Escape behavior of Acartia hudsonica copepods during interactions with scyphomedusae

Calanoid copepods possess remarkable abilities to detect andescape from hydrodynamic disturbances, such as those createdby approaching predators. At the same time, a number of studiesin coastal ecosystems have suggested that gelatinous predators,including medusae in the Class Scyphozoa, exert top-down controlon copepod populations. Although prey escape behavior playsa critical role in predation models, we have relatively littleempirical data on how copepods respond to encounters with scyphomedusae.In this study, I used video to quantify encounter rates andescape behaviors of the copepod Acartia hudsonica during interactionswith two scyphomedusae, Aurelia aurita and Cyanea sp., in twoflow regimes. Escapes were complex, variable and effective.Fewer than 1% of encounters resulted in ingestion. Typically,A.hudsonica avoided contact by responding when predators remainedseveral body lengths (4 to 10 mm) distant and stringing togethermany escape jumps at submaximum velocities (33 to 59 mm s^–1).In addition, copepodite stages behaved passively—or failedto respond—following encounters with medusae more oftenthan did adults. Because escape behavior exhibited by A.hudsonicawas so variable, it is unlikely that medusae capture copepodsusing a single, quantifiable mechanism. A range of responseswithin populations and individuals may be the best strategyfor zooplankton faced with strong predation pressure from avariety of predators.