Introduction

Enormous differences exist among ectothermic animals in optimal body temperatures and breadth of thermal tolerance ranges. Extreme stenothermy, coupled with cold tolerance, is exemplified by highly coldadapted notothenioid fishes of Antarctica, that have a thermal tolerance range of only about 6 °C (from the freezing point of seawater, –1.86 °C, to approximately 4 °C; Somero & DeVries, 1967; Eastman, 1993). In contrast, extreme eurythermy and heat tolerance is exhibited by fishes such as the intertidal goby Gillichthys seta, whose body temperature may range from approximately 8 °C to 40 °C, as a function of both seasonal and diurnal changes in water temperature (Dietz & Somero, 1992). The physiological, biochemical and molecular mechanisms that distinguish stenotherms and eurytherms are likely to play critical roles in establishing biogeographical patterning and in establishing the susceptibility of animals to shifts in ambient temperature, such as are predicted as a consequence of global warming.

This review compares homologous biochemical and physiological systems in stenotherms and eurytherms, and relates interspecific differences in these systems to the thermal optima and tolerance ranges characteristic of the whole organism. In keeping with a central theme of this symposium, namely, the similarities and differences found between evolutionary adaptation to temperature and short-term phenotypic acclimatisation, this review contrasts genetically-fixed traits that are important in setting thermal limits and thermal optima, with more ‘plastic’ traits that provide significantly different phenotypes under different thermal conditions.