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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Comparative shell buffering properties correlate with anoxia tolerance in freshwater turtles

Department of Molecular Pharmacology, Physiology, and Biotechnology, Brown University, Providence, Rhode Island

Submitted 20 July 2006 ; accepted in final form 21 September 2006

Freshwater turtles as a group are more resistant to anoxia than other vertebrates, but some species, such as painted turtles, for reasons not fully understood, can remain anoxic at winter temperatures far longer than others. Because buffering of lactic acid by the shell of the painted turtle is crucial to its long-term anoxic survival, we have tested the hypothesis that previously described differences in anoxia tolerance of five species of North American freshwater turtles may be explained at least in part by differences in their shell composition and buffering capacity. All species tested have large mineralized shells. Shell comparisons included 1) total shell CO₂ concentration, 2) volume of titrated acid required to hold incubating shell powder at pH 7.0 for 3 h (an indication of buffer release from shell), and 3) lactate concentration of shell samples incubated to equilibrium in a standard lactate solution. For each measurement, the more anoxia-tolerant species (painted turtle, Chrysemys picta; snapping turtle, Chelydra serpentina) had higher values than the less anoxia-tolerant species (musk turtle, Sternotherus odoratus; map turtle, Graptemys geographica; red-eared slider, Trachemys scripta). We suggest that greater concentrations of accessible CO₂ (as carbonate or bicarbonate) in the more tolerant species enable these species, when acidotic, to release more buffer into the extracellular fluid and to take up more lactic acid into their shells. We conclude that the interspecific differences in shell composition and buffering can contribute to, but cannot explain fully, the variations observed in anoxia tolerance among freshwater turtles.

calcium; bone buffering; bone carbonate; bone carbon dioxide; bone mineral; pH stat