The grapsid crab Sesarma curacaoense is believed to represent the closest relative to the ancestor which gave rise to an adaptive radiation of endemic freshwater and terrestrial species on the island of Jamaica. Living in mangrove swamps with variable salinity conditions and showing semiterrestrial behaviour, S. curacaoense exhibits in its ecology conspicuous adaptations to non-marine conditions. In laboratory experiments, we studied the salinity tolerance during development from hatching to the end of the first juvenile stage. Successful development through metamorphosis occurred in the full salinity range tested (15-32), although mortality and duration of development were significantly enhanced in brackish water (15). Attempting to understand the physiological basis of this high tolerance of osmotic stress, we measured the osmoregulatory capacity in all larval stages, the first juvenile, and in adult crabs. Our results show that S. curacaoense is, already from hatching, a fairly strong hyper-osmoregulator in dilute media. This capacity increased gradually from hatching throughout larval and juvenile development. In seawater and at an enhanced salt concentration (44), the zoeal stages remained hyper-osmoconformers. The capacity of hyporegulation in concentrated media appeared first in the Megalopa stage and increased thereafter. Adult crabs were observed to be strong hyper-hypo-osmoregulators in a salinity range from at least 1 to 44. The unusually early appearance of strong osmoregulatory capabilities, particularly in dilute media, is interpreted as a physiological preadaptation that should have facilitated the evolutionary process of adaptive radiation in non-marine environments on Jamaica.