The relationship between whole-organism growth and metabolism is generally assumed to be positive and causative; higher metabolic rates support higher growth rates. In Manduca sexta, existing data demonstrate a deviation from this simple prediction: at supraoptimal temperatures for larval growth, metabolic rate keeps increasing while growth rate is decreasing. This mismatch presumably reflects the rising "cost of maintenance" with temperature. Precisely what constitutes this cost is not clear, but we suspect the efficiency with which mitochondria harness oxygen and organic substrates into cellular energy (ATP) is key. We tested this by integrating existing data on M. sexta growth and metabolism with new data on mitochondrial bioenergetics across the temperature range 14°-42°C. Across this range, our measure of mitochondrial efficiency closely paralleled larval growth rates. At supraoptimal temperatures for growth, mitochondrial efficiency was reduced, which could explain the mismatch between growth and metabolism observed at the whole-organism level.

Antarctic fauna are highly adapted to the frigid waters of the Southern Ocean. This study describes the in vitro temperature sensitivity of oxygen consumption rates measured in liver mitochondria from the pelagic nototheniid Pleuragramma antarcticum between 5°C and 35°C. Oxygen fluxes were measured after the addition of milimolar levels of pyruvate, malate, succinate and glutamate (State II, LEAK) and saturating levels of ADP (State III, OXPHOS). State III respiration significantly decreased above 18.7°C. A comparison of the oxidative capacities among P. antarcticum and other notothenioids showed significant differences in state III respiration, where benthic species exhibited about 50% lower rates than P. antarcticum. In addition, state III respiration rates normalized per milligram of mitochondrial protein of P. antarcticum were up to eight times higher than state III rates reported in the literature for other notothenioids. The comparatively high respiration rates measured in this study may be explained by our approach, which engaged both complex I and II under conditions of oxidative phosphorylation. State III rates of independently activated complexes I and II were found to range from 42 to 100% of rates obtained when both complexes were activated simultaneously in the same species. The remarkable tolerance of P.antarcticum OXPHOS toward warmer temperatures was unexpected for an Antarctic stenotherm, and may indicate that thermal sensitivity of their mitochondria is not the driving force behind their stenothermy.3

Thermal regimes can diverge considerably across the geographic range of a species, and accordingly, populations can vary in their response to changing environmental conditions. Both local adaptation and acclimatization are important mechanisms for ectotherms to maintain homeostasis as environments become thermally stressful, which organisms often experience at their geographic range limits. The spatial spread of the gypsy moth (Lymantria dispar L.) (Lepidoptera: Erebidae) after introduction to North America provides an exemplary system for studying population variation in physiological traits given the gradient of climates encompassed by its current invasive range. This study quantifies differences in resting metabolic rate (RMR) across temperature for four populations of gypsy moth, two from the northern and two from southern regions of their introduced range in North America. Gypsy moth larvae were reared at high and low thermal regimes, and then metabolic activity was monitored at four temperatures using stop-flow respirometry to test for an acclimation response. For all populations, there was a significant increase in RMR as respirometry test temperature increased. Contrary to our expectations, we did not find evidence for metabolic adaptation to colder environments based on our comparisons between northern and southern populations. We also found no evidence for an acclimation response of RMR to rearing temperature for three of the four pairwise comparisons examined. Understanding the thermal sensitivity of metabolic rate in gypsy moth, and understanding the potential for changes in physiology at range extremes, is critical for estimating continued spatial spread of this invasive species both under current and potential future climatic constraints.

Thermal regimes in aquatic systems have profound implications for the physiology of ectotherms. In particular, the effect of elevated temperatures on mitochondrial energy transduction in tropical and subtropical teleosts may have profound consequences on organismal performance and population viability. Upper and lower whole-organism critical temperatures for teleosts suggest that subtropical and tropical species are not susceptible to the warming trends associated with climate change, but sub-lethal effects on energy transduction efficiency and population dynamics remain unclear. The goal of the present study was to compare the thermal sensitivity of processes associated with mitochondrial energy transduction in liver mitochondria from the striped mojarra (Eugerres plumieri), the whitemouth croaker (Micropogonias furnieri) and the palometa (Trachinotus goodei), to those of the subtropical pinfish (Lagodon rhomboides) and the blue runner (Caranx crysos). Mitochondrial function was assayed at temperatures ranging from 10 to 40°C and results obtained for both tropical and subtropical species showed a reduction in the energy transduction efficiency of the oxidative phosphorylation (OXPHOS) system in most species studied at temperatures below whole-organism critical temperature thresholds. Our results show a loss of coupling between O2 consumption and ATP production before the onset of the critical thermal maxima, indicating that elevated temperature may severely impact the yield of ATP production per carbon unit oxidized. As warming trends are projected for tropical regions, increasing water temperatures in tropical estuaries and coral reefs could impact long-term growth and reproductive performance in tropical organisms, which are already close to their upper thermal limit.