STILLMAN, J.H.: Physiological Tolerance Limits in Intertidal Crabs

Vertical zonation of intertidal organisms, from the shallow subtidal to the supralittoral zones, is a ubiquitous feature of temperate and tropical rocky shores. The results of many studies suggest that the upper range of a species’ vertical distribution is determined by tolerance limits to abiotic factors, while the lower range is set by biotic interactions among taxa. Organisms that live higher on the shore experience larger daily and seasonal fluctuations in microhabitat conditions, due to their greater exposure to terrestrial conditions during emersion. Here, I summarize work on the physiological tolerance limits of rocky intertidal zone crabs to emersion-related heat stress. Comparative analyses of the adaptive linkage between physiological tolerance limits and vertical distribution are the most powerful when the study species are closely related and occur in discrete vertical zones throughout the intertidal range. For example, each of approximately 45 porcelain crab species of the genus Petrolishes lives in a discrete vertical intertidal zone in one region of the eastern Pacific. In these crabs, whole organism thermal tolerance limits, and thermal limits of heart and nerve function reflect microhabitat conditions. Species living higher in the intertidal zone are more eurythermal than low-intertidal congeners, tropical species have the highest thermal limits, and the differences in thermal tolerance between low- and high-intertidal species is greatest for temperate crabs. Acclimation of thermal limits of high-intertidal species is restricted as compared to low-intertidal species. Thus, because thermal limits of high-intertidal species are near current habitat temperature maxima, global warming could most strongly impact intertidal species.