Metabolic and cardiovascular adjustments of juvenile green turtles to seasonal changes in temperature and photoperiod

We measured activity levels, oxygen consumption, metabolic enzyme activity, breathing frequency, heart rate and blood chemistry variables of juvenile green turtles exposed to a laboratory simulation of subtropical winter and summer temperatures (17-26°C) and photoperiod (10.25 h:13.75 h to 14 h:10 h light:dark). The activity level of turtles had a significant effect on oxygen consumption and breathing frequency but there was no significant change in activity level between the summer and winter simulations. There was a moderate 24-27% decrease in oxygen consumption during exposure to winter conditions compared with summer conditions, but this difference was not statistically significant. Likewise, there was no statistically significant difference in breathing frequency between summer and winter simulations. Exposure to winter conditions did result in a significant decrease in activity of the aerobic enzyme citrate synthase. By contrast, activities of the glycolytic enzymes pyruvate kinase and lactate dehydrogenase were significantly higher in tissue collected during exposure to winter conditions compared with summer conditions. Citrate synthase, pyruvate kinase and lactate dehydrogenase had relatively low thermal dependence over the range of assay temperatures (15-30°C; Q10=1.44-1.69). Heart rate was 46-48% lower during the winter simulation compared with the summer simulation, and this difference was statistically significant. Exposure to winter conditions resulted in a significant decrease in plasma thyroxine and plasma proteins and a significant increase in plasma creatine phosphokinase and hematocrit. Overall, our results suggest that green turtles have a relatively low thermal dependence of metabolic rate over the range of temperatures commonly experienced at tropical to subtropical latitudes, a trait which allows them to maintain activity year-round.