Abstract
The rates of body water loss and the maximum tolerable losses in the three Indo‐Pacific species of land crab (Gecarcoidea lalandii, Cardisoma carnifex and Birgus latro) were similar to those of other terrestrial decapod crustaceans of marine origin. During dehydration, the haemolymph osmolalities and Na and Cl concentrations increased in all species; and in Gecarcoidea and Cardisoma, corresponding increase in urine concentrations were found. Thus the U/B ratios of all three parameters did not change significantly from those of hydrated animals. As in other crabs, a urine hyperosmotic to the haemolymph was not produced during desiccation.
Clearance of the filtration marker inulin from the haemolymph ceased in these two species, so reduced urine production is suggested to be an important mechanism in limiting water loss. No post‐filtration withdrawal of fluid from the antennal gland, which would be indicated by an increase in inulin U/B ration to in excess of unity, appeared to take place under these conditions.
During desiccation the extracellular solute volume in Gecarcoidea and Cardisoma remained relatively constant, in contrast to Birgus, where a significant decrease in volume, equivalent the volume of body water lost, occurred. This suggests that in Birgus the intracellular volume is conserved. The consequence of this in Birgus the intracellular volume decrease on the measurement of clearance rates is discussed. It is suggested, from indirect evidence, that in Birgus the antennal gland continued to produce urine during desiccation.
The increases in haemolymph solute concentrations, while the extracellular spaces remain unchanged, requires a net movement of solutes into the haemolymph in Gecarcoidea and Cardisoma. In Birgus the increase in haemolymph concentration is less than would be expected if it was inversely proportional to the decrease in extracellular volume, implying some removal of solutes from this compartment.