Invalidation of Diphyllobothrium hottai (Cestoda: Diphyllobothriidae) based on morphological and molecular phylogenetic analyses.

Diphyllobothrium hottai Yazaki, Fukumoto & Abe, 1988 was described based on the morphology of adult worms recovered from golden hamsters that had been experimentally infected with plerocercoids obtained from Japanese surf smelts (Hypomesus pretiosus japonicus) and olive rainbow smelts (Osmerus eperlanus mordax). Although D. hottai was considered to be distinct from Diphyllobothrium ditremum (Creplin, 1825), their taxonomic relationship requires further clarification. In our study, D. hottai and D. ditremum obtained from hamsters experimentally infected with plerocercoids isolated from Japanese surf smelts were compared using morphological and molecular methods. The criterion usually used to differentiate between D. hottai and D. ditremum is the difference in the angle between the long axis of the cirrus sac and that of the seminal vesicle. However, we found variation of the angle within the same individual and, one specimen showed both of the different angles that were supposedly unique to each of the species. Furthermore, phylogenetic analysis of the complete sequences of the mitochondrial cytochrome c oxidase subunit 1 and cytochrome b genes revealed that both species were genetically indistinguishable. Therefore, D. hottai is considered to be a junior synonym of D. ditremum.