At 6.4 meters, this is one of the largest saltwater, or estuarine, crocodiles ever measured -- a 27-foot (8.2m) individual was reported killed in the Philippines in 1823, and a 10m one in 1840, but measurements of their skulls by Guinness World Records suggest that these animals were closer to the 6-6.5 meter range. At 8 meters or 6, it's indisputably the world's largest living reptile. The saltwater croc is native to waters stretching from Australia to India, with individuals reported having traveled as far away as the Sea of Japan. They are capable of swimming remarkable distances across open sea.
The saltwater crocodile is an ambush predator in estuarine and freshwater environments, and can travel far up river systems during warm, wet periods of the year; it's been spotted as far inland as Brisbane, Australia. Most tetrapods (a group including birds, mammals, reptiles and amphibians) can't tolerate concentrations of salt inside their body as high as the concentration of salt found in seawater, which means that they need some way to get rid of the extra salt in order to survive in a marine environment. Mammals living in seawater, such as seals or even the massive whales, would die of excess salt intake if they tried to drink the water they swim in. In order to cope, they have a suite of adaptations that resemble the strategies used by animals in the desert: super-efficient kidneys that lose as little water as possible to waste, and the ability to efficiently extract every bit of precious moisture from their food3. The saltwater crocodile doesn't have super-efficient kidneys, nor does it bar the doors against brackish or salty water. Instead, it has salt glands on its tongue, which actively transport sodium and chlorine ions out of the crocodile's blood and secrete a concentrated salt solution2. Sea turtles have very similar tongue glands, which help them to survive in the open sea.
The saltwater crocodile, which goes from entirely freshwater environments (juveniles live exclusively in freshwater rivers and marshes, and adults often move inland to hunt) to extended journeys over the open ocean, can adjust the output of its salt glands to match its needs. The sodium/potassium pump, a transport protein that serves a vital ion-transport role across the animal kingdom, is a primary component of the croc's salt gland. A team of scientists examined the number of sodium/potassium pumps present in saltwater crocodiles that had been acclimated to fresh water for months versus crocodiles that were used to brackish (70% saltwater) environments, and discovered that the crocodiles acclimated to a saltier environment had twice as many as the freshwater crocs1. The saltwater crocodile's remarkable ability to thrive anywhere from Australian rivers to the open ocean may come down to its quick-change tongue glands.
1. Cramp, Rebecca L., Hudson, Nicholas J., and Franklin, Craig E (2010) Activity, abundance, distribution and expression of Na+/K+-ATPase in the salt glands of Crocodylus porosus following chronic saltwater acclimation. Journal of Experimental Biology 213(Pt 8), pp 1301-1308.
2. Grigg, Gordon and Gans, Carl (1993) Morphology And Physiology Of The Crocodylia. Fauna of Australia Vol 2A Amphibia and Reptilia, pp 326-336. Australian Government Publishing Service, Canberra.
3. O’Driscoll, K.J.; Staniels, L.K.; and Facey, D.E.. "Osmoregulation and Excretion". http://www.cartage.org.lb/en/themes/sciences/Zoology/AnimalPhysiology/Osmoregulation/Osmoregulation.htm. Retrieved 9/11/2011.