Genetic Differences in Spatially and Temporally Isolated Populations: Winter and Spring Populations of Pelagic Mollusk Clione (Mollusk: Gymnosomata), Southern Okhotsk Sea, Japan

The pelagic mollusk Clione is a naked pteropod with a sympatric two-species distribution in the southern Okhotsk Sea, Japan, consisting of the morphologically and genetically distinct C. elegantissima and C. okhotensis. Clione elegantissima appears in both winter and spring, and body length differs between the winter coastal population (WCP, January to March, 10–20 mm) and the spring offshore population (SOP, April to July; up to 30 mm). This body size difference and temporal-spatial separation of the populations suggests that the SOP is either a cryptic species or C. limacine drifted from the Subarctic Atlantic Ocean or an interspecies of C. elegantissima resulting from reproductive isolation. We investigated the taxonomic positions of both populations using morphological and genetic analyses and identified both as C. elegantissima with very high genetic similarity. We explain the occurrence of spatio-temporal isolated populations using the water mass dynamics in the Okhotsk Sea. Warm water entering the southern Okhotsk Sea around Japan through the Soya Straits is divided into the Soya Warm Water (SWW: June to November) and the Forerunner of the SWW (FSWW: March to May); cold water entering the Okhotsk Sea around Japan, east of Sakhalin Island, is divided into the East Sakhalin Current Water (ESCW: November to April). The Cold Water Belt (CWB) is frequently formed off the SWW during summer and autumn and comprises upwelling cold water originating from either subsurface water of the Japan Sea off Sakhalin or Okhotsk Sea bottom water. We present the temporal-spatial isolation mechanism of WCP and SOP per the SWW, FSWW, ESCW, and CWB dynamics.