Homeoviscous adaptation

The maintenance of proper cell membrane fluidity is of critical importance for the function and integrity of the cell, essential for the mobility and function of embedded proteins and lipids, diffusion of proteins and other molecules laterally across the membrane for signaling reactions, and proper separation of membranes during cell division. A fundamental biophysical determinant of membrane fluidity is the balance between saturated and unsaturated fatty acids. Regulating membrane fluidity is especially important in poikilothermic organisms such as bacteria, fungi, protists, plants, fish and other ‘cold-blooded’ animals that cannot regulate their own body temperatures. The general trend is an increase in unsaturated fatty acids at lower growth temperatures and an increase in saturated fatty acids at higher temperatures. This compositional adaptation of membrane lipids, called homeoviscous adaptation, serves to maintain the correct membrane fluidity at the new conditions.

Reference:
"Homeoviscous Adaptation-A Homeostatic Process that Regulates the Viscosity of Membrane Lipids in "Escherichia coli"; Proceedings of the National Academy of Sciences USA, February 1, 1974, vol. 71, no. 2, pp. 522-525.