Plasma homeostasis and cloacal urine composition inCrocodylus porosus caught along a salinity gradient

Abstract

Summary. Juveniles of the Estuarine or Saltwater Crocodile, Crocodylus porosus, maintain both osmotic pressure and plasma electrolyte homeostasis along a salinity gradient from fresh water to the sea. In fresh water (FW) the cloacal urine is a clear solution rich in ammonium and bicarbonate and containing small amounts of white precipitated solids with high concentrations of calcium and magnesium. In salt water (SW) the cloacal urine has a much higher proportion of solids, cream rather than white in colour, which are the major route for excretion of potassium in addition to calcium and magnesium. Neither liquid nor solid fractions of the cloacal urine represent a major route for excretion of sodium chloride. The solids are urates and uric acid, and their production probably constitutes an important strategy for water conservation by C. porosus in SW. These data, coupled with natural history observations and the recent identification of lingual salt glands, contribute to the conclusion that C. porosus is able to live and breed in either fresh or salt water and may be as euryhaline as any reptile.

Osmoregulation of the Australian freshwater crocodile, Crocodylus johnstoni, in fresh and saline waters

“…The relevance of this distinction to consideration of salt and water balance in crocodilians (and other taxa) has been outlined by Grigg (1981) and Taplin (1984Taplin ( , 1988. The approximate boundaries between continuously and periodically hypo-and hyper-osmotic waters on the Limmen Bight mainstream at the times of our expeditions are shown in Fig.…”

“…A range of 10 mg.ml -1 between high and low tides would not be surprising based on experience in the Liverpool-Tomkinson River system (Grigg 1981). In the upstream reaches the salinity range is certain to be much narrower.…”

“…Relevant information on the climatic and salinity regimes of northern Australian river systems is provided in Messel et al (1979Messel et al ( -1986, Grigg et al (1980Grigg et al ( , 1986, Grigg (1981), andTaplin (1984). Background information specific to the Limmen Bight River System is provided by Messel et al (1980) and Taplin et al (1985).…”

“…Hatchlings and juveniles are able to maintain plasma homeostasis and grow in salinities of 0-45 mg.ml -1 at least (Grigg et al 1980;Grigg 1981;Taplin 1984Taplin , 1988. This capability derives in part from active lingual salt glands (Taplin and Grigg 1981) which account for a substantial fraction of Na + efflux in sea water (Taplin 1985;Grigg et al 1986).…”

Sea water drinking as a homeostatic response to dehydration in hatchling loggerhead turtles Caretta caretta

“…C. porosus is widespread in estuarine waters of northern Australia and is not uncommon on sea beaches and among outer islands of the northern Great Barrier Reef (personal observation). It is an excellent osmoregulator, capable of maintaining itself homeostatic in salinities of 0-60 + 0 / 00 at a size of only 100-200 g body weight (Grigg, 1981;Taplin, 1984b). This osmoregulatory ability depends on access to abundant food for small crocodiles exposed to highly saline waters.…”