Sodium and water budgets of the fasted estuarine crocodile,Crocodylus porosus, in sea water

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1. Quantitative sodium and water budgets have been constructed for the estuarine crocodile,Crocodylus porosus, unfed in sea water in the laboratory.C. porosus is homeostatic for exchangeable Na in sea water but is unable to achieve homeostasis of plasma Na concentration or total body water when fasted. 2. The lingual salt glands account for 55% of the total Na efflux compared with only 2% excreted by the renal/cloacal complex. The urinary system appears to be the principal route for excretion of K. 3. The integument is essentially impermeable to Na but permeable to water, as in other estuarine/marine reptiles. Much of the Na and water exchange across the skin appears to take place across cephalic epithelia, particularly the buccal mocosa. 4. The permeable buccal mucosa is costly in terms of direct water loss and indirect losses tied to excretion of excess Na. Lowering of buccal permeability may be limited by the necessity to retain some functional glandular tissue and sensory organs in the mouth. 5. There is no evidence to suggest thatC. porosus feeds selectively on invertebrate prey of low Na or high water content despite the apparent selective advantage of minimizing water losses associated with extra-renal Na excretion.