Distributions of three species of Nihonotrypaea (Decapoda: Thalassinidea: Callianassidae) in intertidal habitats along an estuary to open-sea gradient in western Kyushu, Japan

The intertidal distributions and habitats of three congeneric callianassid shrimps [Nihonotrypaea harmandi (Bouvier), N. japonica (Ortmann), and N. petalura (Stimpson)] were surveyed along the estuary to opensea gradient from Ariake Sound through Tachibana Bay to the East China Sea in western Kyushu, Japan. In the light of long-term-averaged salinity (psu) values hi the surface waters (at 5 m depth) in the rainy season, (N. harmandi + N. petalura) and N. japonica mainly occurred in the ranges of 34.0-30.5 (the East China Sea + Tachibana Bay + the outermost part of Ariake Sound) and 30.5-28.5 (the middle part of Ariake Sound), respectively, with their distribution boundary distinct. Using the Venice-system terms for the clas sification of brackish waters, the former species group could be designated as euhaline to mixo-euhaline species, while the latter species as a mixo-polyhaline species. From the viewpoint of the sub strate conditions and their dimensions, the habitats for the three species were basically characterized, as follows: 1) N. harmandi - the relatively small sandflats and sandy beaches of medium - fine sands, with boulder beaches, very ex posed sandy beaches, and mud flats unin habited; 2) N. petalura - small sand patches surrounded by boulders in boul der beaches; and 3) N.japonica - the ex tensive sandflats of medium - fine sands, with boulder beaches, exposed sandy beaches, and mudflats uninhabited.