Osmoregulation of the Nile crocodile, Crocodylus niloticus, in Lake St. Lucia, Kwazulu/Natal, South Africa

Nile crocodiles of three age classes, hatched in captivity and reared in fr esh water, when exposed acutely to water of 17 and 35 ppt NaCl, suffered ma rked dehydration, were lethargic, ceased to feed and lost mass. When expose d to gradually increasing salinities (3-35 ppt), with a short acclimation p eriod at each salinity, crocodiles survived, continued to feed and increase d in mass and size. All age classes had a relatively constant plasma osmola lity across the salinity spectrum. Cloacal urine osmolality varied througho ut the acclimation experiment, but did not increase with increasing salinit y. No significant increase was found in plasma concentrations of any of the osmolytes. There was a trend of decreasing cloacal urine [Na+] and [Cl-] a nd increasing cloacal urine [K+] with increased salinity, indicating that u rine was not an important route for Na+ and Cl- excretion. Crocodiles expos ed to saline conditions maintained relatively constant plasma uric acid con centrations, but urinary uric acid concentrations increased markedly with i ncreasing salinities. This suggests that uric acid is the main constituent of nitrogenous waste excretion in saline exposed Nile crocodiles. As in Cro codylus porosus, C.niloticus has the physiological ability to survive and t hrive in periodically hyper-osmotic environments. However, its euryhalinity is restricted, in that acute exposure to sea water leads to dehydration, b ut with an acclimation period at lower salinities, it survives and thrives in sea water. (C) 2000 Elsevier Science Inc. All rights reserved.