First record of a pea mussel Euglesa milium (Mollusca: Bivalvia: Sphaeriidae) from Japan, with its morphological and genetic characteristics

Euglesa milium is a Holarctic freshwater mussel species characterised by its unique subtrapezoidal shell and elongated perisiphonal suture. In this study, E. milium is recorded for the first time at two sites in Hokkaido, Japan, based on the investigation of the shell, anatomical, and genetic characteristics. The shell and anatomical features of the Japanese specimens were consistent with those of E. milium previously reported, while variations in shell outline shapes were observed between localities. Identical or very similar 16S rRNA haplotypes among Japanese and Russian specimens supported the morphological identification. The species richness of Japanese Sphaeriidae may have been underestimated because of the lack of surveys; therefore, further faunal investigations and taxonomic revisions are required.