WHY DO THE 2 MOST ABUNDANT COPEPODS IN THE NORTH-ATLANTIC DIFFER SO MARKEDLY IN THEIR DIEL VERTICAL MIGRATION BEHAVIOR

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 copepo d Calanus finmarchicus. These two species are of similar size and pigm entation, but differ in morphology, with C. finmarchicus being the mor e elongate. To examine whether this variation in morphology might be l inked to the observed difference in NDVM behaviour, both body morpholo gy and NDVM behaviour were quantified for a further five similar sized copepod species (Pleuromamma robusta, P. abdominalis, C. helgolandicu s, C. glacialis and Neocalanus gracilis). NDVM behaviour was significa ntly less marked in the more elongate species. It is suggested that th e more elongate species have a faster avoidance speed from predators, which leads to a lower susceptibility to predation and consequently th ese species do not need to exhibit marked NDVM behaviour.