Introduction

Of all the cellular responses that occur during thermal acclimation of fish, the most widespread and consistent is a change in the composition of cellular lipids. Since the early part of this century it has been known that cold acclimation leads to increased levels of unsaturated fatty acids, both in depot and cellular lipids. Since that time there have been a very large number of published studies demonstrating this effect in different animal groups, as well as in plants and in microorganisms. It is a matter of common observation that unsaturated fats are more fluid than saturated fats and the idea that the cold-induced increase in unsaturation compensated for the cold-induced rigidification of cellular lipids was implicit even in the earliest studies. This concept, however, was specifically applied to phospholipid cellular membranes and to the thermal acclimation of poikilothermic animals by Johnston & Roots (1961) who ascribed the adaptive significance to the preservation of some unidentified physical condition.

Spectroscopic and calorimetric techniques to quantify the physical properties of order or of molecular motion in phospholipid bilayers became available in the early 1970s and it was not long until they were used to compare the membranes isolated from bacteria grown at different temperatures. Sinensky (1974) showed that the membranes of E. coli grown at low temperatures were more ‘fluid’, as judged by electron spin resonance spectroscopy, than those of bacteria grown at higher temperatures.