PREY SWITCHING BEHAVIOR IN THE PLANKTONIC COPEPOD ACARTIA-TONSA

The copepod Acartia tonsa has 2 different prey encounter strategies. I t can generate a feeding current to encounter and capture immobile pre y (suspension feeding) or it can sink slowly and perceive motile prey by means of mechanoreceptors on the antennae (ambush feeding). We hypo thesized that A. tonsa adopts the feeding mode that generates the high est energy intake rate; i.e. that prey selection changes according to the relative concentrations of alternative prey (prey switching) and t hat the copepods spend disproportionately more time in the feeding mod e that provides the greatest reward. Based on earlier observations, we also hypothesized that turbulence changes food selection towards moti le prey. We tested these hypotheses by examining feeding rates and beh aviour in adult females of A. tonsa feeding in mixtures of 2 prey orga nisms, a diatom (Thalassiosira weissflogii) and a ciliate (Strombidium sulcatum). Our data demonstrate prey switching in A. tonsa, both in t erms of behaviour and in terms of feeding rates on the alternative pre y. The time allocated to ambush and suspension feeding changed with th e composition of the food, and clearance of diatoms was, accordingly, negatively related to the availability of ciliates. In contrast, clear ing of ciliates was almost constant and independent of the availabilit y of the alternative prey (diatoms), probably because this particular ciliate species (in contrast to most other microzooplankters) is unabl e to escape a feeding current and, thus, can also be captured by suspe nsion feeding copepods. Finally, we demonstrate that turbulence favour s the selection of ciliates as prey. We suggest that prey switching by copepods may provide survival windows for microzooplankters during bl ooms 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 indepe ndently of their own food.