Abstract / Introduction / Summary:
Argulus coregoni Thorell, 1864 was collected from two species of fishes in small mountain streams in Akita Prefecture, northern Honshu, Japan. The infected fishes are ayu Plecoglossus altivelis altivelis (Temminck and Schlegel, 1846) (Plecoglossidae) from the Nukazawa River and stream-resident masu salmon Oncorhynchus masou masou (Brevoort, 1856) (Salmonidae) in the Bussha Stream. These collections are the second records of A. coregoni from ayu and river-resident masu salmon, respectively, in Akita Prefecture. Both the Nukazawa River and the Bussha Stream belong to the Yoneshiro River system and represent new collection localities for A. coregoni. The collected specimens of the species are briefly described and particularly characterized by the presence of four or five plumose setae on the posterior margin of the coxa in the first legs. This feature is useful to distinguish A. coregoni from a morphologically similar species, Argulus japonicus Thiele, 1900, which also occurs in Akita Prefecture and has only one plumose seta on the coxa of the first legs.