Abstract / Introduction / Summary:
An ethanol-preserved specimen of argulid branchiuran loaned from the Toyama Science Museum (TOYA, Toyama City, central Japan) is identified as Argulus japonicus Thiele, 1900. The specimen (TOYA-Cr 23805) is an adult female collected in a fish-rearing pond on the campus of the Faculty of Fisheries, Hokkaido University, in Hakodate, southern Hokkaido, on 27 April 1979. There was no record of its host fish. Although the specimen is deformed in shape, it has a single plumose seta near the posterior margin of each coxa of the first legs, which is a character to distinguish A. japonicus from a morphologically similar, congeneric species Argulus coregoni Thorell, 1864 that is known to have 4–9 plumose setae on each coxa. Moreover, the specimen has about 45–50 supporting rods in each marginal membrane of the first maxillae, which corresponds to the number (40–52) of supporting rods reported from A. japonicus. In Hokkaido, this parasite so far has been recorded from common carp Cyprinus carpio Linnaeus, 1758 reared in a carp farm and koi carp C. carpio held by a hobbyist.
