The sentinel crabs, Chaenostoma orientale Stimpson, 1858, and C. crassimanus Stimpson, 1858, were both described from the Ryukyu Islands, Japan, and have both been long considered junior synonyms of C. boscii (Audouin, 1826). After examining specimens of the C. boscii species-complex from East Africa, the Malay Peninsula, Indonesia, New Caledonia, Papua New Guinea, the Philippines, Taiwan, and the Ryukyus, we now consider both C. crassimanus and C. orientale to be valid species, and C. lisae (Poupin & Bouchard, 2010) is found to be a synonym of C. crassimanus, based on the morphology of the carapace, male cheliped and male first gonopod, supported by molecular evidence from mitochondrial cytochrome oxidase subunit I (COI) sequences. Both C. orientale and C. crassimanus are found sympatrically in Taiwan (including Penghu), and possibly in the Ryukyus and New Caledonia. While C. boscii is endemic to the Red Sea, the distribution of C. sinuspersici is limited to the western Indian Ocean (East Africa and the Persian Gulf).
2016 The Carcinological Society of Japan