Distribution and life history of two planktonic copepods, Rhincalanus nasutus and Rhincalanus rostrifrons, in the northwestern Pacific Ocean

To clarify the life histories of two tropical–subtropical Eucalanidae species, Rhincalanus nasutus and Rhincalanus rostrifrons, we investigated the geographical and vertical distributions of their copepodid populations in the northwestern (NW) Pacific Ocean. Zooplankton samples were collected between 15°N and 49°N in the NW Pacific. Rhincalanus nasutus was widely distributed from 20°N to 49°N, whereas R. rostrifrons was restricted to more southerly latitudes (15–37°N). The occurrence of the two species in the surface layer was associated with seasonal spring ontogenetic vertical migration (OVM). Gonadal analysis indicated that, during spring, both species reproduce in the surface layers of the Kuroshio–Oyashio Transition Area (TA) and the slope water area, which is a region of the continental slope situated north of the Kuroshio axis. Our results suggest that both species might have 1 or 2 year life cycles. In comparison, dormant populations occupying the water below the surface layer mainly comprised adult females (50.2% for R. nasutus and 57.7% for R. rostrifrons), which implied that this is an advantageous strategy for this life state in response to irregular increases of primary production in tropical–subtropical oceans. Furthermore, dormancy in deep waters (500–1000 m) by R. nasutus might indicate a strategy to avoid the relatively high predation risks incurred in shallower waters, due to its larger body size. In contrast, the smaller body size of R. rostrifrons facilitates dormancy in shallower waters (200–500 m depth).