What is cold adaptation and how should we measure it?

Cold adaptation encompasses all those aspects of an organism's physiology that allow it to live in polar regions. With the exception of the special case of the need to avoid freezing, it is therefore merely a specific example of the more general temperature compensation needed by all marine organisms. Temperature compensation is a form of homeostasis; the extent to which a given organism has achieved this can only be assessed in those processes which can be studied at the molecular level. Recent studies of polar organisms, primarily fish, have indicated that compensation is not always perfect. Studies of complex integrated processes such as growth or respiration do not necessarily give useful information concerning cold adaptation. Growth, for example, may show compensation at the molecular level but still be slow for other reasons (for example, resource limitation). Respiration is a particularly misleading indicator of temperature compensation, primarily because it represents the summation of many processes each of which may react differently to temperature. The use of respiration rate to assess temperature compensation should be abandoned forthwith.