Feeding behaviour, prey detection and capture efficiency of the copepod Acartia tonsa feeding on planktonic ciliates

The copepod Acartia tonsa Dana exhibits raptorial feeding behaviour when encountering planktonic ciliates. It detects individual ciliates at a distance of 0.1 to 0.7 mm from the first antennae, and requires no physical contact. Detection distance is correlated with prey size, and the probabhty of successful captures declines with distance from the first antennae. When A. tonsa encounters the ciliate Mesodiniun~ rubrum, the ciliate responds with rapid swimming motion, which reduces the probability of capture. Feeding experiments, in which A. tonsa was offered different concentrations of planktonic ciliates, showed that ciliates were cleared at concentrations as low as 0.3 pg C 1-' The observed differences in clearance for planktonic chates (15 to 86 m1 d-') may be explained in terms of detection &stance and ciliate swimming behaviour. Experiments with mixtures of ciliates and the microflagellate Cryptomonas baltjca indicated that A. tonsa allocated more tlme to suspension-feeding behaviour at high concentrations of microflagellates. The potential of A. tonsa to switch between raptorial and suspension-feeding behaviour may be a key in explaining their ubiquity in coastal waters.