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To better understand the life history and ontogenetic vertical migration of Neocalanus gracilis (Dana), geographical and seasonal change in the copepodid population structure was inves- tigated in the western North Pacific. Multi-layered zooplankton samples were collected from 1000 m depth to the surface at 21 stations (ca. 15 to 48°N, 127°E to 159°W) from September 2001 to June 2006 and from 200 m depth to the surface at a fixed station (30°N, 138°E) from January 2003 to January 2004. N. gracilis was distributed in the southern part of the sampling area <36°N. Early copepodid stages (C1 or C2) were always collected regardless of season, suggesting no seasonality in reproduc- tion. In water columns of from 0 to 1000 m depth, 93% of the total copepodids occurred between the surface and 200 m, and diel vertical migration (DVM) was not clearly observed, except for non- feeding C6 males. Males were mainly distributed in deep layers (<300 m depth) during the daytime, whereas they migrated to shallower layers during night, where the females were present. Our results suggest that a key factor controlling male DVM is a trade-off between mating with females and pre- dation risk in the upper layer. The species also showed small-scale ontogenetic vertical migrations (OVM) of both developmental ascent (C1 to C3) and descent (C5 to C6). The relatively deeper distri- bution of Stage C1 individuals exhibiting oil droplets in their bodies implies non-feeding early devel- opmental stages, which is beneficial in avoiding predatory risk and surviving in oligotrophic tropical waters. On an evolutionary time scale, these adaptive traits to oligotrophic environments may have allowed the genus to extend into the subarctic Pacific, which is characterized as a 'high nutrient, low chlorophyll' area, and the OVM pattern might be modified to synchronize the life history to the seasonality of local production.

Life history and ontogenetic vertical migration of Neocalanus gracilis in the western North Pacific Ocean