Predation vulnerability of planktonic copepods: consequences of predator foraging strategies and prey sensory abilities

We investigated the vulnerability of 2 copepod species (Eurytemora affinis and Temora longicornis) to predation by predators with different foraging m odes, three-spined stickleback Gasterosteus aculeatus juveniles and mysid s hrimps Neomysis integer. Copepods were videofilmed escaping from predators and from an artificial flow field, and the results were used in a model of hydrodynamic disturbance generated by a predator. The copepods detected mys ids from a significantly larger distance than they detected sticklebacks (0 .45 and 0.24 cm, respectively). Consequently, the capture success of the st icklebacks was higher than that of mysids. In the case of sticklebacks fora ging on E. affinis, copepod reaction distance was significantly correlated with stickleback approaching speed; sticklebacks captured a copepod only if they were able to slowly approach to within a strike distance of <0.1 cm f rom the prey. Also, there was a major difference between the vulnerabilitie s of the 2 prey species: the capture success of sticklebacks was 92 % with T. longicornis and 53 % with E. affinis. This corresponded with experiments with artificial flow, where the threshold fluid velocity gradient elicitin g an escape response in copepods was 4 times higher in T. longicornis than in E. affinis (8.2 and 2.1 s(-1), respectively). The hydrodynamic model acc urately predicted the positive relationship between stickleback approaching speed and copepod reaction distance, as well as the difference between the 2 copepod species. This suggests that, by using simple artificial flow exp eriments, we can rank various zooplankton species according to their escape capabilities, and thus predict their vulnerability to predation by small f ish with different motility patterns. In contrast, the model did not confor m with observations on mysids. Apparently, the hydrodynamic disturbance cre ated by a mysid is not related to its swimming speed, but to some other fac tor, such as the beat rate of swimming appendages.