Abstract / Introduction / Summary:
One adult male of Argulus japonicus Theile, 1900 was collected from the body surface near the pectoral fin of a koi carp Cyprinus carpio Linnaeus, 1758 (Cypriniformes: Cyprinidae) reared by a carp hobbyist in a garden pond in Akiota, Hiroshima Prefecture, western Honshu, Japan. The male collected is briefly described. This is the second specimen-based record of A. japonicus from Hiroshima Prefecture, where the species was previously found from silver crucian carp Carassius sp. in the Kurose River. Based on the literature published between 1900 and 2023, this paper also reviews the biology of A. japonicus infecting common carp and koi carp in Japan. This parasite has been recorded from common carp and koi carp reared in 12 prefectures (Hokkaido, Ibaraki, Tokyo, Kanagawa, Ishikawa, Shiga, Mie, Osaka, Hyogo, Hiroshima, Yamaguchi, and Kagoshima) and from wild common carp in two prefectures (Miyagi and Shimane). This indicates that A. japonicus often utilizes captive common carp and koi carp as its hosts and is one of the important parasites of these fishes in Japan. In ponds rearing common carp in central Japan, free-swimming individuals of A. japonicus are highly abundant in warm water months but decrease in early winter. This species usually overwinters as eggs, but a few adults may survive in winter months. A secondary bacterial infection has been suggested to result from A. japonicus infection.