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The effects of seasonal cold exposure on the metabolism and behaviour of juvenile green sea turtles (Chelonia mydas) Southwood, Amanda Leah

Abstract

Field and laboratory studies were conducted to determine the metabolic response to seasonal cold exposure in juvenile green sea turtles (Chelonia mydas). Field studies were conducted with turtles at Heron Island, Australia during the summer and winter. Field metabolic rate (FMR) of turtles was determined using the doubly labeled water technique, and diving patterns were recorded using data loggers. Laboratory experiments were conducted with captive-reared juvenile green turtles exposed to a thermal regime which mirnicked seasonal temperature changes at subtropical latitudes. Oxygen consumption (V0₂), breathing frequency (/B), heart rate (/H), and blood flow distribution were determined at 26°C and after 4 - 16 weeks exposure to 17°C. Temperature effects on maximal activity of oxidative and glycolytic enzymes were determined in skeletal muscle tissue obtained from turtles at Heron Island and from captive turtles. Turtles at Heron Island showed seasonal differences in diving patterns, but FMRs during the summer (85.0 kJ-kg⁻¹-day⁻¹) and winter (66.5 kJ-kg⁻¹-day⁻¹) were not significantly different. Likewise, V02 of captive turtles was not significantly different during exposure to 26°C (0.44 ml-min⁻¹-kg⁻¹) and 17°C (0.34 ml-mm⁻¹-kg⁻¹). Q10 for metabolic rate of turtles at Heron Island (1.66) and in captivity (1.33) was lower than Qf 0 observed in acute studies of temperature effects on metabolism of green turtles (2.1-2.7). There was no significant difference in /B during exposure to 17°C and 26°C, however / H was significantly lower at 17°C than at 26°C. The mismatch between V02 and / H at low temperatures may be offset by adjustments in stroke volume or blood oxygen carrying capacity to ensure that O₂ supply meets demand. Regional blood flow distribution was not significantly different at 17°C and 26°C. Activity of oxidative and glycolytic enzymes showed a low thermal dependence. Compensation in oxidative enzyme activity did not occur. However, a compensatory increase in glycolytic enzyme activity was observed for captive turtles during exposure to 17°C. Results suggest that a combination of low thermal dependence and thermal acclimation of metabolic and physiological variables allow juvenile green turtles to remain active over the range of temperatures experienced seasonally at tropical and subtropical latitudes.

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