Temperature influences the physiology of poikilotherm organisms such as fish in two general ways: (a) it determines the rates of chemical reactions (i.e. influences biochemical reaction rates by a factor of two to three for every change of 10°C); and (b) it shifts the equilibria between the formation and disruption of the non-covalent interactions that stabilise biological structures. Accordingly, temperature can affect fish cardio-circulatory function at least at two biological levels: at the metabolic level influencing the rate of enzyme catalysis and direction of metabolic flow, and at the morpho-functional level influencing the composition and dynamic interrelationships of the various components of the cardiovascular apparatus.

The heart, like the myotomal musculature, is a chemo-mechanical converter of energy, made up of excitable and contractile units. Therefore, its rhythmical performance is particularly sensitive to temperature. However, as in other excitable tissues, adaptive mechanisms operate also in the cardiovascular system by reshaping it metabolically and structurally to confer a degree of independence from the intrinsic effects of temperature. Despite its limited and yet mostly descriptive nature, the available information indicates that the cardio-circulatory biology of fish mirrors, to some extent, the differences in the thermal tolerance limits experienced by the different teleost groups. Thus, eurythermal species in general show a remarkable ability to compensate cardiovascular performance for the direct consequences of seasonal changes in environmental temperature. Other species, such as the stenothermal Antarctic fishes that live in a nearly constant temperature, have developed cardio-circulatory adaptations for functioning within a very restricted thermal regime (Hazel & Prosser, 1974; Cossins & Bowler, 1987).