A variant form of Megacalanus longicornis (Copepoda: Megacalanidae) from deep waters off Southern California

Megacalanus longicornis (Sars, 1905) is a mesopelagic copepod widely distributed in the oceans. Specimens collected in deep coastal waters off California and at other sites in the eastern Pacific bear sharply edged cephalic crests. These crests have not been found in other regions. Close examination of body form and details of limbs revealed no other distinguishing characters between crested and uncrested forms. The very large size of Megacalanus calls special attention to morphologic details, including massive hooks borne on the bases of the first swimming legs, maculae cribrosae (presumed to be sensory structures), clavate setae on segment 9 of each male antennule, and externally opening glands on exopodite segments of the swimming legs.