Water Balance and Urine Production in the Australian Arid-zone Crab Holthuisana

The permeability to tritiated water (hourly exchange fraction = 0-164) and the rate of urine flow (0-47% body weight/day) are lower than recorded in other freshwater decapods. The calculated net osmotic water flux (2-71 % body weight/day) is 5 times the water output in the urine, indicating that there is extrarenal excretion of water. Water balance is maintained in the absence of urine production, again indicating an extrarenal excretory site. Water turnover is greater for crabs in burrows than for crabs kept in air of 98% R.H., indicating relatively favourable conditions of moisture availability at the base of the burrow.