Two regions of the world ocean are characterised by extremely cold temperatures: the polar oceans and the deep sea. In each of these regions, water temperatures usually are close to 0°C. In the polar oceans, especially in the waters surrounding Antarctica, temperatures may rise seasonally by no more than a few hundredths of a degree centigrade above the freezing point of sea water, −1.86°C (Eastman, 1993). Deep-sea temperatures generally are also very stable, with the notable exception of waters found at the deep-sea hydrothermal vents, where animals may encounter temperatures between approximately 2°C and at least 40°C (Fustec, Desbruyeres & Juniper, 1987; Johnson, Childress & Beehler, 1988; Dahlhoff et al, 1991). Thus, ectothermic species living in polar and typical deep-sea waters are characterised by some of the lowest and most stable body temperatures found in any species, in any environment. One would predict that many physiological and biochemical systems of shallow-living polar species and deep-sea species would manifest similar adaptations to low and stable temperatures.

Despite facing similar thermal conditions, deep-sea species and shallowliving polar species encounter other environmental differences that may lead to divergent patterns of adaptation. Hydrostatic pressure is one important difference between shallow and deep cold water habitats. Pressure rises by approximately 0.1 MPa (∼1 atm) with each 10 m increase in depth, so deepliving organisms encounter pressures of up to 100 MPa (=∼1000 atm) in the deepest trenches of the ocean. Because most physiological and biochemical systems are sensitive to pressure (Siebenaller, 1991; Somero, 1993), adaptations to high pressure, as well as to low temperature, would be expected to characterise diverse physiological and biochemical traits of deep-sea species.