PREDATOR-INDUCED CHANGES IN THE DIEL FEEDING CYCLE OF A PLANKTONIC COPEPOD

An in situ manipulation experiment examined the effect of a diurnally foraging, zooplanktivorous fish, the three-spine stickleback Gasterosteus aculeatus Linnaeus, on the diel feeding cycle of the planktonic marine copepod Acartia hudsonica Pinhey. Copepods in the presence of fish had a significantly lower level of daytime gut fullness, and consequently a sharpened diel cycle of grazing, than did copepods in the absence of fish. The reduced level of copepod feeding could not be attributed to reduced availability of phytoplankton, such as has been previously suggested for vertically migrating animals residing at depth during the day. Rather, these changes in the feeding behavior of the copepods stemmed directly from the presence of predators. Due to reductions in motion and gut pigmentation, reduced daytime feeding may have adaptive value in reducing the copepods' risk of predation by making them less conspicuous to visual predators. Our results, combined with those of earlier studies, indicate that diel cycles of feeding and vertical migration can be separate, independent behaviors, although both likely have the same adaptive significance-predator evasion.