Cave-dwelling crabs of the genus Karstarma from lava tubes of the volcano 'Piton de la Fournaise', in Réunion Island, with description of a new species and redescription of Karstarma jacksoni (Balss, 1934) from Christmas Island (Decapoda, Brachyura, Sesarmidae).

Sesarmid crabs of the genus Karstarma Davie Ng, 2007 are reported for the first time in the Western Indian Ocean: they were discovered in the lava tubes of the volcano 'Piton de la Fournaise', Réunion Island. A new species, morphologically similar to Karstarma jacksoni (Balss, 1934) from Christmas Island, Eastern Indian Ocean, is recognized and described. A redescription of K. jacksoni is also provided. A second species is recognized, but being represented in the collection by a single juvenile, it cannot be identified to species level until more collections are made in the lava tubes. [Zoobank: http://zoobank.org/NomenclaturalActs/C70A95B2-D667-491D-BD35-5DF32E3FFB08].