Ontogenetic and seasonal variation in the diel vertical migration of the copepods Metridia lucens and Metridia longa

Plankton samples collected and analyzed by the Continuous Plankton Recorder survey were used to examine the length of time spent near the surface in the North Atlantic and the North Sea by three closely related groups of zooplankton (copepodite stages 1-4 Metridia spp., copepodite stages 5-6 Metridia lucens, and copepodite stages 5-6 Metridia longa). For all three groups, the mean daily length of rime spent near the surface in each month of the year covaried seasonally with day length. In addition, the amount of time spent near the surface varied significantly between the three groups, being longest for the copepods of smallest body size (C1-C4 Metridia spp.) and shortest for the copepods of largest body size (C5-C6;M longa). These results support the suggestion that diel vertical migration serves to reduce the risk of mortality from visually orienting predators.