Mechanisms of homeostasis during long-term diving and anoxia in turtles

A major challenge for ectothermic vertebrates in high latitudes is survivin g winters under sub-freezing conditions. Freshwater turtles seek underwater sites that enable them to avoid freezing of body fluids, but this strategy subjects them to Long periods of breath-holding. In hypoxic or anoxic envi ronments, species such as the painted turtle (Chrysemys picta) can survive long periods of anaerobic metabolism by a combination of metabolic depressi on and effective acid buffering. The large mineral reserve of the turtle's shell and bone provides much of the buffering power. This species is often found buried in anoxic mud or in ponds that may become anoxic. In aerated w ater, species such as the map turtle (Graptemys geographical and the softsh ell turtle (Apalone spinifera) can satisfy all their metabolic oxygen requi rements via uptake from water. These species normally select hibernating si tes with dependably high aquatic PO,. The painted turtle can supply all or most of its oxygen needs from aquatic respiration after several weeks of su bmergences, but it is not certain whether this is accomplished by metabolic depression or by increased oxygen uptake.