Thermal physiology and vertical zonation of intertidal animals: optima, limits, and costs of living.

Temperature's pervasive effects on physiological systems are reflected in the suite of temperature-adaptive differences observed among species from different thermal niches, such as species with different vertical distributions (zonations) along the subtidal to intertidal gradient. Among the physiological traits that exhibit adaptive variation related to vertical zonation are whole organism thermal tolerance, heart function, mitochondrial respiration, membrane static order (fluidity), action potential generation, protein synthesis, heat-shock protein expression, and protein thermal stability. For some, but not all, of these thermally sensitive traits acclimatization leads to adaptive shifts in thermal optima and limits. The costs associated with repairing thermal damage and adapting systems through acclimatization may contribute importantly to energy budgets. These costs arise from such sources as: (i) activation and operation of the heat-shock response, (ii) replacement of denatured proteins that have been removed through proteolysis, (iii) restructuring of cellular membranes ("homeoviscous" adaptation), and (iv) pervasive shifts in gene expression (as gauged by using DNA microarray techniques). The vertical zonation observed in rocky intertidal habitats thus may reflect two distinct yet closely related aspects of thermal physiology: (i) intrinsic interspecific differences in temperature sensitivities of physiological systems, which establish thermal optima and tolerance limits for species; and (ii) 'cost of living' considerations arising from sub-lethal perturbation of these physiological systems, which may establish an energetics-based limitation to the maximal height at which a species can occur. Quantifying the energetic costs arising from heat stress represents an important challenge for future investigations.