Calanus finmarchicus and Related Forms

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.