Why do the two most abundant copepods in the North Atlantic differ so markedly in their diel vertical migration behaviour?

A total of 55 113 plankton samples, collected between 1948 and 1992 in the North Atlantic by the Continuous Plankton Recorder (CPR) survey, were used to show that normal diel vertical migration (NDVM) behaviour was far more marked in the copepod Metridia lucens than in the copepod Calanus finmarchicus. These two species are of similar size and pigmentation, but differ in morphology, with C. finmarchicus being the more elongate. To examine whether this variation in morphology might be linked to the observed difference in NDVM behaviour, both body morphology and NDVM behaviour were quantified for a further five similar sized copepod species (Pleuromamma robusta, P. abdominalis, C. helgolandicus, C. glacialis and Neocalanus gracilis). NDVM behaviour was significantly less marked in the more elongate species. It is suggested that the more elongate species have a faster avoidance speed from predators, which leads to a lower susceptibility to predation and consequently these species do not need to exhibit marked NDVM behaviour.