User:Nica88/Painted turtle

Anoxia tolerance
During the winter months, painted turtles become ice-locked and spend their time in either hypoxic (low oxygen) or anoxic (no oxygen) regions of the pond or lake. Painted turtles essentially hold their breath until the following spring when the ice melts. As a result, painted turtles rely on anaerobic respiration, which leads to the production of lactic acid. However, painted turtles can tolerate long periods of anoxia due to three factors: a depressed metabolic rate, large glycogen stores in the liver, and sequestering lactate in the shell and releasing carbonate buffers to the extracellular fluid.

The shell of an adult painted turtle has the largest concentration of carbonate content recorded among animals. This large carbonate content helps the painted turtle buffer the accumulation of lactic acid during anoxia. Both the shell and skeleton release calcium and magnesium carbonates to buffer extracellular lactic acid. A painted turtle can also sequester 44% of total body lactate in their shell. Despite the shell's large buffering contribution, it does not experience any significant decrease in mechanical properties under natural conditions.

The duration of anoxia tolerance varies depending on the sub-species of painted turtle. The western painted turtle (C. picta bellii) can survive 170 days of anoxia, followed by the midland painted turtle (C. picta marginata) which can survive 150 days, and finally the eastern painted turtle (C. picta picta) which can survive 125 days. Differences in anoxia tolerance are partially attributed to the rate of lactate production and buffering capability in painted turtles. Furthermore, northern populations of painted turtles have a higher anoxia tolerance than southern populations.

Other anoxia tolerant freshwater turtles include: the southern painted turtle (Chrysemys dorsalis), which can survive 75-86 days of anoxia, the snapping turtle (Chelydra serpentina), which can survive 100 days under anoxia, and the map turtle (Graptemys geographica), which can survive 50 days of anoxia. One reason for the difference in duration between more anoxia-tolerant species and less anoxia-tolerant species is the turtle's ability to buffer lactic acid accumulation during anoxia.

Unlike adult painted turtles, hatchlings can survive only 40 days, but still exhibit high anoxia tolerance and freeze tolerance compared to other hatchling species (30 days for Chelydra serpentina, and 15 days for Graptemys geographica) due to cold winters.