Macrobrachium ustulatum is the sister species of Macrobrachium australe, and they have been historically confused because of their morphological similarity. The northern edge of the distribution range of both species is Japan: M. australe has been recorded from the Ryukyu Islands to a wide area of Honshu, while M. ustulatum has only two records in Japan. Therefore, the distributional records and habitat information of M. ustulatum are lacking. We report a total of 10 specimens of M. ustulatum collected from three localities in the Ryukyu Islands. The environments in which these specimens were collected suggest that the habitat of this species is a weakly flowing environment in the middle reach of a river. This differs slightly from M. australe, which prefers stagnant environments. Furthermore, we have succeeded in proposing new distinguishing characteristics for both species.
Figure: Macrobrachium ustulatum (top) and M. australe (bottom). Photo by the authors.
