the species are not likely to be confounded". Taxonomic history Seychellum alluaudi was first described in 1893 by Alphonse Milne-Edwards and Eugène Louis Bouvier, as a species in the genus Deckenia, D. alluaudi; the specific epithet commemorates Charles A. Alluaud, who had collected the specimens they used. Unaware of that description, Mary J. Rathbun described "Deckenia cristata" in 1894, in a paper published in the Proceedings of the United States National Museum; her description was based on specimens donated to the United States National Museum by William Louis Abbott, who had travelled to the Seychelles in 1890. At the time, the only other species in the genus was D. imitatrix from the coast of East Africa, and a second species (D. mitis), also from East Africa, was added in 1898. Rathbun synonymised D. cristata and D. alluaudi herself in 1906. The three species of Deckenia at that time were considered to be sufficiently distinct from other crabs to warrant placement in a separate subfamily (Deckeniinae) or family (Deckeniidae). In 1995, Peter K. L. Ng, Zdravko Števčić and Gerhard Pretzmann revised the family Deckeniidae, as then circumscribed, and concluded that "D. alluaudi" could not be accommodated with the other species of Deckenia in the same genus, or even in the same family. They described a new genus, Seychellum, and the species took on its current name. They placed Seychellum in the family Gecarcinucidae, leaving Deckenia as the only genus in the family Deckeniidae, and restricting its distribution to the African mainland. Following a number of phylogenetic studies, Cumberlidge et al. reduced that taxon to a subfamily of the larger family Potamonautidae, and Seychellum is again considered a part of it. Distribution and biogeography Seychellum alluaudi is endemic to the granitic high islands in the inner group of the Seychelles, in the western Indian Ocean. It is found on the four largest granitic islands – Mahé, La Digue, Silhouette and Praslin – and lives in mountain streams that flow through rainforest. The closest relatives of Seychellum are the two species of Deckenia from the African mainland. The two-lobed shape of the last segment of the mandibular palp, which had been used to argue for a close relationship between Seychellum and Indian crabs of the family Gecarcinucidae, does not appear to be a reliable phylogenetic character. The presence of a strictly freshwater species on the Seychelles is hard to explain, biogeographically. Several possible