Escape behavior of Acartia hudsonica copepods during interactions with scyphomedusae

Calanoid copepods possess remarkable abilities to detect and escape from hydrodynamic disturbances, such as those created by approaching predators. At the same time, a number of studies in coastal ecosystems have suggested that gelatinous predators, including medusae in the Class Scyphozoa, exert top-down control on copepod populations. Although prey escape behavior plays a critical role in predation models, we have relatively little empirical data on how copepods respond to encounters with scyphomedusae. In this study, I used video to quantify encounter rates and escape behaviors of the copepod Acartia hudsonica during interactions with two scyphomedusae, Aurelia aurita and Cyanea sp., in two flow regimes. Escapes were complex, variable and effective. Fewer than 1% of encounters resulted in ingestion. Typically, A.hudsonica avoided contact by responding when predators remained several body lengths (4 to 10 mm) distant and stringing together many escape jumps at submaximum velocities (33 to 59 mm s–1). In addition, copepodite stages behaved passively—or failed to respond—following encounters with medusae more often than did adults. Because escape behavior exhibited by A.hudsonica was so variable, it is unlikely that medusae capture copepods using a single, quantifiable mechanism. A range of responses within populations and individuals may be the best strategy for zooplankton faced with strong predation pressure from a variety of predators.