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Naturalists have often marveled at the ability of some air-breathing vertebrates to remain underwater for long periods of time (that is, "long" from man's reckoning). Seals, penguins, porpoises, and whales are all noted fro their ability to use oxygen stored in various ways to permit them to "stay under" for many minutes.

The return of vertebrates, which evolved on land, to an aquatic existence has been a much discussed subject from an evolutionary standpoint. That there are many advantages to life underwater is easily seen simply by the fact that, according to fossil records, terrestrial life arose from the sea. Water is such a necessary component in protoplasms that it took eons of evolutionary change to produce organisms capable of surviving outside of it. To the vertebrate amniote, though, having gained physiological independence from an aquatic existence, competition from other land-dwellers and the relatively stable and rich environments found underwater make a return to their primeval dwelling place a viable alternative.

However, after millions of years of adapting to life on land, many of the features "taken for granted" by aquatic creatures had been lost. These animals had developed legs and had lost fins. The streamlined shape necessary for aquatic mobility had in most cases been drastically modified. Most importantly, they had developed a dependence on atmospheric oxygen. The surfaces for gas exchange had been modified to facilitate this dependence, and the relative richness of the oxygen supply in the air allowed metabolic rates to be raised, even to the point of producing what we know as warm-blooded animals.

Thus, the animal attempting to readjust to an aquatic existence encounters many adaptive obstacles. However, many species of amniores (reptiles, birds, mammals) have made this return in varying degrees. Perhaps of these three classes, the reptiles have more aquatic or semi-aquatic species than either of the other two groups. Of the reptiles, the turtles (order Chelonia) are the most aquatic in habitats. Many are wholly aquatic, and most of the group are at least semi-aquatic.

It is the purpose of this paper to examine some of the recent work done of one phase of the turtles' "retrogressive evolution"--their adaptation to life underwater without gills.

Turtles breathe air by means of lungs, as do other amniotes. What permits turtles to stay underwater for hours at a time? How does a turtle survive buried in the mud at the bottom of a pond for the duration of the winter months? Even more intriguing is the ability of turtles injected with cyanide (which stops oxidative processes in the mitochondria) to survive for twenty hours.

Recent (and some not-so-recent) work has examined some of the adaptive mechanisms of some turtles and has illuminated many of the anatomical and physiological features which permit chelonians to be so well adapted to hypoxic and anoxic environments.



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