Vertical distribution, population structure and life cycle of Neocalanus cristatus (Crustacea : Copepoda) in the Oyashio region, with notes on its regional variations

Vertical distribution and population structure of Neocalanus cristatus were investigated at Site H in the Oyashio region from September 1996 through October 1997 to evaluate their life cycle mode. Additional temporary samplings were also made at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea, as a basis for regional comparison of life cycles of this species. At Site H, N. cristatus spawned throughout the year below 500 m depth, with a peak from October to December. The resulting eggs and nauplii floated/migrated upward, and formed an abundance peak of Copepodite Stage 1 (C1) in the surface layer in February. In the surface layer, the C1 developed and reached C5 by early June through a phytoplankton bloom which occurred in mid-March to end of June. The C5 migrated to deeper layers in July and August, where they molted to adults. Apparently, the developmental time from C5 to adults was highly variable (>1 month), and some might overwinter. The life cycle of N. cristatus appeared to be annual for the major portion of the population. Taking into account sampling season, temporal changes in Vertical distribution and population structure data collected from regions other than Site H, there was a close correlation in the timing of the life cycle over the entire subarctic Pacific, but the reproduction season (April to June) was observed to be different in the Okhotsk and Japan Sea populations. Regional comparison of prosome length of C5 individuals, including those in the Bering Sea, indicated significantly larger sizes of specimens from the Japan Sea and Okhotsk Sea, as compared with those from the entire subarctic Pacific. Possible causes for regional variability in life cycle patterns and body sizes are discussed.