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

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 betweenregion 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.

The paradox of Metridia spp. egg production rates: a new technique and measurements from the coastal Gulf of Alaska

“…Similar to observations made for M. lucens (McLaren et al 1989), over-wintering females are small compared to other generations, and those females completing development during the spring bloom are largest. In contrast, results from this study are more consistent with a single year life cycle for M. okhotensis, rather than a 2 y life cycle proposed for this species at slightly colder temperatures (Padmavati et al 2004).…”

“…The 2 species differ in that multiple generations of M. pacifica occur annually in the Gulf of Alaska, while only 1 generation occurs for M. okhotensis. Sampling was not conducted frequently enough to conclusively determine whether 2 (Hirakawa & Imamura 1993, Padmavati et al 2004) or 3 (Batchelder 1985) generations per year occur for M. pacifica in the study area; however, based on the sustained reproductive activity, patterns of female abun-dance, and knowledge of temperature-dependent growth in the laboratory (Padmavati & Ikeda 2002), 3 are most likely. Similar to observations made for M. lucens (McLaren et al 1989), over-wintering females are small compared to other generations, and those females completing development during the spring bloom are largest.…”

Lipid and fatty acid/alcohol compositions of the subarctic copepods Neocalanus cristatus and Eucalanus bungii from various depths in the Oyashio region, western North Pacific

“…For E. bungii: U: "upper" and L: "lower" depth (see Table 1). (Padmavati et al, 2004). These copepods accumulate oil as droplets or in an oil sac after the spring bloom season in the Oyashio region, and use their lipid stores for overwintering and reproduction and probably also for buoyancy.…”

East-west differences in population structure and vertical distribution of copepods along 47^|^deg;N in the subarctic Pacific in June 2009

“…However, the occurrence of C1 stages at these stations indicates that M. pacifica was not in dormancy. Metridia pacifica has been reported to be dormant in the Sea of Japan (Hirakawa & Imamura 1993) and the Oyashio region (Padmavati et al 2004). Contrary to these reports, active diel vertical migration during summer in the present region (Kobari et al 2008) suggests that there is no dormancy during summer in this region.…”

“…The life cycles of the abundant calanoid copepods in the subarctic Pacific (Neocalanus cristatus (Krøyer, 1848), N. plumchrus (Marukawa, 1921), N. flemingeri Miller, 1988, Eucalanus bungii Giesbrecht, 1893and Metridia pacifica Brodsky, 1950 have been studied in both the east (Miller et al 1984, Miller & Clemons 1988, Mackas et al 1998 and the west regions (Kobari & Ikeda 1999, 2001a, b, Tsuda et al 1999, Padmavati et al 2004, Shoden et al 2005). Regional differences in abundance, population structure, body size and lipid accumulation at epipelagic depths (0-200 m) have been examined (Dagg 1991, Tsuda et al 2001, Batten et al 2003, 2006, Kobari et al 2003b).…”