypothesized that A. tonsa adopts the feeding mode that generates the highest energy intake rate; i.e. that prey selection changes according to the relative concentrations of alternative prey (prey switching) and that the copepods spend disproportionately more time In the feeding mode that provides the greatest reward. Based on earlier observations, we also hypothes~zed that turbulence changes food selection towards motile prey. We tested these hypotheses by examining feeding rates and behaviour in adult females of A. tonsa feeding in mixtures of 2 prey organisms, a diatom (Thalassiosira welssflogjj) and a ciliate (Strombidium sulcatum). Our data demonstrate prey switching in A. tonsa, both in terms of behaviour and in terms of feeding rates on the alternat~ve prey. The time allocated to ambush and suspension feeding changed with the composition of the food, and clearance of diatoms was, accordingly, negatively related to the availability of ciliates. In contrast, clearing of ciliates was almost constant and independent of the availability of the alternative prey (diatoms), probably because this particular ciliate species (in contrast to most other microzooplankters) is unable to escape a feedlng current and, thus, can also be captured by suspension feeding copepods. Finally, we demonstrate that turbulence favours the selection of cillates as prey. We suggest that prey switching by copepods may provide survival windows for microzooplankters during blooms of net phytoplankton because predation pressure from the copepods is then less. This may explain why microzooplankton populations often peak concurrently with net phytoplankton blooms and apparently independently of their own food.

No abstract

Many protist plankton are mixotrophs, combining phototrophy and phagotrophy. Their role in freshwater and marine ecology has emerged as a major developing feature of plankton research over recent decades. To better aid discussions, we suggest these organisms are termed “mixoplankton”, as “planktonic protist organisms that express, or have potential to express, phototrophy and phagotrophy”. The term “phytoplankton” then describes phototrophic organisms incapable of phagotrophy. “Protozooplankton” describes phagotrophic protists that do not engage in acquired phototrophy. The complexity of the changes to the conceptual base of the plankton trophic web caused by inclusion of mixoplanktonic activities are such that we suggest that the restructured description is termed the “mixoplankton paradigm”. Implications and opportunities for revision of survey and fieldwork, of laboratory experiments and of simulation modelling are considered. The main challenges are not only with taxonomic and functional identifications, and with measuring rates of potentially competing processes within single cells, but with decades of inertia built around the traditional paradigm that assumes a separation of trophic processes between different organisms. In keeping with the synergistic nature of cooperative photo- and phagotrophy in mixoplankton, a comprehensive multidisciplinary approach will be required to tackle the task ahead.

The copepod Acartia tonsa exhibits 2 different feeding modes: when feeding on small phytoplankton cells it sets up a feeding current and acts as a suspension feeder; when feeding on motile prey it acts as an ambush feeder. We examined experimentally the effects of small-scale turbulence on feeding rates in these 2 modes. The different feeding behaviours were triggered by offering the copepods diatoms Thalassiosira weissfloqii and ciliates Strombidium sulcatum, respectively. Turbulence at 5 different intensities (energy dissipation rate, E, between 4 X I O -~ and 3.7 X 10' cm2 s -~) was generated by an oscillating grid. In ambush feeding mode, low (realistic) intensities of turbulence (E = 1 0 '~ to 1 0 -~ cm2 s -~) enhanced clearance rates by up to a factor of 4 above those observed in calm water Higher intensities of turbulence (E = 10-' to 10' cm2 s -~) resulted in a depression of clearance rates, although the rates were still significantly higher than those observed in calm water. The depression of clearance rates at high turbulence intensities was due partly to a decline in capture success, but mainly to a decrease in reactive distance, because turbulence interferes with prey perception by disturbing the hydrodynamical signal generated by motile prey. The negative effects were evident only at turbulence intensities exceeding those normally encountered by A tonsa in its natural habitat. In suspension feeding mode, low intensities of ambient turbulence (E = I O -~ to 10-2 cm2 s-" had negligible effects on clearance rates, while at higher turbulence intensities (E = 10-I to 10' cm2 s -~) we observed a negatlve effect (depression of clearance rate). The negative effects become evident when ambient turbulent fluid shear approaches the maximum shear rate of the copepod's feeding current, and we hypothesize that at these intensities the feeding current is eroded. Again the negative effects were observed only at turbulence intensities higher than those typically experienced by A. tonsa in the sea. The differential response to turbulence of the 2 feeding behaviours, including the negative effects, were accurately predicted by encounter rate and feeding behaviour models proposed by korboe & Saiz (1995; Mar Ecol Prog Ser 122:135-145). Because feeding behaviour is specific to the prey (phytoplankton vs mot~le prey), and because ambush-mode feeding is much more dependent on turbulence than suspensionmode feeding, our findings suggest that prey selection in A. tonsa may be partly governed by turbulence in the ocean. This may explain why rnicrozooplankton at times dominates the diet of A. tonsa and other copepods, even though it is numerically scarce relative to phytoplankton in the environment.

We report the effects of small-scale turbulence on the feeding rates of the marine copepod Oithona davisae. Laboratory experiments were conducted under a range of turbulence dissipation rates between 10 Ϫ4 and 10 1 cm 2 s Ϫ3 . Net enhancements of feeding were observed only at the lowest, whereas negative net effects appeared only at the highest, turbulence intensities. These results contrast with expectations from an encounter-based model for this copepod species that predicted positive feeding enhancements at all turbulence intensities. This disagreement suggests the presence of detrimental effects at moderate and high turbulence intensities, very likely driven by either a lower mechanosensor perception capability or lower capture success. In comparison to other ambush copepods, O. davisae appears much more sensitive to the presence of turbulence, which might be the result of its strict ambush behavior, whereas copepods like Acartia tonsa or Centropages typicus, which can switch into different feeding modes, appear to benefit more from turbulence. The response of O. davisae feeding to turbulence in our experiments agrees with recent field observations on changes in the vertical distribution of Oithona as a function of wind-driven turbulence events. Hence, O. davisae seems to choose those depths where small-scale turbulence favors feeding.