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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 ''Least Concern'' 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 constract 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 "''フランス語: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".〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Seychellum」の詳細全文を読む スポンサード リンク
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