Geographic variations in dorsal and anal ray counts of juvenile Japanese flounder, Paralichthys olivaceus, in the Japan Sea

Meristic counts of early and late wild settled juveniles of Japanese flounder, Paralichthys olivaceus (Paralichthyidae), in Wakasa Bay, Japan, were compared, with specimens collected from 26 sites spread along ca. 1700 km of coastline facing the Japan Sea. Mean dorsal and anal ray counts of the early settled group at Kyoto were significantly larger than those of the late settled group. A discontinuous geographic boundary in mean dorsal and anal ray counts was found around Wakasa Bay and the Noto Peninsula with both means significantly larger in the southern group than in the northern group. Mean ray counts of the early and late settlers at Kyoto were similar to those of the southern and northern groups, respectively. The effect of water temperature on meristic characters was tested on two groups of laboratory-reared juveniles. Mean dorsal and anal ray counts were larger in juveniles reared at higher water temperatures. The early and late settler groups were estimated to hatch in late February and late April, respectively, representing negative response of meristic characters to water temperatures if these two groups hatched out in Wakasa Bay. Differences in the dorsal and anal ray counts between the early and late settled groups at Kyoto can, therefore be attributed to genotypic, not phenotypic variation. This suggests the possibility of transport of larvae of the early group from areas further south to Wakasa Bay by the warm Tsushima Current, a branch of the Kuroshio, which flows northeastward along the coast of the Japan Sea, particularly when the temperature gradation between the south and north areas during these seasons is considered.