A new monogenean species, Gyrodactylus ajime n. sp. (Gyrodactylidae), parasitic on Niwaella delicata (Niwa), an endemic loach of Japan.

A new gyrodactylid species, Gyrodactylus ajime n. sp., is described from the skin and fins of an endemic and commercially important loach Niwaella delicata (Niwa) (Cypriniformes: Cobitoidea: Cobitidae), from the upper reach of the River Yura in Ashiu, Kyoto Prefecture, Japan with scanning electron microscopy observations and molecular data. This species can be distinguished from congenerics by characteristics of the sickle of the marginal hook: the sickle proper is composed of two straight sections and a curved section; its short point extends beyond the limit of the toe and is directed downwards; and the upper corner of heel angular is slightly raised. There are 31 species and subspecies of native loaches (Cobitoidea) known from Japan, of which 25 are red-listed nationally, and two are commercially important. I reviewed all reports of monogeneans from Japanese loaches, and found that only five fish species have been recorded as hosts of these parasites. This suggests that the parasite fauna of these fishes is poorly described. It is urgent to thoroughly describe the monogenean parasites of loaches in Japan as some of the parasite species may be threatened with co-extinction because their hosts are threatened, and to understand any negative impacts on commercially important fish species.