Comparative analysis on the genetic population structures of the deep-sea whelks <Emphasis Type="Italic">Buccinum tsubai</Emphasis> and <Emphasis Type="Italic">Neptunea constricta</Emphasis> in the Sea of Japan

The population structures of Buccinum tsubai and Neptunea constricta were compared using partial sequences of the mitochondrial COI gene. B. tsubai included four genetically distinguishable subpopulations (Hokkaido, Yamagata-Toyama, Yamato Bank, and San’in), which is consistent with the results of our previous study using mitochondrial 16SrRNA sequences. The genetic structure of N. constricta was also clear, but the lineage sorting corresponding to the geographical distribution was not as clear as that of B. tsubai. The difference between the genetic structures of these two species might be due to a difference in either their dispersal abilities or the historic distribution patterns in the Sea of Japan. The fossil record and nested clade analyses support the latter alternative. The four subpopulations of B. tsubai are estimated to have diverged 0.42–1.46 million years ago, providing support for the existence of intermediate water of normal salinity and oxygen concentration capable of sustaining B. tsubai between the low-salinity surface water and anoxic deep water during the Pleistocene glacial periods. The genetic diversity of the Yamato Bank subpopulation of B. tsubai was lower than that of the other subpopulations, which is likely attributable to the small size of the Yamato Bank subpopulation.