Temperature is considered to be one of the most important abiotic factors shaping marine ecosystems due to its major impact on all biological processes. Therefore, low or high temperature extremes characterise the limits of geographical distribution of many species, and global change has already caused a change in the distribution of species (Southward, Hawkins & Burrows, 1995). An investigation of marine ectotherms surviving in seasonally or permanently cold ocean environments and their comparison with ectotherms from temperate and warm waters should help to reveal those biochemical or physiological mechanisms which determine geographical distribution limits. These studies should also reveal which molecular, cellular and systemic functions have been shifted to levels compatible with the steadystate maintenance of all life-sustaining processes in the cold. In permanent cold, the latter must also include growth and reproduction, whereas during seasonal cold exposure these processes may be suspended.

Adaptation to cold started from life forms that evolved in warm waters (e.g. Arntz, Brey & Gallardo, 1994; Thiel, Pörtner & Arntz, 1996). Therefore, the characteristics of the adaptational process must be seen in the light of this evolutionary trend. Cold adaptation then becomes a special physiological feature rather than a basic ability of all life forms. If life conquered the cold after having evolved in warm waters, the question arises about what were the limiting factors in this adaptational process and how would these limiting factors affect the whole organism, thereby preventing an easy access to cold ocean environments.