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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Madagascar(590,000 km2) today lies about 400 km from the east coast of southern Africa (Mozambique) and became separated from Africa 165-121 Myr ago and from the Seychelles and India 88-63 Myr ago (Rabinowitz et al., 1983; Storey et al., 1995). The long period of isolation of Madagascar’s original Gondwanan fauna together with additions by dispersal events over the years have produced a unique endemism on this island that includes an impressive number of species of freshwater crabs. The seven endemic genera and fourteen endemic species of Malagasy freshwater crabs are unevenly distributed on the island and are most diverse in Antsiranana Province in northern Madagascar, an area with both dry and moist forests that includes the Tsaratanana and Marojejy mountains (Ng and Takeda, 1994; Cumberlidge and Sternberg, 2002; Cumberlidge et al., 2004; Cumberlidge et al., 2005; Reed and Cumberlidge, 2006a; Daniels et al., 2006; Cumberlidge et al., 2008). The tropical rainforest on the mountainous eastern side of the island has a lower freshwater crab species diversity, while crabs are completely absent from vast areas the island on the western side (where the vegetation consists of dry deciduous forests) and in the far south (which is arid spiny desert) (Cumberlidge et al., 2004).

The fourteen species of Malagasy freshwater crabs are so striking in their morphological variety that they have been assigned to seven genera (Cumberlidge et al., 2000; Cumberlidge and Sternberg, 2002; Reed and Cumberlidge, 2006a; Cumberlidge et al., 2008). They are also sufficiently morphologically divergent from the freshwater crab fauna elsewhere in the Afrotropical region to have been assigned to two different families by some authors (Bott, 1960, 1965, 1969a, 1970a; Ng et al., 1995). These authors argued that the Malagasy freshwater crabs comprised two separate lineages, one group with supposedly African affinities that they placed in the Potamonautidae, and the other group with supposedly Indian and Seychellian (Gondwanan) affinities that they placed in the Gecarcinucidae. This arrangement meshed with the view that it was the fragmentation of the Gondwanan supercontinent that carried the freshwater crabs to their present position on Madagascar and not transoceanic rafting (Bott, 1960, 1965; Ng et al., 1995; Ng and Rodriguez, 1995). Vicariance hypotheses require that freshwater crabs found on Gondwanan continental fragments have phylogenetic relationships that are congruent with a shared history of continental break-up. However, this two-family hypothesis for the Malagasy freshwater crabs was not supported by the morphological study of Cumberlidge and Sternberg (2002), or by the molecular study of Daniels et al. (2006), which both concluded that all Malagasy taxa form a single monophyletic lineage that was part of the African Potamonautidae (Cumberlidge et al., 2008). Daniels et al. (2006) identified the sister group of the Malagasy freshwater crabs to be a clade consisting entirely of African genera that were included in the African Potamonautidae by Cumberlidge and Sternberg (2002) and Cumberlidge et al. (2008). The consensus phylogeny (based on molecular and morphological evidence) is not congruent with the affinities predicted by Gondwanan break-up, and supports African origins for the Malagasy freshwater crabs (Daniels et al., 2006; Cumberlidge et al., 2008). In addition, the molecular divergence estimates provided by Daniels et al. (2006) indicated that the Malagasy freshwater crabs diverged from their African relatives about 43.5-39 Mya, long after the separation of Madagascar from Africa. The available data indicate that the Malagasy freshwater crabs most likely descended from a transoceanic migrant from Africa during the Eocene (Palaeogene) when the distance between the coast of Africa and northwest Madagascar would have been considerably shorter than it is today (400 km).

Klaus et al. (2006) also proposed that freshwater crabs reached Madagascar and the Seychelles from Africa in the past by rafting across oceanic distances that were shorter than today, but they differed in their estimates of the timing of this event and in the way it may have happened. Klaus et al. (2006) argued that freshwater crabs reached these islands during the Oligocene 28 Mya when sea levels were 150-200 m lower than today and when significant portions of the continental shelves as well as the Mascarene Plateau, Seychelles Bank, and Chagos/Laccadive Plateau were emergent. Klaus et al. (2006) also suggested that freshwater crabs may have reached Madagascar and the Seychelles from Africa indirectly by rafting via intervening small emergent islands and seamounts that served as ‘stepping stones’. Those authors further suggested that freshwater crabs in the Seychelles reached India via the Chagos/Laccadive Plateau during the Oligocene but this is considered unlikely here because Daniels et al. (2006) found no close relationship between any Afrotropical genera (Potamonautidae) and Indian and southeast Asian genera (representing the Gecarcinucidae and Parathelphusidae).