5.4.1 Summary of Section 5
Several anatomical and biochemical adaptations to living in very cold water have evolved in polar fish, particularly those of the southern oceans, which have evolved in isolation for many millions of years. Cold, turbulent water is rich in oxygen. One family of fairly large fish lacks blood pigments but its blood is less viscous and it has additional respiratory surfaces. Many fish have cryoprotectants in the blood and other body fluids, and the muscles of some contain numerous mitochondria and are adapted to use lipid as fuel in preference to carbohydrate. Many polar fish tolerate only a narrow range of temperatures and quickly die if exposed to water even slightly warmer than that in which they normally live. The fatty acid composition of the tissue (the ‘fatty acid profile’) provides information about the animals' recent diet, which varies greatly with season and location.