Life histories of Neocalanus flemingeri and Neocalanus plumchrus (Calanoida: Copepoda) in the western subarctic Pacific

Abstract

Life cycles of large suspension-feeding copepods were investigated in the western subarctic gyre (Oyashio area). Two populations were recognized in Neocalanus flemingeri by body size, life cycle and timing of ontogenetic migration. Small forms (same size as original description; adult female ca. 3.6 mm in prosome length) have a 1-year life cycle and occurred at the surface between December and June. At least some individuals of large forms (adult female ca. 4.5 mm in prosome length) have a biennial life cycle with winter dormancy as Stage 4 copopodites (C4) and adult females. The young copepodites of the large form occur in the surface water later than the small forms, then grow to C4 with full deposition of lipid. The following year, the C4 individuals start grazing in January, and sink to deep layers at the same time as the small forms. The life cycle of Neocalanus plumchrus was identical to the original description made for the Alaskan gyre population, although the recruitment to the surface layer during copepodite stages occurred 2 weeks to a month later than in other areas of the subarctic Pacific. Temporal partitioning of surface habitat utilization between N. plumchrus and the small form of N. flemingeri was clearly established, but overlap was observed between N. plumchrus and the large form of N. flemingeri.