Vertical distribution, life cycle and body allometry of two oceanic calanoid copepods (Pleuromamma scutullata and Heterorhabdus tanneri) in the Oyashio region, western North pacific Ocean

Diel and seasonal vertical distribution, life cycle and body allometry of Pleuromamma scutullata and Heterorhabdus tanneri were investigated in the Oyashio region during September 1996 through October 1997. Monthly samples were collected with 0.1 mm mesh closing nets towed through five discrete depths between the surface and less than or equal to 2000 m. Copepodite stages 1 through 6 of P. scutullata and 3 through 6 of H. tanneri were collected effectively with the nets. Day-night samplings during December 1996, April and October 1997 revealed both species ascend at night. Seasonal differences in the vertical distribution patterns of both copepods were insignificant, and P. scutullata was distributed at a much shallower depth (250-500 m) than H. tanneri (600-850 m). By tracing the sequential changes in abundance peaks of each copepodite stage, both species are considered to produce one generation per year. The main reproduction season was estimated to occur in spring for P. scutullata and in winter for H. tanneri. The relationships between prosome length and wet weight, dry weight, and ash-free dry weight indicated that the increase in weight per molt was much greater in H. tanneri than in P. scutullata. These dissimilar features are discussed in relation to the dissimilar food habits of these two species and the production cycle of food organisms in the Oyashio region.