Crabs supplied with deionized water (DIW) and food maintained constant haemolymph osmolality, [Na] and [Cl]. Drinking of 300, 600 or 1000 mosmol kg-1 sea water (300SW, 600SW, 1000SW) elevated Na and Cl concentrations, which restabilized by 12 days on the 300SW and 600SW regimens but continued to rise on the 1000SW regimen. [Ca] increased slightly on all regimens. [K] changed only on the 1000SW regimen. Haemolymph [Mg] was poorly regulated on all saline regimens, doubling after 47 days on the 1000SW regimen. Urine was always isosmotic to haemolymph, had twice the [K] and a lower [Ca]. Changes in urinary ion concentrations paralleled those in the haemolymph, indicating that the antennal organs are unimportant in ionic regulation. Changes in ionic composition of the branchially modified urine (P) reflected drinking water concentrations on a mole-for-mole basis (except for [Mg]), confirming the regulatory role of P. Water and ion budgets indicate reingestion of 70–90 % of filtered urine. Branchial ion uptake and reingestion allow variable reclamation of 70–99.9 % of filtered Na, Ca, Mg and Cl and 37–96 % of K. Crabs drank DIW daily (mean rate, 16.2 g kg-1 day-1) and released P intermittently (intervals ranged from less than 1 to more than 6 days; mean 5.8 g kg-1 day-1). Provision of 300SW and 600SW doubled drinking rate and trebled P output. Intense initial drinking of 1000SW (58.1 g kg-1 day-1) was not maintained. Return to DIW after 1000SW stimulated very high drinking rates initially (119 g kg-1 day-1). Birgus can vary P composition rapidly and widely (less than 100 mosmol kg-1 and 20 mmol l-1 [NaCl] to more than 1000 mosmol kg-1 and 500 mmol l-1 [NaCl] within 1 day) and sometimes produce P that is hyperosmotic to the haemolymph. Production of P hyperosmotic to the drinking water and increased drinking rate permit a gain of osmotically free water to balance evaporation when drinking 300SW and 600SW, but not 1000SW. Adjustments to the volume and composition of the P appear to be more important than ‘behavioural osmoregulation’.

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