Three semiterrestrial crab species of the genus Armases from Jamaica were shown to be strong osmotic regulators keeping internal media at very constant concentrations over all ambient salinities tested. Special attention was paid to Armases roberti (H. Milne Edwards, 1853),
a crab restricted to the Caribbean and exclusively found along the banks of freshwater rivers. All three species were able to cope with experimental ambient salinities ranging from 0 to 45‰. Osmotic capacities of the freshwater species Armases roberti were compared with those
from the marine supralittoral, A. miersii (Rathbun, 1897) and A. ricordi (H. Milne Edwards, 1853), and with those from other brachyuran crabs reported from brackish or fresh water. While the three species of Armases were shown to hyper- and hyporegulate, most of the other
freshwater crabs only regulate in dilute media becoming isosmotic in higher salinities. It is argued that the ability of Armases roberti to regulate in hypersaline media and the comparatively high osmotic hemolymph concentration when hyperregulating in fresh water (ca. 680 mOsm kg-1)
reflects a fairly recent invasion of the freshwater habitat from the marine supralittoral.
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