Relationship between intermediate host taxon and infection by nematodes of the genus Rhabdochona

We describe the intermediate and definitive hosts of the fish nematodes Rhabdochona coronacauda and R. denudata honshuensis and discuss the relationships between parasitism and the feeding habitats of their intermediate hosts. We found that the principal intermediate hosts of the two nematodes were filter-feeding mayflies of the genera Ephemera, Photamanthus and Isonychia. Ephemera strigata seemed to be the most important intermediate host of these nematodes. Adult R. coronacauda were found mainly in Hemibarbus longirostris and Rhinogobius flumineus, which are benthic fishes that feed on benthic aquatic insects, including E. strigata. For R. coronacauda, therefore, the feeding habits of the definitive hosts facilitate host alternation by this species. However, adult R. denudata honshuensis were found in cyprinids. In particular, Zacco temmincki was the principal natural definitive host in our study area. Since Z. temmincki is a swimming predator, E. strigata nymphs that burrow in the substrate are not the main prey of this species. This indicates that the transmission of R. denudata honshuensis hardly occurs from E. strigata nymph to Z. temmincki, suggesting another, unknown transmission route.