Geothelphusa dehaani
Geothelphusa dehaani, commonly known as Japanese freshwater crab or sawagani, is a species of freshwater crab in the family Potamidae, endemic to Japan. It inhabits clear mountain streams and is known for its small size, nocturnal behavior, and role as an indicator of healthy freshwater ecosystems. The species is also a culinary delicacy in Japanese cuisine.
Taxonomy
Geothelphusa dehaani was first described by Adam White in 1847. It belongs to the genus Geothelphusa in the family Potamidae, which includes freshwater crabs. It is one of two freshwater crab species native to Japan, alongside the mitten crab. Historically, it was known as Thelphusa dehaani and Thelphusa japonica.Description
Geothelphusa dehaani is a small crab with a carapace width of 20 to 30 mm. Its body color varies by region, including purple-black, red-brown, and gray-blue, with chelipeds often dark-yellow or milky white. The carapace is smooth, and color variations include a notable blue variant.Distribution and habitat
Geothelphusa dehaani is endemic to Japan, distributed from Aomori Prefecture in northern Honshu to Kagoshima Prefecture in southern Kyushu, including the Ryukyu Islands and islands like Sado, Oki, Tanegashima, and Yaku. It inhabits clear, unpolluted mountain streams, valleys, and swamps, often under rocks and fallen trees in water. Its reliance on clean water makes it an indicator species for healthy freshwater ecosystems.Biology and ecology
Geothelphusa dehaani is primarily nocturnal but may be active during the day on rainy or cloudy days. It is active from March to November, hibernating under rocks in water during winter. As an omnivore, it feeds on small insects, snails, earthworms, plants, and fallen leaves, contributing to nutrient cycling in its ecosystem. On humid days, it may venture into forests and climb trees in parks.Parasites and public health
Geothelphusa dehaani serves as an intermediate host for lung flukes such as Paragonimus westermani, Paragonimus wmiyazakii, and P. skrjabini miyazakii. Human consumption of raw or undercooked infected crabs can lead to paragonimiasis, presenting symptoms like pleural effusion and pneumothorax.Leeches from the family Praobdellidae have also been found feeding on this species, which is rare for crustacean hosts.