Occurrence of the Japanese potamid crab <i>Geothelphusa dehaani</i> in tidal rivers flowing into Ariake Sea

Potamid crabs are primary freshwater crabs that complete their life cycle independently from the marine environment. Geothelphusa dehaani, the only primary freshwater crab found in most temperate areas of Japan, generally inhabits the upper region of rivers or mountain streams. However, in the present survey some G. dehaani were found in the floodplains of upper tidal rivers even with some crabs inhabiting the brackish area in four rivers flowing into the Ariake Sea. This suggests that G. dehaani occasionally occurs in various environments and is tolerant to the saline environment to some degree so that their distribution is not simply limited to the narrow mountain stream area.