Salt gland

The salt gland is an organ for excreting excess salts. It is found in elasmobranchs, seabirds, and some reptiles. In sharks, salt glands are found in the rectum, but in birds and reptiles, they are found in or on the skull, in the area of the eyes, nostrils or mouth. In crocodiles, the salt is excreted through the tongue. Such glands work by active transport via sodium-potassium pump that moves salt from the blood into the gland, where it can be excreted as a concentrated solution. Salt glands function to maintain salt balance and allow marine vertebrates to drink seawater.

The aviary salt gland is coupled with a counter-current exchange mechanism that preserves the high concentration of salt in the blood plasma so that most of the salt leaving the gland is returned to the incoming blood plasma, allowing the concentration buildup within the gland by active transport.

The need for salt excretion in reptiles (such as marine iguanas and sea turtles) and birds (such as petrels and albatrosses) stems from the fact that their kidneys are much less efficient than those of mammals. Unlike the skin of amphibians, reptile and bird skin is impermeable to salt, meaning that the transition to a tougher skin meant a loss in salt-releasing ability. The evolution of a salt gland would have allowed early reptiles and birds to eat aquatic plants and animals, who have high salt concentrations. This does not, however, explain the evolution of the gland in the elasmobranchs, suggesting convergent evolution.

Some theories suggest that mammalian tear ducts and sweat glands may be evolutionarily related to salt glands. Human tears are high in potassium, lending support to this theory; however, most phylogeneticists disagree with this idea.