Genetic Structure of <scp><i>Eudorella emarginata</i></scp> (Peracarida: Cumacea): Effects of Topography and Historical Anoxic Events in the Sea of Japan

Geohistorical events are among the most important factors determining population genetic structure. The Sea of Japan is an intriguing area because of its connection to neighboring seas via shallow straits (< 140 m deep) and the occurrence of deep-water anoxic events during glacial periods. Despite repeated anoxic events, species with low dispersal capabilities have been reported at depths deeper than the straits in both the Sea of Japan and the Pacific Ocean. We focused on one such species, <i>Eudorella emarginata</i> (Cumacea, Peracarida). This species is expected to have limited dispersal capabilities owing to its lack of a planktonic larval stage. Phylogenetic analyses based on mitochondrial and nuclear DNAs revealed three distinct clades within individuals of <i>E. emarginata</i> distributed in Japanese waters, suggesting the existence of sibling species. Only one clade included individuals from both the Sea of Japan (519-1024 m deep) and the Pacific Ocean (490-1504 m deep). Further analyses focused on this clade to explore the relationship between population dynamics and geohistorical events. The effects of glacial anoxia have been suggested in Sea of Japan populations; however, such an effect has not been detected in a population nearby the Tsugaru Strait. We hypothesized that the inflow of oxygen-rich seawater from the Tsugaru Strait maintained suitable conditions in the area facing the strait during glacial periods. Future studies should implement genome-wide strategies to investigate this hypothesis by focusing on high-resolution genetic structure.