Osmoregulation in Crocodilians

Abstract

Recent crocodilians live primarily in freshwater habitats. However two species ( Crocodylus acutus and C. porosus ) are estuarine specialists; two others ( C. niloticus and C. johnstoni ) that are primarily found in fresh water, have estuarine populations. Routes of uptake of water and sodium include drinking, feeding and associated incidental drinking, and integumental and buccal diffusion. Routes of loss include faeces-cloacal fluid, lingual salt glands, integumental and buccal diffusion, and respiratory loss. The least understood route of salt and water exchange is that of the oral and buccal epithelia, which are much more permeable to water and sodium than the general integument. The freshwater alligator ( Alligator mississippiensis ) osmoregulates in a manner typical for an amphibious reptile. Body sodium turnover is low and the general integument is quite low in permeability to sodium. Water turnover is more rapid (in terms of molar exchange) but still relatively low for an aquatic reptile. Most water exchange occurs across the integument and buccal epithelia. The presence of lingual salt glands in freshwater crocodilians remains enigmatic, as does the failure of these exocrine glands in estuarine species to respond to saline loading. Secretion does occur after injection of the parasympathetic stimulant methacholine chloride. The “salt water crocodile” ( C. porosus ) possesses a suite of osmoregulatory adaptations similar to those found in other estuarine reptiles. Water and sodium balance are maintained primarily by an extremely low general permeability to sodium, by economies in water loss, and by excretion of excess sodium by the lingual salt glands. Further work is needed to examine newly hatched C. porosus , and the possibility of ontogenetic change in lingual gland function in C. acutus. The importance of incidental drinking of sea water during feeding (recently discovered in turtles) needs to be evaluated in crocodilians. The use of osmoregulatory data in interpretation of the evolutionary history of the genus Crocodylus needs to be viewed with caution. The hypothesis that all species of Crocodylus originated from the transoceanic migration of a saline-tolerant form may not be the most parsimonious explanation.