Salt Gland

The salt gland is an organ for excreting excess salts. It is found in elasmobranchs (sharks, rays, and skates), seabirds, and some reptiles. Sharks' glands are found in their rectum, birds' and reptiles' in or on the skull in the area of the eyes, nostrils or mouth.

Salt glands maintain salt balance and allow marine vertebrates to drink seawater. Active transport via sodium-potassium pump action moves salt from the blood into the gland, where it is excreted as a concentrated solution.

The aviary salt gland is coupled with a counter-current exchange mechanism that preserves high salt concentration in the blood plasma; most removed salt 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) reflects their having much less efficient kidneys than mammals. Unlike the skin of amphibians, that of reptiles and birds is impermeable to salt, preventing its release.

The evolution of a salt gland in early reptiles and birds allowed them to eat aquatic plants and animals with high salt concentrations. This evolutionary development does not account for the gland in elasmobranchs, suggesting convergent evolution.

Some theories suggest mammalian tear ducts and sweat glands may be evolutionarily related to salt glands. While human tears are high in potassium, most phylogeneticists disagree with the association.