Abstract / Introduction / Summary:
An ovigerous female of cymothoid isopod was collected in June 2021 from the branchial cavity of a Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846), in coastal waters of the Sea of Okhotsk at Yubetsu, Hokkaido, northern Japan. This female’s body length (21.2 mm) falls within the range of that (20.5–27.5 mm) of Mothocya sajori Bruce, 1986 and concave lateral margins of coxae are similar to those of the same species reported in its original description. However, the posterior margin of pereonite 7 is weakly recessed and resembles that of Mothocya parvostis Bruce, 1986. Thus, the female collected is not identified as M. sajori or M. parvostis but reported herein as Mothocya sp. This result indicates that the criteria for distinguishing these two species given in their original descriptions are not useful.