Abstract / Introduction / Summary:
A female specimen of argulid branchiuran was collected from the first gill arch of an unidentified crucian carp Carassius sp. (Cypriniformes: Cyprinidae) in the moat of Fukui Castle ruins in Fukui City, Fukui Prefecture, central Japan, on 8 August 2025. This specimen was deformed in shape but is identified as Argulus japonicus Thiele, 1900 based on its gross morphology and the presence of a single plumose seta on the posterior margin of the first leg coxa. It was impossible to count the number of supporting rods in the marginal membrane of the first maxilla. This collection represents a new prefecture record for A. japonicus in Japan
