OSMOTIC TOLERANCE AND REGULATION IN CRABS FROM A HYPERSALINE LAGOON

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Abstract
Populations of the crabs Hemigrapsus oregonensis and Pachygrapsus crassipes were found inhabiting a lagoon which had been isolated from the open sea by a sand bar for months and had attained a hypersaline condition. Blood and urine from these crabs were sampled and analyzed periodically throughout the following months during which time the salinities rose to 190% of normal. Contrary to previous findings on short-term experiments, Hemigrapsus was found capable of hypo-osmotic regulation in salinities as high as 175% sea water. Pachygrapsus was regulating in salinities higher than 185% sea water and survived Hemigrapsus in 190% sea water. Although Pachygrapsus was shown to be a stronger osmotic regulator than Hemigrapsus, it is a weaker regulator of Na. Pachygrapsus, on the other hand, is a stronger regulator of Mg than Hemigrapsus, but the antennary glands of these two species concentrate Mg about equally in the urine. It is suggested that greater water permeability in Hemigrapsus precludes excretion of sufficient volumes of urine for strong Mg regulation. The possible role of coastal lagoons in the evolution of land crabs is discussed.