The freshwater Brachyura (Decapoda: Potamidae, Gecarcinucidae, Sesarmidae) of the Anambas and Natuna Islands, Kepulauan Riau Province, Indonesia

We report on the freshwater crabs of the families Potamidae, Gecarcinucidae, and Sesarmidae in the archipelago of Anambas and Natuna Islands in Indonesia. Five genera (one new) and eight species (four new) are now recorded from the islands. For the Potamidae, a new genus and a new species, Natunamon indonesicumn. sp., is described from Natuna; the taxon has diagnostic characters intermediate between IsolapotamonBott, 1968 and IbanumNg, 1995 from Borneo. A new species of Isolapotamon, I. remotumn. sp., is also described from Anambas; this is the first record of the genus outside Borneo and Mindanao, Philippines. For the Gecarcinucidae, the rare Balssiathelphusa natunaensisBott, 1970 is collected for the first time since its description, although the type was collected over 120 years ago; the species is redescribed in detail and the composition of the genus discussed. The widely distributed gecarcinucid, Parathelphusa maculataDe Man, 1879 is also recorded from Anambas and Natuna. Four species of Sesarmidae are reported: Geosesarma insulare Ng, 1986 (from Anambas), G. teschi Ng, 1986 (from Natuna), G. anambasn. sp. (from Anambas) and G. hashimin. sp. (from Natuna). Geosesarma hashimin. sp. is an arboreal species with a suite of atypical characters that suggest its present classification in Geosesarma needs to be reassessed.