A Possible New Salt Gland in a Marine Homalopsid Snake (Cerberus rhynchops)

Abstract

The dog-faced water snake, Cerberus rhynchops, appaiently has a small premaxillary salt gland, which is not homologous with previously known lachrymal (turtles), nasal (lizards) or posterior sublingual (sea and file snakes) salt glands. Cerberus is a mangrove-dwelling snake that has adapted to a marine existence independently of the hydrophiids and the acrochordids. The premaxillary salt gland of Cerberus is small (approximately 0.005% body wt) and excretes sodium at about 15 ,umoles/100 g body wt. h. When salt adapted, the glandular fine structure of the principal cells is very similar to that of other ophidian salt glands. However in fresh water striking changes in ultrastructure take place, indicating that the gland is probably readapting to serve a different function. The salt gland in Cerberus probably functions only during periods of dehydration, as the plasma Na concentration rises above 150 mM. The skin of Cerberus is virtually impermeable to Na, and overall Na influx (all oral) is extremely low, about 10 ,umoles/100 g h. Almost all water exchange with sea water is through the skin, and net loss is relatively low. Cerberus can hydrate by drinking fresh water when it is available, and survive many months of sea water immersion before gradual dehydration causes the activation of the premaxillary salt gland. Even when fully active the gland can excrete only very limited amounts of electrolytes in comparison with some other marine reptiles. However the presence of even such a small source of extrarenal electrolyte excretion must be important to fasting snakes or those denied access to fresh or brackish drinking water for prolonged periods.