VARIABILITY OF DIEL VERTICAL MIGRATION IN THE MARINE PLANKTONIC COPEPOD PSEUDOCALANUS-NEWMANI IN RELATION TO ITS PREDATORS

We report results of a 3-yr field study of the vertical distributions and diel vertical migration (DVM) of Pseudocalanus newmani in the central basin of Dabob Bay, Washington, USA. Our results include two novel findings. First, a statistically significant relationship exists between strength of DVM in P. newmani and the potential predation impact of its planktonic invertebrate predators. Second, a strong "normal" DVM (up at night, down during the day), unique for P. newmani in 5 yr of sampling at this locale, occurred at a time when the zooplanktivorous fish Ammodytes hexapterus was unusually abundant and preying on the copepod; this DVM may have been induced by the fish. DVM behavior of P. newmani was highly variable, with changes in behavior commonly occurring on a time scale of weeks; in one case the copepod switched from a normal migration pattern to a reverse migration pattern (down at night, up during the day) in less than 5 wk. These observations, combined with those of previous research, indicate that P. newmani has an exceptionally diverse repertoire of migration behavior, any particular expression of which is most likely manifested by individual copepods exercising phenotypic behavioral plasticity in response to potential predation.