Capture mechanisms used by the lobate ctenophore, Mnemiopsis leidyi, preying on the copepod Acartia tonsa

Abstract

Although the lobate ctenophore Mnemiopsis leidyi is an influential planktonic predator, the mechanisms enabling it to capture its characteristically wide range of prey have not been systematically examined. We recorded interactions between free-swimming M.leidyi and two stages (nauplii, adults) of the calanoid copepod Acartia tonsa in order to determine a mechanistic explanation of this feeding process. Prey encounter with Mnemiopsis involved two different processes. The first depended on fluid motions created by the nearly continuous beating of cilia lining the four auricles. These cilia created a low-velocity flow in which A.tonsa nauplii were entrained (94% of naupliar encounters) and transported past the oral lobes onto the tentillae (oral tentacles). The nauplii, although capable of rapid escape responses, generally appeared to be insensitive to the current in which they were carried. The second process relied upon the collision of swimming prey with the inner surfaces of the oral lobes and was not obviously influenced by the auricular feeding currents. Adult A.tonsa were rarely entrained in the auricular flow, but, instead, propelled themselves into contact with the oral lobes (97% of adult encounters). Both prey capture processes functioned simultaneously. The synergistic functioning of these processes probably explains the broad patterns of prey ingestion found by in situ studies of Mnemiopsis feeding.