The Japanese crayfish, Cambaroides japonicus, is found on the islands of Hokkaido and northern Honshu in association with 12 species of Cirrodrilus, a genus of ectosymbiotic branchiobdellidans. To clarify the distributional ranges of these species, we examined museum-deposited specimens and collected branchiobdellidan assemblages from 68 locations on Hokkaido and 60 on northern Honshu. Molecular analyses (mt COI) were performed on a part of the specimens examined. 

The present investigation found 10 morphologically distinct species; however, Cirrodrilus cirratus and C. uchidai were combined into a single taxon, Cirrodrilus cirratus complex, following molecular analyses in which the two taxa nested together in a phylogenetic tree. The following 6 species: Cirrodrilus digitatus, C. sapporensis, C. nipponicus,C. ezoensis, C. homodontus and C. cirratus complex were recorded from Hokkaido, while C. aomorensis, C. tsugarensis and C. iwakiensis were found on Honshu. The presence of Cirrodrilus digitatus (= C. inukaii) and C. cirratus complex (C. uchidai) on Honshu resulted from the translocation of crayfish from Hokkaido, but finding C. tsugarensis and C. iwakiensis on Hokkaido is currently inexplicable. No significant difference in the species composition was found between eastern and western regions on Hokkaido, where divergent lineages have been detected in the Japanese crayfish host. Morphological and molecular studies suggest that six species with dissimilar dorsal and ventral jaws form a monophyly, in which four species were found on Hokkaido and two on Honshu. The present reassessment supports the hypothesis that ectosymbiotic branchiobdellidans had undergone diversification into multiple species prior to the host's divergence on Hokkaido and northern Honshu. 

Two species previously reported from Hokkaido, Cirrodrilus megalodentatus and C. makinoi were not found in the present study. It is possible that the two species are rare, or that they are intraspecific variants of the respective, similarly appearing C. ezoensis and C. homodontus.