MEPS 270:181-198 (2004)  -  doi:10.3354/meps270181

ABSTRACT: Vertical distribution and population structure of Metridia pacifica and M. okhotensis were investigated at Site H in the Oyashio region (western subarctic Pacific) to evaluate their life cycle modes. Although recruitment of M. pacifica continued throughout the year in the top 250 m of the water column, 2 pronounced generations were recognized. The first generation was characterized by rapid development during the spring phytoplankton bloom (generation length: 2-3 mo), and the second by slow development (generation length: 9-10 mo) with overwintering at Stage 5 in deeper-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 during the phytoplankton bloom. Developmental sequences of M. okhotensis could not be resolved very clearly, largely because of extremely low abundance of Copepodite Stages 2 and 3 (C2-C3). As a tentative life cycle scenario, the C1 produced during one year was considered to overwinter and develop to C5 through the phytoplankton bloom of the next year. These C5 overwinter and molt to C6 and reproduce 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 closely associated with phytoplankton bloom. For overwintering stages of both species, a large deposition of lipid in the body and its gradual decrease toward the onset of phytoplankton bloom of the next year was seen. While no comparable life cycle data is presently available for M. okhotensis, the present results of M. pacifica are compared with the results from other regions, showing a large between-region variation in the life cycle patterns of this species. Further, the present results are discussed in the context of broad overwintering mechanisms of pelagic copepods living in high-latitude seas.

KEY WORDS: Metridia pacifica · Metridia okhotensis · Vertical distribution · Life cycle · Oyashio · Western North Pacific