SOMERO, George N.: Thermal Physiology of Intertidal Animals: Optima, Limits, and Adaptive Plasticity

Studies of congeneric species with different vertical distributions along the subtidal to intertidal gradient have revealed pervasive physiological adaptations that establish thermal optima and limits for function. Some adaptive differences are genetically fixed, whereas other traits exhibit adaptive plasticity during acclimatization or acclimation. Protein thermal stability and function reflect fine-scale adaptation to temperature across this vertical gradient. Temperature differences of only a few degrees C are adequate to favor selection for adaptive change. Only minor variations in amino acid sequence, which occur exclusively outside the catalytic site, establish these adaptations. Membrane-based systems have substantial adaptive plasticity, which varies between stenotherms and eurytherms. The thermal limits of mitochondrial respiration vary adaptively among species and between warm- and cold-acclimated conspecifics and are correlated with changes in lipid physical state. Thermal sensitivity of action potential generation differs adaptively among congeners, and may play a role in setting upper thermal limits. The absolute upper thermal limit of eukaryotes is predicted to lie near 65 ° C and may be set by limitations in membrane stability. Some physiological systems, e.g., heart function and protein synthesis, may have upper thermal limits lying close to current habitat temperatures. The proximity of lethal temperatures to habitat temperatures is closest for mid-to high-intertidal species. Thus, these species may be most threatened by global warming. The thermal sensitivities of physiological processes and the costs of repairing thermal damage to cells play important roles in setting vertical distribution patterns in the rocky intertidal zone.