New records of a freshwater fish parasite Argulus japonicus (Branchiura: Argulidae) from northern Honshu, Japan, with a note on its occurrence in a brackish water lake

One and two specimens of Argulus japonicus Thiele, 1900 were collected from the body surface of big-scaled redfin Pseudaspius hakonensis (Günther, 1877) (Cypriniformes: Leuciscidae) in the Babame River, Akita Prefecture, and Lake Jusan, Aomori Prefecture, northern Honshu, Japan, respectively. This is two new prefecture records in Japan and a new host record for A. japonicus. Lake Jusan is the northernmost collection locality of this parasite in Japan recorded from wild fishes. As Lake Jusan is a brackish water lake and big-scaled redfin is known to migrate from fresh to salt waters, and since A. japonicus completes its life cycle in fresh waters, we infer that the two individuals of A. japonicus parasitized the big-scaled redfin in an inflowing river, then this fish was collected along with the surviving argulid individuals after migrating to the lake. The collection of A. japonicus in Lake Jusan represents the second record for the species from brackish water lakes in Japan.