Seychellum

Seychellum alluaudi
Conservation status
Least Concern (IUCN 3.1) [1]
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Decapoda
Infraorder:	Brachyura
Family:	Potamonautidae
Subfamily:	Deckeniinae
Genus:	Seychellum Ng, Števčić & Pretzmann, 1995
Species:	S. alluaudi
Binomial name
Seychellum alluaudi (A. Milne-Edwards & Bouvier, 1893)
Synonyms [2]
Deckenia alluaudi A. Milne-Edwards & Bouvier, 1893 Deckenia cristata Rathbun, 1894

Seychellum alluaudi is a species of freshwater crab endemic to the Seychelles. It lives in rainforest streams on the granitic high islands, where it may be abundant. It grows to a carapace width of around 50 mm (2.0 in), and is almost quadrangular in outline. It was described in the East African genus Deckenia in 1893 and 1894, and separated off as the monotypic genus Seychellum in 1995. It is listed as Least Concern on the IUCN Red List, although little is known about its biology. Several hypotheses have been published to explain how the species reached the Seychelles.

Description

Seychellum alluaudi is the only truly freshwater crab in the Seychelles; all the other true crabs in the islands have marine larvae.^[3] Adult specimens reach a carapace width of 30–52 millimetres (1.2–2.0 in),^[4] and a carapace length about 80% of the width.^[5] The carapace is almost quadrangular and quite flat, in constract to the more rounded outline in Deckenia.^[4] Its surface is rough, with scattered tubercles, and is divided into distinct regions by a series of grooves.^[4] Whereas Deckenia has distinctly flattened walking legs, those of Seychellum are normal.^[4]

Taxonomic history

External identifiers for Seychellum
EOL	3049668
NCBI	168678
WoRMS	213018

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.^[6] 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.^[5] At the time, the only other species in the genus was D. imitatrix from the coast of East Africa, and a second species, also from East Africa, was added in 1898.^[2] Rathbun synonymised D. cristata and D. alluaudi herself in 1906.^[4] 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).^[4]

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.^[4] 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.^[4][7] That taxon is now considered a subfamily of the larger family Potamonautidae, and Seychellum is again considered a part of it.^[2]

Conservation status

Of the five criteria assessed by the International Union for Conservation of Nature for its IUCN Red List, four cannot be assessed in Seychellum alluaudi because of a lack of information.^[1] Under the remaining criterion, which measures the species' geographic range, the species would qualify as Endangered if there were was a "decline in habitat quality".^[1] It is, however, abundant in places, and is present within a protected area, and is therefore listed as "Least Concern".^[1]

Distribution and biogeography

Praslin, the second largest island in the Seychelles, is one of the islands where Seychellum lives.

Seychellum lives in streams among montane rainforest.

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 Deckenia from the African mainland, which together form the subtribe Deckeniina.^[8] The bilobed form 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.^[8]

The presence of a strictly freshwater species on the Seychelles is hard to explain, biogeographically.^[8][9] 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 continental drift;^[3] Gondwana is believed to have split up 160 million years ago,^[8] with the Seychelles separating from the Indian Plate around 68 to 65 million years ago.^[9] An ancient origin would be supported by the absence of freshwater crabs from oceanic islands in the same area, such as Mauritius and Réunion.^[9]
• Finally, the ancestor of Seychellum may have travelled across the western Indian Ocean by rafting, perhaps at a time when the 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.^[10] The time of divergence between Seychellum and Deckenia has been estimated independently at 27 to 8 million years ago^[11] and 12 to 5.5 million years ago.^[10] 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.^[12]

The most recent research favours trans-oceanic dispersal, but some uncertainty remains.^[10]

