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

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Discussion

The origins of species of freshwater crabs found on continental and oceanic islands elsewhere in the world, such as the Neotropics (Rodriguez and López, 2003) and southeast Asia (Brandis, 2002), are comparable to the findings presented here for the Afrotropical region. For example, one species of Trichodactylidae (Dilocarcinus dentatus (Randall, 1839)) from Trinidad and four species of Pseudothelphusidae are present on continental islands in three areas of Central and South America (Rodriguez and López, 2003). The disjunct distributions of widespread species such as Eudaniela garmani (Rathbun, 1898) on Trinidad and Margarita Island as well as the Venezuelan mainland have been attributed to lower sea levels during the Pleistocene that united these islands with the coast. Similarly, the presence of the endemic species Microthelphusa odaelkae (Bott, 1970c) on Trinidad, and the endemic Potamocarcinus roatanensis on Roatan Island off the coast of Honduras have been attributed to prior land access during lowered sea levels in the Tertiary followed by isolation when sea levels returned to higher levels.

Six endemic species of pseudothelphusids are found on oceanic islands in the Caribbean (five species of Epilobocera on Cuba, Hispaniola, Puerto Rico, and La Croix) (Rodriguez and Williams, 1995), and Guinotia dentata (Latreille, 1828) in the Lesser Antilles islands of Guadeloupe, Dominica, Martinique, St. Lucia and St. Vincent (Rodriguez and López, 2003). Rodriguez (1986) viewed the presence of crabs on these oceanic islands to be the result of a primitive Gondwanan distribution made disjunct by Mid-Cretaceous vicariant events associated with continental break-up. Finally, the presence of the endemic Hypolobocera gorgonensis von Prahl (1983) on Gorgona island off the Pacific coast of Colombia has been attributed to accidental dispersal through natural rafts because the island is separated from the mainland by a channel that is 200 to 1,000 m deep, which precludes a connection during minimal sea levels in the Quaternary (Rodriguez and López, 2003).

In southeast Asia the distribution patterns of species of Potamidae found on major continental islands of the Sunda shelf (e.g., Sumatra, Java, Borneo, the Philippines, Okinawa, Hainan) are thought to reflect the palaeogeographic and palaeoclimatic conditions of this region during the Tertiary and Quaternary at times when low sea levels created land bridges between islands (Brandis, 2002). Similar conclusions were reached to explain the distribution patterns of species of potamid freshwater crabs found on islands in the Mediterranean (Brandis et al., 2000).

In conclusion, species of freshwater crabs found on islands in the Afrotropical region (and elsewhere in the world) are either not unique, or are endemic at either the species or genus level. This in turn depends on the island’s geological history: whether it is part of the continental shelf, an oceanic island of volcanic origin, or a former part of the ancient continent of Gondwana.