Urine production rate and water balance in the terrestrial crabs Gecarcinus lateralis and Cardisoma guanhumi.

1. The rate of urine production in the terrestrial crabs Gecarcinus lateralis (Freminville) and Cardisoma guanhumi Latreille maintained on s.w.-moist sand was measured by inulin clearance. 2. The inulin U/B ratios in both species rose to about unity after 48 h, and remained at this level. It was concluded that neither species withdrew water from the primary urine. 3. The relatively high rate of inulin clearance in G. lateralis (10-69% body wt day-1) was considerably reduced when animals were transferred to dry sand (2-15% body wt day-1). 4. The rate of water loss on dry sand was measured in G. lateralis and changes in the haemolymph and urine osmolality recorded. 5. The prevention of possible reabsorption of water from voided urine did not appear to affect significantly the rate of water loss in conditions of water deprivation. 6. The maintenance of urine production rates measured on s.w.-moist sand would double the water loss of G. lateralis in dry conditions.