Sodium balance in the green turtle,Chelonia mydas, in seawater and freshwater

Summary Sodium balance in the green turtle,Chelonia mydas, has been investigated in both seawater and freshwater. In seawater the unidirectional Na efflux is 131 M · 100 g^–1 · h^–1, over 90% of which is via the head region and less than 5% each is via the cloaca and the integument. After 17 days in freshwater the efflux of Na has declined by 97% and the majority is via the cloaca. The integumental efflux in freshwater is less than that in seawater indicating that a change in skin permeability, trans-skin electrical potential or pattern of blood flow has taken place. Although there are indications that Na is actively extracted from freshwater by the green turtle, this species faces a net loss of Na of the medium and the blood Na concentration falls significantly. When transferred from seawater to freshwater the turtle's orbital salt gland is turned off within 25 h after transfer. The salt gland does not become functional until 400 h after transfer from freshwater to seawater.