Abstract / Introduction / Summary:
Male and female specimens of Argulus coregoni Thorell, 1864 were collected from the body surface of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) cultured at a trout farm in Aomori Prefecture, the northernmost prefecture in Honshu, the largest island of Japan. This represents a new prefecture record for A. coregoni in Japan. The specimens of A. coregoni are characterized by five or six plumose setae near the posterior margin of the first leg coxa and 59–65 supporting rods in the marginal membrane of the first maxilla in both sexes; two protrusions adorned with small spines and one digitiform projection on the ventro- and dorsoposterior margins of the second leg coxa, respectively, in male; and one plumose seta on the posterior margin of that coxa in female. Aomori Prefecture is one of the six prefectures in the Tohoku Region, and the present collection of A. coregoni also represents its second record from captive fishes in this region, where it previously infected masu salmon O. masou masou (Brevoort, 1856) farmed in Akita Prefecture.
