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 copepod Acartia tonsa and the feeding-current-generating copepod Temora longicornis. The copepods were offered a mixed diet of the dinoflagellate Heterocapsa triquetra and the ciliate Balanion comatum of similar cell size. The dinoflagellate was offered at a constant concentration of 10-15 cells mL(-1), whereas the ciliate was offered at a variety of concentrations, ranging from 7 to 57 cells mL(-1). Copepods with different feeding modes possess different mechanisms for prey detection, suggesting that the two copepods would respond differently to the two prey types. Both copepods had significantly higher clearance rates on the highly motile ciliate than on the less motile dinoflagellate. In encounters between A. tonsa and its prey, we argue that this is due to the higher hydromechanical signal generated by the ciliate. The advection feeding copepod T. longicornis fed on the two prey according to their relative concentrations; in this case, we suggest that although B. comatum is capable of detecting feeding-current-generating predators, the feeding current velocity generated by T. longicornis is greater than the escape velocity of this ciliate.