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Seychellum alluaudi is a species of freshwater crab endemic to the Seychelles, and the only true freshwater crab in that country. It lives in rainforest streams on the archipelago's granitic high islands. Although it may be abundant, little is known about its biology. If its habitat were to decline in quality, S. alluaudi might become endangered, but it is currently listed as Vulnerable on the IUCN Red List.
Seychellum alluaudi was described as a species of Deckenia in 1893 and 1894, and later split off into the monotypic segregate genus Seychellum. Its closest relatives are the two species currently in Deckenia, both of which are found in East Africa. Several hypotheses have been published to explain how Seychellum reached its isolated location, including submerged "" and ancient vicariance, although some means of transport across the open ocean is considered the most likely explanation.
Adults are dark yellow to brown in colour, and have a quadrangular carapace with a width of around . The claws are unequal in size. Seychellum differs from Deckenia in a number of characters, including the lengths of the legs and antennae, and the fact that Deckenia species have flattened legs, compared to those of Seychellum.
Adult specimens reach a carapace width of , and a carapace length about 80% of the width. The carapace is almost quadrangular and quite flat, in contrast to the more rounded outline in its closest relative, Deckenia. Its surface is rough, with scattered , and is divided into distinct regions by a series of grooves. A raised ridge runs sinuously along the front of the carapace, interrupted by a groove in the centre and by grooves towards each side of the carapace. The colour of Seychellum is described as " jaune noirâtre, qui devient franchement marron dans la moitié antérieure de la carapace" ("dark yellow, which becomes quite brown in the anterior part of the carapace").
The antennae are minute (smaller than those of Deckenia), and the chelae (claws) are unequal in size. Whereas Deckenia has distinctly flattened walking legs, those of Seychellum are normal. Overall, Rathbun concluded that Seychellum alluaudi "differs from Deckenia ... in so many other respects that the species are not likely to be confounded".
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.
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, biogeography. Several possible explanations have been proposed.
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