Taxonomic Position and Distribution of Unio biwae (Bivalvia : Unionidae)

Abstract

There are two species belonging to the genus Unio in Japan. They resemble each other in the shell morphology, but color of glochidia is different : buff in Unio douglasiae nipponensis and milky white in U. biwae. In the present study, species identification was carried out based on this character. Although the pseudocardinal tooth on the right valve was thicker in U. biwae than in U. douglasiae nipponensis on the average, the ranges of variation were overlapped largely. Moreover, experiments proved that hybridization between them occurred easily when they reared together. U. biwae was only found in and around Lake Biwa, while U. douglasiae nipponensis was never found there, though it was distributed in the lower reaches of the water system of Lake Biwa. As there was no distinct difference in the shell morphology and no overlap of distribution between them, it may be better to regard U. biwae as another subspecies of U. douglasiae. This subspecies U. douglasiae biwae is endemic in Omi Basin.