Comparison of vertebrate salt-excreting organs.

Many marine vertebrates must excrete NaCl against a substantial chemical gradient to remain hypoionic to the environment. Salt secretion appears to be mediated by "chloride cells" found in the gills of many fish or in salt glands in elasmobranchs, reptiles, and birds. The ultrastructure of the cells is similar in all animals possessing them, as is the composition of the fluid they produce, and a common mode of function is suggested. However, the extant flux-force data cannot be reconciled with a single mechanism. They appear to suggest that various combinations of Na+-, K+-, and Cl--transport systems can operate in different epithelia, and that epithelial permeabilities may also differ. However, a solution approximately seawater in monovalent ion concentrations is produced in every case.