Calanus finmarchicus and Related Forms

Abstract

THE copepod Calanus finmarchicus has been more thoroughly investigated than any other marine plankton organism. Some systematists have recognized in the material from European¹ and eastern American waters² two distinct species, C. finmarchicus and C. helgolandicus; but the distinguishing characters given have been too uncertain to receive universal acceptance³. In consequence, it has been assumed in most studies of the copepod—its population changes, physiology, behaviour, etc.—that the material used consists of a single species. Recently⁴, it was pointed out that two forms, corresponding to the two species, could be readily distinguished in the female and copepodite stage V by the structure of the basipod of the fifth swimming feet, though their status, whether species or sub-species, remained undecided. It has alsobeen shown⁵ that the eggs of helgolandicus are bigger (165–180 µ in diameter) than those of finmarchicus (135–155 µ). Following upon the more definite means of identification, it is becoming recognized that in future research the two forms should be distinguished⁶.