Shell morphology and genetic characteristics of Corbula (Potamocorbula) spp. (Bivalvia: Corbulidae) from the waters adjacent to Japan, Korea and San Francisco Bay

Corbula (Potamocorbula) spp. are natively distributed in the waters adjacent to Japan and Korea. These populations are distinguished by 6 morphological characteristics. How- ever, they have high geographic variation, and it is quite difficult to distinguish among them. In the present study, we investigated the geographic distribution of the morphological and genetic characteristics of Corbula (Potamocorbula) spp. collected from the waters adjacent to Japan, Korea and San Francisco Bay, USA. Shell morphology was inferred through canonical discrimi- nant analysis (CDA) of 8 shell characteristics, and genetic characteristics were analysed using the DNA sequences of mitochondrial cytochrome c oxidase subunit 1 (CO1) and nuclear internal tran- scribed spacer 2 (ITS2) regions. The results of CDA showed that the samples from Ganghwa-do in western Korea could be clearly separated from those of all the other localities. The molecular phy- logenetic tree and haplotype network that were constructed from the CO1 and ITS2 regions, respectively, did not show clear groupings among the collected samples, except for 1 group com- posed of samples from Ganghwa-do. In addition, no differences in CO1 amino acid sequences could be found among any of the samples examined, including the Ganghwa-do samples. These facts suggest that Corbula (Potamocorbula) spp. distributed in the waters adjacent to Japan and Korea were separated into 2 groups. However, these 2 groups were genetically closely related, and it seems appropriate to treat them as 2 distinct species. The classification of the subgenus Potamocorbula should be re-examined at the species level.