Contributions to Zoology, 77 (2) - 2008Neil Cumberlidge: Insular species of Afrotropical freshwater crabs (Crustacea: Decapoda: Brachyura: Potamonautidae and Potamidae) with special reference to Madagascar and the Seychelles
Oceanic ‘Gondwanan’ islands

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The Seychelles

The Seychelles are a group of more than a hundred granitic and coralline islands, cays, and atolls in the Indian Ocean that extend between 160 and 1,300 km north of Madagascar, 480 and 1,600 km east of Africa (Tanzania, Kenya, and Somalia), and some 3,000 km from India. The long isolation of the Seychelles (75 million years) and the great distance from the mainland has resulted in a highly endemic flora and fauna that includes Seychellum alluaudi a morphologically unique genus and species of freshwater crab (Ng et al., 1995). This species is found only on the four largest granitic Gondwanan islands (Mahé, Praslin, Silhouette and La Digue) (with a combined area of only 454 km2) that lie 1,600 km off the coast of Africa, and it is absent from the more recent but still isolated coralline Seychelles islands (Ng et al., 1995). The presence of the same species of freshwater crab on these four remote islands in the Seychelles is probably the result of their more recent isolation from each other by present day high sea levels. Until recently, the granitic Seychelles consisted of a much larger Gondwanan landmass (55,000 km2) known as the Seychelles Bank, when past sea levels were more than 100 m lower than at present. Today most of this land is submerged and only the terrestrial communities on the hilltops (where Seychellum is found) survived, presumably because the higher elevations remained above the surface when the oceans rose to their present levels.

How Seychellum came to be on the Seychelles in the first place has been the subject of disagreement over the years. Authors that discount the possibility of oceanic dispersal by freshwater crabs (Rodriguez, 1986; Ng et al., 1995; Ng and Rodriguez, 1995) have explained the presence of these decapods on the Seychelles by simple vicariance, arguing that freshwater crab ancestors already lived on this part of Gondwana before it fragmented. On the other hand, the morphological and molecular evidence that can now be brought to bear over the question of the origins of Seychellum favors accidental overseas dispersal over vicariance (Daniels et al., 2006; Cumberlidge et al., 2008). For example, the closest affinities of Seychellum are with ancestral East African Deckeniinae rather than with African Potamonautinae or Indian Gecarcinucidae (Daniels et al., 2006; Klaus et al., 2006; Cumberlidge et al., 2008). The phylogenetic relationships between Seychellian freshwater crabs and those from Madagascar, Africa, and India therefore do not fit the temporal framework of Gondwanan fragmentation in the Mesozoic whereby Madagascar-Seychelles-India split first from Africa, followed millions of years later by the break-up of the Seychelles and India. In addition, the divergence time estimates for the Afrotropical clade (78.6-75.03 Mya), the Seychellum/Deckenia clade (75 Mya), and the Malagasy clade (43.5-39 Mya) indicate Upper/Late Cretaceous or Eocene origins for these lineages (Daniels et al., 2006) that are not congruent with the dates for continental break-up. The fact that Seychellum and Deckenia (East Africa)today occur on opposite sides of a deep oceanic barrier argues for overseas dispersal rather than vicariance. The common ancestor of these two genera was presumably living on the African mainland (where Deckenia still occurs) (rather than Madagascar or India)and was carried to the Seychelles by accidental transoceanic dispersal. The distance between the coast of Africa and the granitic Seychelles today is about 1,600 km, but this would have been considerably shorter in the Upper/Late Cretaceous.