Scyphomedusae have been recognized in recent years as important predators in coastal and estuarine environments, but the factors affecting vulnerability of zooplankton to these cruising jellyfish remain poorly understood. In particular, models fail to account for the ingestion of fast-escaping prey such as calanoid copepods. Analysis of gastric contents of Aurelia aurita, Cyanea sp. and Chrysaora quinquecirrha from field collections and mesocosm experiments showed that despite morphological differences, all three species fed on Acartia spp. copepods; C. quinquecirrha selected adult over copepodite stages. The goal of this research was to determine which characteristics of Acartia spp. copepods make them susceptible to predation by scyphomedusae. I compared feeding rates by medusae on live and immobilized (heat-killed) adult and copepodite stages in a planktonkreisel while simultaneously videotaping predator-prey interactions. The continuous-flow planktonkreisel allowed manipulation of freely swimming predator-prey pairs yet avoided many of the problems associated with small-container experiments.

The results contained some surprises. First, size had a dramatic impact on the vulnerability of copepods, influencing contact and retention rates. Small size provided a refuge for copepods trying to elude capture by jellyfish. This effect must be incorporated into predictive models. In addition, scyphomedusae digested smaller copepodites faster than larger adults, a difference that influenced calculations of feeding rates and prey selection. Second, although copepod behavior decreased feeding rates on adults and mitigated the liability associated with larger size, it did not significantly change feeding rates on copepodites. Consequently, I expected adults and copepodites to respond differently to encounters with scyphomedusae. The salient distinction was that copepodites failed to escape, or responded passively, more often than did adults. Third, escape velocities of copepods corresponded poorly to their vulnerability to medusae. Instead, copepod escape behavior was complex, variable, and effective. Fewer than 1% of encounters resulted in ingestion. Escapes typically involved multiple, sometimes-misdirected jumps at submaximum velocities. Because copepods exhibit wide variability in escape behavior, scyphomedusae may not capture them using a single, quantifiable mechanism. A range of responses may be the best strategy for copepods faced with strong predation pressure from a variety of predators.