Sodium regulation in the freshwater/land crabHolthuisana transversa

Summary Holthuisana transversa maintains sodium balance at very low external concentrations (7–25 M). This is achieved by the high affinity of the uptake mechanism (K _m=0.18 mM), a low rate of sodium loss (0.17 mol g^–1h^–1) and a large increase in the rate of sodium uptake after depletion. The site of sodium absorption is the gills and extraction of sodium from respired water by sodium-depleted crabs can be as high as 28%. Crabs tolerate direct transfer to 80% sea-water and, following acclimation, the sodium and calcium levels of the haemolymph are maintained above those of the medium while the level of magnesium is significantly lower.