Detection and molecular analysis of Philopinna higai (Digenea: Didymozoidae) from domestically introduced Sarcocheilichthys fishes in the Tohoku region, Japan.

Philopinna higai is a species of Didymozoidae (Digenea: Hemiuroidea). The definitive hosts of this parasite only belong to the fish genus Sarcocheilichthys. Sarcocheilichthys fishes are endemic to Lake Biwa and southwestern Japan and were introduced into the northeastern (Tohoku) region. However, P. higai parasitism has not been investigated in the Tohoku region. In this study, we surveyed the distribution of P. higai in the Tohoku region and sequenced 28S rDNA (994 bp) and cytochrome oxidase subunit 1 (CO1) gene (721 bp) of P. higai. We also sequenced mitochondrial cytochrome b (581 bp) of Sarcocheilichthys fishes from the Tohoku region and Lake Biwa. Our findings confirmed the distribution of P. higai in all seven surveyed river systems in the four prefectures of the Tohoku region. The 28S rDNA sequence of P. higai did not differ among regions, whereas 10 haplotypes of CO1 were identified and clustered into two major clades. The haplotypes of Sarcocheilichthys fishes introduced in the Tohoku region were identical to the dominant haplotypes in Lake Biwa. Thus, P. higai from Lake Biwa and the Tohoku region were genetically the same species, although genetically differentiated populations formed in the Tohoku region.