Osmoregulation and the transition from marine to freshwater and terrestrial life: a comparative study of Jamaican crabs of the genus Sesarma

Osmotic regulation was studied in nine crabs of the genus Sesarma SAY, 1817 and in Metopaulias depressus (Brachyura: Grapsidae: Sesarminae) from Jamai ca. All but one of these species are endemic to this island, evolving remar kable life histories that are completely independent from the sea. Despite exclusively relying on fresh water in their natural habitats, endemic speci es could be acclimated to sea water. Hemolymph osmotic concentrations in fr esh water were found to be related to the evolutionary and ecological depen dence on this medium. Ail species showed strong hyperregulatory capacities in fresh water and dilute media. In sea water, the mangrove species Sesarma curacaoense hyporegulated while the non-marine species were isosmotic with the external medium. A model proposed by LITTLE (1989, 1990) to discern ev olutionary routes taken during the invasion of terrestrial habitats based o n osmoregulation, cannot be applied to our results and others obtained for brachyuran crabs. An alternative model is therefore proposed, in which the presence or absence of hyporegulatory capacities is the only possible crite rion to discriminate between terrestrial colonization via fresh water or th e marine supralittoral in crabs.