Remarkable decline of the fish-parasitic nematode Raphidascaris gigi endemic to Lake Biwa

Abstract

　Raphidascaris gigi Fujita, 1928 is a parasitic nematode endemic to Lake Biwa. This species has a broad host range and can parasitize various fish species, including Plecoglossus altivelis (Osmeridae), Oncorhynchus masou subsp. (Salmonidae) and Opsariichthys uncirostris (Xenocyprididae). It uses zooplankton as an intermediate host or a paratenic host. It was once one of the most common fish parasites in Lake Biwa. However, in a fish parasite survey conducted in 2022, we found no R. gigi in 535 fish from 36 species. A parasitological survey using P. altivelis specimens collected between 2007 and 2023, and specimens of paratenic host Leptodora richardi (Cladocera: Leptodoridae) collected during 2004 and 2022, revealed that R. gigi has been absent from both hosts since 2017. Only one nematode was detected in P. altivelis collected in 2023. Therefore, while R. gigi does not completely extinct from the lake, its population has substantially declined since 2017.