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. S. 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 "land bridges" 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. Description Seychellum alluaudi is the only truly freshwater crab in the Seychelles; all the other true crabs in the islands have marine larvae. It is common for freshwater crabs to have large eggs, and this is also seen in Seychellum; its eggs are around in diameter, which Rathbun described as "very large". 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 tubercles, 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 "" ("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