Species information
Scientific name: Geothelphusa tenuimanus
Japanese name: ヒメユリサワガニ
Chinese name: 細掌澤蟹
English name: Geothelphusa tenuimanus
Origin: Okinawajima
Ecology and description
Geothelphusa tenuimanus is a rather small freshwater crab with a carapace about 3cm wide. Its walking legs are very long and slender, and its color varies by individual from purple to brown. It lives in the limestone areas of Okinawajima, mainly in the north and south, and has also been recorded in Okinawa City in the central part. It spends the day under limestone rocks or in caves and comes out at night, preferring slightly moist but not waterlogged soil. Its populations are small in each area.
Crabs of the genus Geothelphusa do not release their eggs into the sea. The mother crab looks after them until the young crabs undergo their first molt. The eggs of Geothelphusa tenuimanus are about 4–5mm in diameter and number only about 25, the largest in diameter and the fewest among the Geothelphusa species of Japan, and the hatched juveniles are also large. Most freshwater crabs release their juveniles into water, but this crab places them directly on land, which shows how well it is adapted to life away from water.
Among the freshwater crabs of Okinawajima, it is the most terrestrial, and it is a very unusual and valuable example of evolution. Fossils have also been found, so it has value for scientific research.
References:
Miyake, S. & Minei, H. (1965). A new fresh-water crab, Potamon (Geothelphusa) tenuimanus sp. nov., from Okinawa-jima, the Ryukyu Islands. Journal of the Faculty of Agriculture, Kyushu University, 21(4), 377–383.
Naruse, T., Karasawa, H., Shokita, S., Tanaka, T. & Moriguchi, M. (2003). A first fossil record of the terrestrial crab, Geothelphusa tenuimanus (Miyake & Minei, 1965) (Decapoda, Brachyura, Potamidae) from Okinawa Island, Central Ryukyus, Japan. Crustaceana, 76(10), 1211–1218.
Naruse, T., Shokita, S. & Nagai, T. (2005). Taxonomy of the terrestrial crab Geothelphusa tenuimanus (Crustacea: Decapoda: Brachyura: Potamidae) of Okinawa Island, Central Ryukyus, Japan. Species Diversity, 10, 171–184.
Field photographs taken during our tours, with observation dates
