Life cycle, population structure and vertical distribution of Metridia spp. (Copepoda: Calanoida) in the Oyashio region (NW Pacific Ocean)

摘要

Vertical distribution and population structure of Metridia pacifica and M. okhotensiswere investigated at Site H in the Oyashio region (western subarctic Pacific) to evaluate their lifecycle modes. Although recruitment of M. pacifica continued throughout the year in the top 250 m ofthe water column, 2 pronounced generations were recognized. The first generation was characterizedby rapid development during the spring phytoplankton bloom (generation length: 2–3 mo), andthe second by slow development (generation length: 9–10 mo) with overwintering at Stage 5 indeeper-layers (up to 1000–2000 m). In contrast, M. okhotensis was consistently found at depth(250–1000 m) throughout the year, except for excursions to the subsurface layer for spawning duringthe phytoplankton bloom. Developmental sequences of M. okhotensis could not be resolved veryclearly, largely because of extremely low abundance of Copepodite Stages 2 and 3 (C2–C3). As a tentativelife cycle scenario, the C1 produced during one year was considered to overwinter and developto C5 through the phytoplankton bloom of the next year. These C5 overwinter and molt to C6 andreproduce during the phytoplankton bloom of the following year. Thus, the generation length of M.okhotensis is estimated as 2 yr. For both species, active spawning and subsequent growth are closelyassociated with phytoplankton bloom. For overwintering stages of both species, a large deposition oflipid in the body and its gradual decrease toward the onset of phytoplankton bloom of the next yearwas seen. While no comparable life cycle data is presently available for M. okhotensis, the presentresults of M. pacifica are compared with the results from other regions, showing a large betweenregionvariation in the life cycle patterns of this species. Further, the present results are discussedin the context of broad overwintering mechanisms of pelagic copepods living in high-latitude seas.