About Geothelphusa dehaani (White, 1847)
Geothelphusa dehaani is a small crab species, with a carapace width ranging from 20 to 30 mm (0.8 to 1.2 inches). Its body color varies across different regions, including purple-black, red-brown, and gray-blue; its chelipeds (claws) are often dark yellow or milky white. The carapace is smooth, and a distinct blue color variant is among the recorded color variations. This species is endemic to Japan, where it is distributed from Aomori Prefecture in northern Honshu to Kagoshima Prefecture in southern Kyushu. Its range also covers the Ryukyu Islands, as well as smaller islands including Sado, Oki, Tanegashima, and Yaku. It lives in clear, unpolluted mountain streams, valleys, and swamps, and is most often found under submerged rocks and fallen trees. Because it depends on clean water, it serves as an indicator species for healthy freshwater ecosystems. Geothelphusa dehaani is mainly nocturnal, but may be active during the daytime on rainy or cloudy days. It is active between March and November, and hibernates under submerged rocks during the winter. It is an omnivore that feeds on small insects, snails, earthworms, plants, and fallen leaves, and contributes to nutrient cycling in its native ecosystem. On humid days, individuals may move into forests and climb trees in parks. Reproduction takes place in summer. Females lay around 50 eggs, which they carry on their abdomen until the eggs hatch. This species has direct development: eggs hatch directly into juvenile crabs, with no planktonic larval stage. The estimated lifespan of Geothelphusa dehaani is 10 years.
