Introduction

Temperature is a critical determinant of physiological performance in ecotherms. For the majority of fish and gill-breathing aquatic invertebrates, thermal fluctuations in the environment change body temperature as a result of the rapid thermal equilibration across gill surfaces. Thermal fluctuations affect physiological processes both through effects on reaction rates as well as on the equilibria determining the noncovalent interactions that stabilise macromolecules and membranes. In response to either short- or long-term thermal fluctuations, individual organisms often adjust their exact biochemical composition as well as the rates of physiological and metabolic processes. Adaptation to different thermal habitats on an evolutionary time-scale is likely to have favoured qualitative and quantitative modifications in biochemical and physiological properties.

As the centres of oxidative phosphorylation, mitochondria are critical sites of adenosine triphosphate (ATP) provision in aerobic tissues. Whereas mitochondria are implicated in anabolic and catabolic pathways in liver, kidney and other organs, the primary role of mitochondria in oxidative muscle fibres is the provision of ATP for muscle contraction. Given the importance of locomotion in foraging, migration, prey capture and predator avoidance, both sustained and burst locomotor capacity are likely to have been modified during evolutionary adaptation to new thermal environments. These considerations suggest that the thermal sensitivity of mitochondria in oxidative fibres is likely to have changed during thermal adaptation on an evolutionary time-scale. During cold acclimation of fish, the mitochondrial volume density of muscle increases, much as observed in response to endurance training in mammalian muscle.