Abstract / Introduction / Summary:
Toyama Bay is an inlet of the southern Sea of Japan, and its coastal water is known as the locality where two paratypes of Mothocya parvostis Bruce, 1986 were collected from Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846). In this study, 13 ovigerous females of Mothocya sp. were collected from the branchial cavity of 16 individuals of the same host species in Toyama Bay off Matsunami, Noto, Ishikawa Prefecture, central Japan, on 17 May 2019. The body lengths of one and four females are 13.1 and 21.0–22.3 mm, respectively, which fall within the ranges of body length of ovigerous females of M. parvostis (11.0–15.0 mm) and M. sajori Bruce, 1986 (20.5–27.5 mm) given in their original descriptions. However, the body length of the remaining eight females shows a range of 15.5–19.3 mm, which does not correspond to the above body length ranges of the two species. Moreover, smaller and larger individuals from these eight females are morphologically similar to M. parvostis and M. sajori, respectively. This indicates that isopods of the genus Mothocya infecting Japanese halfbeak cannot be always separated into the two species based on the previously reported criteria (the shape of the body, coxae and the posterior margin of pereonite 7, and the body length range) but can be regarded as a single species.