CHEMICAL, MECHANICAL AND VISUAL CUES IN THE VERTICAL MIGRATION BEHAVIOR OF THE MARINE PLANKTONIC COPEPOD ACARTIA-HUDSONICA

Recent experimental evidence in both marine and freshwater systems ind icates that predators can induce vertical migration behavior in indivi dual zooplankters, yet the specific cues by which zooplankters sense t heir predators appear to vary. In situ manipulation experiments were c arried out with enclosed populations of the marine planktonic copepod Acartia hudsonica to re-examine the potential role of chemical cues in the behavior of A.hudsonica, and to test explicitly for the role of m echanical or visual stimuli in triggering vertical migration behavior in this species. Adult female copepods were induced to vertically migr ate (descend) when exposed to fish mimics during the day, but no such response occurred when the copepods were exposed to fish mimics during the night. Moreover, copepods exhibited no changes in vertical distri bution when exposed to water which, having recently held a natural pre dator (the threespine stickleback, Gasterosteus aculeatus), was presum ed to be laden with predator-produced chemical exudates. Predator-medi ated mechanical or visual cues, or a hierarchy of both, are responsibl e for eliciting vertical migration behavior in adult female A.hudsonic a. These results, together with those of other investigations demonstr ating the inducing role of chemical exudates. indicate that the stimul i eliciting vertical migration in zooplankton can be expected to vary between species.