Transoceanic dispersal by freshwater crabs
It would seem that oceanic dispersal, rather than Gondwanan break-up and a simple vicariant history, produced present-day freshwater crab distributions on the three oceanic ‘Gondwanan’ islands in the Western Indian Ocean, and on the two volcanic islands in the Atlantic considered here. Island colonization preceded by accidental transoceanic dispersal of freshwater crabs to oceanic islands requires that these strictly freshwater decapods survive both periods of submergence in sea water and exposure out of water for two or more weeks. The fact that freshwater crabs are salt-intolerant and are never found naturally in saltwater habitats argues for their accidental displacement perhaps during storms violent enough to dislodge large masses of vegetation that would be carried down rivers and out to sea. Transoceanic dispersal would also require favorable ocean currents, and successful colonization on arrival would require the ability to establish viable populations in the freshwater ecosystems of the new island habitat.
A number of authors have offered their opinions on the physiological resistance of freshwater crabs to high salinity, and have concluded that in general gecarcinucoids are salt tolerant but that potamoids are not (Bott, 1969b, 1970b; Haig, 1984; Klauss et al., 2006; Yeo et al., 2008). However, there have been only a few actual studies on the physiological response of freshwater crabs to salt water and their survival times in different salinities, and those that are available contradict the above opinions. For example, experiments on species of Potamoidea from Africa and Europe demonstrated that freshwater crabs do indeed have the ability to osmoregulate in brackish and full strength sea water, and that they can survive for between one to two weeks (Shaw, 1958a, 1958b; Morris and Van Aardt, 1998). The recent comparative study of groups of potamids and parathelphusids from Thailand exposed to sea water for different periods of time found no major differences between these families in either survival time or osmoregulatory ability (Esser, 2007; Esser and Cumberlidge, in prep.).
Esser (2007) and Esser and Cumberlidge (in prep.) found that potamid and parathelphusid freshwater crabs are indeed capable of short-term survival for at least two weeks when exposed to sea water, which would be long enough to survive a short oceanic journey clinging to drifting vegetation. Indeed, many species of African freshwater crabs are semi-terrestrial, have well developed air-breathing capabilities, are resistant to desiccation, and do not require permanent submergence in fresh water (Cumberlidge, 1986, 1991, 1999). This is especially true for the majority of Deckeniinae that includes Deckenia, Seychellum, Afrithelphusa, and Globonautes plus the Malagasy genera, all of which have pseudolungs (Cumberlidge and Sternberg, 2002; Cumberlidge et al., 2008). If adequately protected against desiccation, these animals are at least potentially capable of surviving for the relatively long periods away from fresh water involved in crossing distances of up to 500 km (Cumberlidge, 1986, 1991, 1999). A single ovigerous female freshwater crab landing on an island with freshwater ecosystems available would potentially be able to ensure a viable number of males and female offspring needed for successful colonization. This is because she typically carries between 200 and 500 fertilized eggs in her abdominal brood pouch, and because each egg eventually releases a hatchling crab without producing any of the free-living larval stages seen in marine crabs.
Transoceanic dispersal by rafting should be viewed as a rare and accidental occurrence in animals that avoid prolonged contact with sea water, despite the physiological ability of freshwater crabs to be able to withstand periods spent drifting on the open ocean clinging to vegetation, and then to be able to successfully colonize the new island habitat. Overseas dispersal by rafting has also been has been proposed to explain the origins of a range of vertebrates (amphibians, reptiles, and mammals) currently found on the Seychelles and Madagascar (Raxworthy, 2002; Vences et al., 2003, 2004; Yoder et al., 2003; Poux et al., 2005; Yoder and Nowak, 2006).