Thermal acclimation in fish: conservative and labile properties of swimming muscle

Given the rapid thermal equilibration of most fish with their environment, thermal compensation of metabolic and contractile properties is essential for the maintenance of locomotory capacities over a wide range of temperatures. The response of fish swimming performance, contractile properties of isolated fibers, myosin ATPase activity, and metabolic systems for ATP generation to short- and long-term changes in temperature have received sufficient study to allow one to identify certain constrained and labile properties. Sustained swimming performance and its components generally have their optimal performance and lowest thermal sensitivity within the range of temperatures frequently encountered by the organism. These principles are particularly well established for isolated enzyme systems. Furthermore, swimming performance and most of its components demonstrate thermal compensation on the evolutionary time scale. Temperature acclimation also leads to compensatory responses which, while quite species-specific, consistently increase the capacity for sustained swimming at low temperatures. The position of the thermal optimum for locomotion in relation to the width of a species' tolerance limits aids in predicting the species' capacity for thermal compensation during acclimation. Goldfish (Carassius auratus) and common carp (Cyprinus carpio), which tolerate temperatures 25–30 °C below their optimum for locomotion, show thermal compensation in terms of contractile properties, myosin ATPase activity, the proportion of red fibers in their axial musculature, and the levels of aerobic enzymes in their musculature. By contrast, striped bass (Morone saxatilis) and chain pickerel (Esox niger), which have lower optimal temperatures for locomotion, only increase the proportion of red fibers and (or) the levels of aerobic enzymes with cold acclimation. Finally, lake whitefish (Coregonus clupeaformis), which have their optimal temperature for locomotion at 12 °C, show none of these responses. Given that when thermal compensation occurs, aerobic enzymes in red muscle generally increase, the capacity of red muscle to generate ATP seems more temperature sensitive than other metabolic or contractile properties. Whether this compensatory response serves to counteract the effect of temperature on diffusive exchange between mitochondria and the cytoplasm or its effect on the catalytic capacity of aerobic metabolism remains to be established.