Abstract / Introduction / Summary:
An adult male of Argulus japonicus Thiele, 1900 was found in a sample of zooplankton collected by horizontal towing of a plankton net in the near surface waters off the west coast (36º22′13.9″N, 140º27′15.9″E) of Lake Senba (33.2 ha, 1.2 m in maximum water depth), Mito City, Ibaraki Prefecture, central Japan, on 15 July 2023. The male is briefly described and characterized by a single plumose seta on the posterior margin of the coxa of the first leg, two protrusions adorned with small spines on the ventroposterior margin of the coxa of the second leg, and 45 or 47 supporting rods in the terminal suction cup-like segment of each first maxilla. Many common carp Cyprinus carpio Linnaeus, 1758 (Cypriniformes: Cyprinidae) were found swimming near the collection site to feed on artificial foods given by citizens, and this fish species has been frequently documented as one of the hosts utilized by A. japonicus in Japan. It is thus likely that the male of A. japonicus was detached from one of the common carp gathering for feeding and then collected by the plankton net. In Ibaraki Prefecture, A. japonicus has been reported only from common carp or mirror carp C. carpio reared at a fisheries experimental station, and the present collection represents the first record of the parasite from the natural waters in this prefecture.
