VULNERABILITY OF THE COPEPOD ACARTIA-TONSA TO PREDATION BY THE SCYPHOMEDUSA CHRYSAORA QUINQUECIRRHA - EFFECT OF PREY SIZE AND BEHAVIOR

Although scyphomedusae have received increased attention in recent yea rs as important predators in coastal and estuarine environments, the f actors affecting zooplankton prey vulnerability to these jellyfish rem ain poorly understood. Current models predicting feeding patterns of c ruising entangling predators, such as Chrysaora quinquecirrha (Desor, 1948), fail to account for the selection of fast-escaping prey such as copepods. Nevertheless, our analysis of gastric contents of field-col lected medusae showed that this scyphomedusa fed selectively on the ca lanoid copepod Acartia tonsa (Dana, 1846) and preferentially ingested adult over copepodite stages. We measured feeding rates in a planktonk reisel while simultaneously videotaping predator-prey interactions. C. quinquecirrha consumed adult A. tonsa ten times faster than copepodit es. Differences in prey behavior, in the form of predator-prey encount er rates or post-encounter escape responses, could not account for the elevated feeding rates on adults. Prey size, however, had a dramatic impact on the vulnerability of copepods. In experiments using heat-kil led prey, feeding rates on adults (1.5 times longer than copepodites) were 11 times higher than on copepodites. In comparison, medusae inges ted heat-killed prey at only two to three times the rats of live prey. These results suggest that during scyphomedusan-copepod interactions, prey escape ability is important, but ultimately small size is a more effective refuge from predation.