Oceanic volcanic islands
While there is more than one explanation (overseas dispersal or vicariant origins) for how freshwater crabs came to be on oceanic ‘Gondwanan’ islands, there is little debate about the presence of freshwater crabs on oceanic volcanic islands: it is assumed that crabs reached there by overseas dispersal. However, it would seem that large stretches of saltwater present a formidable, but not entirely impenetrable, barrier to freshwater crab dispersal.
Príncipe and São Tomé
The volcanic islands of Príncipe (128 km2) and São Tomé (836 km2) (São Tomé and Príncipe) lie 250 km and 300 km from the mainland and both have forested mountains that reach 948 and 2,024 m asl respectively. Príncipe rose out of the sea about 31 Mya (Oligocene), while São Tomé dates back to about 14 Mya (Miocene). The presence of an endemic species of potamonautid freshwater crab on each of these islands (Potamonautes principe on Príncipe, and Potamonautes margaritarius on São Tomé) means that each species is the product of a separate dispersal event from the mainland, presumably by natural rafts. Colonization of each island separately is the most likely because there is no morphological evidence that these species are closely related to each other and therefore no reason to assume that there was any island-to-island transfer (the distance between the closest islands is 146 km). While both taxa are endemic at the species level, the genus is distributed widely not only in the nearby coastal countries but also throughout sub-Saharan Africa (Cumberlidge, 1999; Cumberlidge et al., 2002). Measey et al. (2007) have recently proposed that amphibians and other salt-intolerant animals found on oceanic islands in the Gulf of Guinea reached there by rafting aided by favorable surface currents and by surface waters of reduced salinity (rather than by storms, birds, or rafts alone). Those authors argued that low-salinity ‘freshwater paths’ in the surface layers of the ocean (formed either by catastrophic events or by wet periods in climatic history) lasted long enough to be capable of transporting salt-intolerant continental animals to isolated oceanic islands.