References

1. ^ ^{a b c d e f} Neil Cumberlidge (2008). "Seychellum alluaudi". IUCN Red List of Threatened Species. Version 2011.1. International Union for Conservation of Nature. http://www.iucnredlist.org/apps/redlist/details/134959. Retrieved September 27, 2011. 
2. ^ ^{a b c} P. K. L. Ng, D. Guinot & P. J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world" (PDF). Raffles Bulletin of Zoology 17: 1–286. http://rmbr.nus.edu.sg/rbz/biblio/s17/s17rbz.pdf. 
3. ^ ^{a b c d} Janet Haig. "Land and freshwater crabs of the Seychelles and neighbouring islands". In David Ross Stoddart. Biogeography and Ecology of the Seychelles Islands. Springer. pp. 123. ISBN 978-90-6193-107-2. http://books.google.co.uk/books?id=hAu6qogRHloC&pg=PA123. 
4. ^ ^{a b c d e f g h} P. K. L. Ng, Z. Števčić & G. Pretzmann (1995). "A revision of the family Deckeniidae Ortmann, 1897 (Crustacea: Decapoda: Brachyura: Potamoidea), with description of a new genus (Gecarcinucidae: Gecarcinucoidea) from the Seychelles, Indian Ocean". Journal of Natural History 29 (3): 581–600. doi:10.1080/00222939500770201. 
5. ^ ^{a b} Mary J. Rathbun (1894). "Descriptions of two new species of crabs from the western Indian Ocean, presented to the National Museum by Dr. W. L. Abbott" (PDF). Proceedings of the United States National Museum 17 (979): 21–24. http://si-pddr.si.edu/dspace/bitstream/10088/13360/1/USNMP-17_979_1894.pdf. 
6. ^ A. Milne-Edwards & E. L. Bouvier (1893). "Sur une nouvelle espèce du genre Deckenia (Hilgendorf) recueilli par M. Alluaud aux îles Seychelles". Annales des Sciences Naturelles, Zoologie. 7th series 15: 325–336. 
7. ^ Saskia A. E. Marijnissen, Sven Lange & Neil Cumberlidge (2005). "Revised distribution of the African freshwater crab genus Deckenia Hilgendorf, 1868 (Brachyura, Potamoidea, Deckeniidae)" (PDF). Crustaceana 78 (7): 889–896. JSTOR 20107558. http://faculty.nmu.edu/ncumberl/PDFs/MARIJNISSENETAL-05_DECKENIA.PDF. 
8. ^ ^{a b c d} Neil Cumberlidge, Richard M. von Sternberg & Savel R. Daniels (2008). "A revision of the higher taxonomy of the Afrotropical freshwater crabs (Decapoda: Brachyura) with a discussion of their biogeography" (PDF). Biological Journal of the Linnean Society 93 (2): 399–413. doi:10.1111/j.1095-8312.2007.00929.x. http://webb.nmu.edu/Departments/Biology/SiteSections/Faculty/Bios/Pubpdfs/Cumberlidge_etal_2008.pdf. 
9. ^ ^{a b c} Savel R. Daniels, Neil Cumberlidge, Marcos Pérez-Losada, Saskia A. E. Marijnissen & Keith A. Crandall (2006). "Evolution of Afrotropical freshwater crab lineages obscured by morphological convergence" (PDF). Molecular Phylogenetics and Evolution 40 (1): 227–235. doi:10.1016/j.ympev.2006.02.022. PMID 16621611. http://decapoda.nhm.org/pdfs/10438/10438.pdf. 
10. ^ ^{a b c} Savel R. Daniels (2011). "Reconstructing the colonisation and diversification history of the endemic freshwater crab (Seychellum alluaudi) in the granitic and volcanic Seychelles Archipelago". Molecular Phylogenetics and Evolution 61 (2): 534–542. doi:10.1016/j.ympev.2011.07.015. PMID 21824522. 
11. ^ Sebastian Klaus, Christoph D. Schubart, Bruno Streit & Markus Pfenninger (2010). "When Indian crabs were not yet Asian – biogeographic evidence for Eocene proximity of India and Southeast Asia". BMC Evolutionary Biology 10: 287. doi:10.1186/1471-2148-10-287. PMC 2949875. PMID 20849594. http://www.biomedcentral.com/1471-2148/10/287. 
12. ^ Sebastian Klaus, Christoph D. Schubart & Dirk Brandis (2006). "Phylogeny, biogeography and a new taxonomy for the Gecarcinucoida Rathbun, 1904 (Decapoda: Brachyura)" (PDF). Organisms, Diversity & Evolution 6 (3): 199–217. doi:10.1016/j.ode.2005.09.006. http://decapoda.nhm.org/pdfs/31279/31279.pdf. 

External links

• Andreas Karge (May–June 2008). "Seychellen 2008" (in German). Süßwassergarnelen – Freshwater shrimps. http://www.caridea.de/archiv/reisen/seychellen/seychellen.htm.  – includes a photograph of Seychellum