Adaptations to a Terrestrial Existence by the Robber Crab Birgus Latro: VI. The Role of the Excretory System in Fluid Balance

Primary urine is formed by filtration in the antennal organ of Birgus latro L. Urine isosmotic with the haemolymph is released into the anterior branchial chambers where substantial reabsorption of water and ions may occur. Some of the branchial fluid is ingested and the remainder (final excretory fluid, P) is released. Crabs supplied with fresh water have a low drinking rate (1.82 ml 100 g−1 day−1). Primary urine is partially reabsorbed (27%) in the antennal organ and urine flow (4.48 ml 100 g−1 day−1) is significantly lower than filtration rate (5.77 ml 100 g−1 day−1). The volume of P released is small in crabs drinking fresh water (0.45 ml 100 g−1 day−1) and the fluid is dilute (≈25 mmol l−1 NaCl). The difference between P flow and drinking rate (1.37 ml 100 g−1 day−1) represents evaporative and faecal water losses. Provision of saline drinking water (300, 600 or 1000 mosmol kg−1 sea water) doubles rates of drinking, filtration and urine flow and increases P flow fourfold. Evaporative/faecal water loss remains constant. Reabsorption of salts from the P rapidly decreases when saline media are provided for drinking.