Homeostasis of plasma electrolytes, water and sodium pools in the Estuarine Crocodile, Crocodylus porosus, from fresh, saline and hypersaline waters.

1. Total body water and exchangeable Na pools have been measured in hatchling and juvenile Estuarine Crocodiles captured from a wide range of salinity (0-64‰). Plasma electrolyte concentrations are presented for hatchlings over the same range of salinity. 2. Plasma electrolyte concentrations in hatchlings are constant across the entire salinity range studied. Hatchlings and juveniles, up to 5 kg body weight, maintain constant weight-corrected total body water pools but show a decline in exchangeable sodium pools with increasing salinity, suggesting a shift in the distribution of electrolytes or water between extra- and intra-cellular fluid compartments. 3. Both water and Na pools scale allometrically with body weight (allometric coefficients of 0.984 and 0.944 respectively). Expression of weight-specific pool sizes in units of ml/100 g or mmol/kg is, therefore, potentially misleading. Demonstration of homeostasis with respect to pool size depends upon the expression of pool size in units of ml or mmol per unit length and upon detailed consideration of weight/length and volume/length scaling relationships. The implications of these findings for future studies of the ecology of C. porosus in saline habitats are discussed.