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

Abstract

Abstract. An unusual saltwater population of the "freshwater" crocodilian, Crocodylus johnstoni, was studied in the estuary of the Limmen Bight River in Australia's Northern Territory and compared with populations in permanently freshwater habitats. Crocodiles in the river were found across a large salinity gradient, from fresh water to a salinity of 24 mg.ml -1 , more than twice the body fluid concentration. Plasma osmolarity, concentrations of plasma Na + , Cl -, and K + , and exchangeable Na + pools were all remarkably constant across the salinity spectrum and were not substantially higher or more variable than those in crocodiles from permanently freshwater habitats. Body fluid volumes did not vary; condition factor and hydration status of crocodiles were not correlated with salinity and were not different from those of crocodiles from permanently fresh water. C. johnstoni clearly has considerable powers of osmoregulation in waters of low to medium salinity. Whether this osmoregulatory competence, extends to continuously hyperosmotic environments is not known, but distributional data suggest that C. johnstoni in hyperosmotic conditions may require periodic access to hypoosmotic water. The study demonstrates a physiological capacity for colonisation of at least some estuarine waters by this normally stenohaline freshwater crocodilian.

Active airflow of the paranasal sinuses in extinct crocodyliforms: Evidence from a natural cast of the thalattosuchianDakosaurus andiniensis

“…It could be expected that these forms were euryhaline. Some populations of teleosauroids and basal metriorhynchoid species inhabiting brackish, estuarine, or even littoral habitats, could have been able to tolerate limited exposure to sea water by selectively drinking only fresh water (like estuarine populations of Alligator mississippiensis ; Jackson, Butler, & Brooks, 1996; Taplin, Grigg, Beard, & Pulsford, 1999; Cramp et al, 2008) or by salt‐secreting glands of low secretory capacity combined with selective drinking (like Crocodylus porosus , C. acutus , and C. johnstoni ) (Taplin & Grigg, 1981; Taplin, Grigg, & Beard, 1983).…”

Osmoregulation by the broad-snouted caiman, Caiman latirostris , in estuarine habitat in southern Brazil

“…The Nile Crocodile C. niloticus, is not uncommon in saline waters and has functional salt glands even in FW (Taplin and Loveridge, 1988). Australia's so-called "freshwater crocodile" similarly has functional salt glands which play an important role in the maintenance of plasma homeostasis in waters of 0-24 ppt in estuarine habitats (Taplin et al 1993). …”