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Lactic Acid Buffering by Bone and Shell in Anoxic Softshell and Painted Turtles

We tested two hypotheses: first, that the inferior anoxia tolerance of the softshell turtle, Apalone spinifera, compared to the western painted turtle, Chrysemys picta bellii, is related to its less mineralized shell, and second, that turtle bone, like its shell, stores lactate during prolonged anoxia. Lactate concentrations of blood, hindlimb bone, and shell were measured on normoxic Apalone and Chrysemys and after anoxic submergence at 10°C for 2 and 9 d, respectively. Blood and shell concentrations of Ca2+, Mg2+, Na+, K+, and inorganic phosphate (Pi; for shell only) were also measured. Because a preliminary study indicated lactate distribution in Chrysemys throughout its skeleton during anoxia at 20°C, we used hindlimb bones as representative skeletal samples. Apalone shell, though a similar percentage of body mass as Chrysemys shell, had higher water content (76.9% vs. 27.9%) and only 20%–25% as much Ca2+, Mg2+, CO2, and Pi. When incubated at constant pH of 6.0 or 6.5, Apalone shell powder released only 25% as much buffer per gram wet weight as Chrysemys shell. In addition, plasma [Ca2+] and [Mg2+] increased less in Apalone during anoxia at an equivalent plasma lactate concentration. Lactate concentrations increased in the shell and skeletal bone in both species. Despite less mineralization, Apalone shell took up lactate comparably to Chrysemys. In conclusion, a weaker compensatory response to lactic acidosis in Apalone correlates with lower shell mineralization and buffer release and may partially account for the poorer anoxia tolerance of this species.