Patterns of Prey Selection in the Cruising Calanoid Predator Euchaeta elongata

The invertebrate predators of marine and freshwater plankton assemblages typically employ encounter tactics that enable investigators to characterize them as either ambushers or cruisers. In this study, we examine the implications of cruising tactics on the prey selection patterns exhibited by the marine calanoid copepod Euchaeta elongata. Patterns and mechanisms of prey selection exhibited by Euchaeta were investigated in the laboratory with predation experiments and videotaped observations of predator and prey swimming behavior. Prey types offered to Euchaeta included each of the developmental stages of Calanus pacificus and the adults of Pseudocalanus sp. Calanus' vulnerability to Euchaeta predation steadily increased through the naupliar stages, reached a peak in the early copepodid stages, and declined rapidly after the molt to the fourth copepodite. Pseudocalanus adults were as vulnerable to Euchaeta predation as the early copepodites of Calanus. The behavioral mechanisms underlying the observed patterns of prey selection are discussed and compared with the results from other investigations of selective planktivory by marine calanoid copepods. It is concluded that prey selection by Euchaeta is largely determined by differences in prey vulnerability; some evidence for active behavioral choice was observed, however.