Osmoregulation in the diamondback terrapin, Malaclemys terrapin centrata.

Survival of vertebrates in hypertonic environments such as the sea is opposed by diffusional loss of water and gain of sodium, and is aided by physical barriers to such diffusion along with the ability to transport ions actively. The energy expenditure for active transport obviously must be related to the efficiency of the physical barriers that impede diffusion. The description of these processes in the primarily aquatic fish has received much more attention than in secondarily aquatic tetrapods. In marine fish homeostasis is achieved by active extrusion of sodium through extrarenal channels, and Schmidt-Nielsen & Fange (1958) have shown that in some marine turtles an orbital gland ('salt gland') can fulfil this function by excreting solutions hypertonic to sea water. Circumstantial evidence indicates that electrolyte excretion through this gland in the marine turtle Chelonia mydas is far greater than from the kidney (Holmes & McBean, 1964). However, the precise roles of the outer integument, the gut and the kidneys in osmotic regulation of reptiles living in a marine environment have not been reported. The present work describes the water, salt, and energy metabolism of an estuarine turtle both in fresh water and in marine situations. The diamondback terrapin, Malaclemys centrata, lives in estuaries but has not been found in areas beyond tidal influence. Thus it must contend with a variety of osmotic conditions, from sea water to almost completely fresh water, a situation which contrasts with that of most other aquatic turtles, which are either exclusively marine or fresh water in habitat.