The Comparative Physiology of Diving in North American Freshwater Turtles. I. Submergence Tolerance, Gas Exchange, and Acid-Base Balance

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Abstract
The responses to prolonged diving in a representative of each of the four families of North American freshwater turtles (Chelydridae, Emydidae, Kinosternidae, Trionychidae) were investigated. For turtles submerged at 10 C in anoxic (N₂-equilibrated) and normoxic (aerated) water, we found: (1) Survival times in anoxia (days) were 2.6 (Trionyx), 5.2 (Sternotherus), 8.5 (Chelydra), and 17.0 (Chrysemys); the corresponding survival times in normoxia were >100, >100, 14.2, and 29.3. The differences between survival in anoxic and normoxic water were significant in all species except Chelydra. (2) The acid-base responses of Trionyx and Sternotherus were similar, apparently as a result of the efficient use of extrapulmonary gas exchange in these species. The lack of such efficient aquatic respiration was evident in both Chelydra and Chrysemys, whose acid-base responses were similar to each other but very different from those of the other species. (3) Buffer capacities ([HCO₃⁻]/pH) of the blood were -8.81 (Chelydra), -12.6 (Chrysemys), -14.0 (Trionyx), and -17.0 (Sternotherus). These values are well below those of diving mammals and birds. (4) Hematocrit increased in all noncatheterized turtles. After 100 days of normoxic submergence, the increase in Trionyx was from 26.0% to 34.9%; (5) Turtles of all four species died when the blood pH fell by about 1.0 pH unit, regardless of whether the acidosis was metabolic or combined respiratory and metabolic. Ecological considerations of these findings are discussed, particularly with regard to the assumed long-term submergence associated with hibernation in the northern portions of the ranges of these species. We suggest that the superior anaerobic capabilities of freshwater turtles may be the result of selection for long-term wintertime submergence rather than for warm-weather diving.

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