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
Continental islands

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Zanzibar (Unguja), Mafia, and Pemba

A single species of potamonautid, Potamonautes obesus, is found on the islands of Zanzibar, Mafia, and Pemba (Tanzania) and across a wide area of East Africa that includes the lowland coastal regions of Somalia, Kenya, Tanzania, and Mozambique (Reed and Cumberlidge, 2006b). Zanzibar (1,660 km2) and Mafia (394 km2) are nearshore continental shelf islands that lie 35 km and 48 km respectively from the coast of Tanzania. Today they are separated by shallow seas but in the recent past they were connected to the mainland of Africa (50,000 years ago and again 10,000 years ago) (Kingdon, 1989). It is likely that the ancestors of P. obesus may have reached Zanzibar and Mafia at a time when low sea levels permitted overland dispersal. Pemba (984 km2) the third of the Tanzanian spice islands, is located 56 km off the coast and is much older than the other Tanzanian islands, dating back to the Miocene; it is separated from the mainland continental shelf by an 800 m deep trench that is at least several million years old (Kingdon, 1989). The presence of P. obesus on Pemba (which would not have been connected to the mainland even in the Tertiary or Quaternary when sea levels were much lower than they are today) suggests possible transoceanic dispersal to this island perhaps by natural rafts.