Predator-induced diet vertical migration in a planktonic copepod

Abstract

Predator evasion is the most commonly hypothesized reason for diel vertical migrations undertaken by a wide variety of planktonic organisms in lakes and seas, yet direct evidence remains elusive. We tested the predation hypothesis by exposing enclosed populations of a marine copepod Acartia hudsonica to caged or free-ranging individuals of their natural predator, the planktivorous fish Gasterosteus aculeatus . After little more than a week, adult copepods changed their vertical distribution and migration behavior depending on the presence or absence of predation. Only free-ranging fish induced vertical migration in the copepod population. Caged fish had no effect, indicating that vertical migration was not a simple chemically mediated response of copepods to the predator. Rather, copepods seemed to react to the presence of predators by other means, perhaps visual or mechanical stimuli, and to exhibit a downward escape response which, because encounters with visually orienting fish occur chiefly in the daytime, effectively limited the copepods' occurrence in the upper water column to the night-time hours. Alternatively, because fish imposed heavy mortality on copepods, it is possible that selective predation altered the proportions of individuals with fixed, genetically determined migration behaviors. We suggest experiments to distinguish these alternatives.