Seychellum
| Seychellum alluaudi | |
|---|---|
| |
| Drawing of dorsal view of Seychellum alluaudi | |
Scientific classification 
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Order: | Decapoda |
| Suborder: | Pleocyemata |
| Infraorder: | Brachyura |
| Family: | Deckeniidae
|
| Genus: | Seychellum Ng, Števčić & Pretzmann, 1995 |
| Species: | S. alluaudi
|
| Binomial name | |
| Seychellum alluaudi (A. Milne-Edwards & Bouvier, 1893)
| |
| Synonyms [2] | |
| |
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
Adults are dark yellow to brown in colour, and have a quadrangular carapace with a width of around 50 mm (2 in). 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
Adult specimens reach a carapace width of 30–52 millimetres (1.2–2.0 in),[6] and a carapace length about 80% of the width.[5] The carapace is almost quadrangular and quite flat, in contrast to the more rounded outline in its closest relative, Deckenia.[6] Its surface is rough, with scattered tubercles, and is divided into distinct regions by a series of grooves.[6] 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.[5] 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").[7]
The antennae are minute (smaller than those of Deckenia), and the chelae (claws) are unequal in size.[5] Whereas Deckenia has distinctly flattened walking legs, those of Seychellum are normal.[6] Overall, Rathbun concluded that Seychellum alluaudi "differs from Deckenia [...] in so many other respects that the species are not likely to be confounded".[5]
Taxonomic history
Seychellum alluaudi was
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.[6] 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.[6][8] Following a number of phylogenetic studies, Cumberlidge et al.[9] reduced that taxon to a subfamily of the larger family Potamonautidae, and Seychellum is again considered a part of it.[2]
Distribution and biogeography
Seychellum alluaudi is endemic to the granitic high islands in the inner group of the Seychelles, in the western Indian Ocean.[1] It is found on the four largest granitic islands – Mahé, La Digue, Silhouette and Praslin – and lives in mountain streams that flow through rainforest.[1]
The closest relatives of Seychellum are the two species of
The presence of a strictly freshwater species on the Seychelles is hard to explain, biogeographically.[9][10] Several possible explanations have been proposed.
- In 1902, prior to the development of plate tectonics, Arnold Edward Ortmann proposed that land bridges formerly connected the Seychelles to other land masses.[3]
- It has also been suggested that the ancestors of Deckenia and Seychellum lived on a landmass comprising Seychelles and the African mainland, which then separated by
- Finally, the ancestor of Seychellum may have travelled across the western Seychelles Bank was larger due to lower sea levels.[3] The greater extent of the Seychelles Bank may also explain how the species is now found on four separate islands in the Seychelles archipelago.[11] The time of divergence between Seychellum and Deckenia has been estimated independently at 27 to 8 million years ago[12] and 12 to 5.5 million years ago.[11] Seychellum is reported to be more tolerant of salt water than other families of freshwater crabs, which may have allowed it to survive rafting between the African continent and the Seychelles Bank.[13]
The most recent research favours trans-oceanic dispersal, but some uncertainty remains.[11]
Conservation status
Of the five criteria assessed by the
References
- ^ . Retrieved 12 November 2021.
- ^ Raffles Bulletin of Zoology. 17: 1–286.
- ^ ISBN 978-90-6193-107-2.
- PDF). Freshwater Forum. 21: 3–26.
- ^ .
- ^ .
- ^ a b A. Milne-Edwards & E. L. Bouvier (1893). "Sur une nouvelle espèce du genre Deckenia (Hilgendorf) recueillie par M. Alluaud aux îles Seychelles" [On a new species of the genus Deckenia (Hilgendorf) collected by Mr. Alluaud in the Seychelles]. Annales des Sciences Naturelles, Zoologie. 7th series (in French). 15: 325–336.
- JSTOR 20107558. Archived from the original(PDF) on 2011-07-20.
- ^ .
- ^ PMID 16621611.
- ^ PMID 21824522.
- PMID 20849594.
- .
External links
- Andreas Karge (May–June 2008). "Seychellen 2008". Süßwassergarnelen – Freshwater shrimps (in German). – includes a photograph of Seychellum