Abstract / Introduction / Summary:
An ovigerous female and a male of the cymothoid isopod Mothocya sajori Bruce, 1986 were collected each from left and right branchial cavities of one of the five Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846), purchased in the fresh fish section of a supermarket in Shizuoka City, Shizuoka Prefecture, central Japan. The fish were commercially caught in coastal Pacific waters of Chiba Prefecture, central Japan. The female and the male of M. sajori collected are briefly described. The female caused a deep dent on the wall of the branchial cavity. In Japan, Mothocya parvostis Bruce, 1986 has been reported from the branchial cavity of Japanese halfbeak and is known to closely resemble M. sajori. A taxonomic issue in identification of these cymothoids is discussed by reviewing the literature on the two species.