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 indicates that predators can induce vertical migration behavior in individual zooplankters, yet the specific cues by which zooplankters sense their predators appear to vary. In situ manipulation experiments were carried 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 mechanical or visual stimuli in triggering vertical migration behavior in this species. Adult female copepods were induced to vertically migrate (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 distribution when exposed to water which, having recently held a natural predator (the threespine stickleback, Gasterosteus aculeatus), was presumed to be laden with predator-produced chemical exudates. Predator-mediated mechanical or visual cues, or a hierarchy of both, are responsible for eliciting vertical migration behavior in adult female A.hudsonica. These results, together with those of other investigations demonstrating the inducing role of chemical exudates. indicate that the stimuli eliciting vertical migration in zooplankton can be expected to vary between species.