Too hot or too cold: the biochemical basis of temperature-size rules for fish and other ectotherms

Abstract

The well-established temperature-dependence of growth parameters and maximum sizes of fish and other water-breathing ectotherms (WBEs) form the basis for various “temperature-size rules” for fish and WBEs. Numerous adaptationist interpretations of these rules exist, but their biochemical basis is largely ignored. One fundamental, but frequently overlooked component of the mechanism that leads to temperature-size rules, is that proteins only “work” if their native quaternary structure (or native folding) is maintained. However, proteins have half-lives are U-shaped functions of temperature, which means that higher or lower than optimal temperatures increase their rates of spontaneous denaturation in aqueous solutions, i.e., within body cells. Proteins that lose their quaternary structures cease to function and, in most cases, need to be resynthesized. Thus, protein denaturation may explain why the metabolic rates of fish and other ectotherms increase with temperatures, both above 4 °C, the temperature at which hydrogen bonding in water is the strongest and hydration of protein nonpolar groups the weakest, and below 4 °C, the regime of “cold denaturation.” Considering the biochemical basis of temperature-size rules for fish and other WBEs would enable biologists to better understand adverse consequences of climate warming for marine and freshwater biodiversity.