A comparative analysis of the upper thermal tolerance limits of eastern Pacific porcelain crabs, genus Petrolisthes: Influences of latitude, verticalzonation, acclimation, and phylogeny

Marine intertidal organisms are subjected to a variety of abiotic stresses, including aerial exposure and wide ranges of temperature. Intertidal speci es generally have higher thermal tolerance limits than do subtidal species, and tropical species have higher thermal tolerance limits than do temperat e species. The adaptive significance of upper thermal tolerance limits of i ntertidal organisms, however, has not been examined within a compar- ative context. Here, we present a comparative analysis of the adaptive significan ce of upper thermal tolerance limits in 20 congeneric species of porcelain crabs, genus Petrolisthes, from intertidal and subtidal habitats throughout the eastern Pacific. Upper thermal tolerance limits are positively correla ted with surface water temperatures and with maximal microhabitat temperatu res. Analysis of phylogenetically independent contrasts (from a phylogeneti c tree on the basis of the 16s rDNA gene sequence) suggests that upper ther mal tolerance limits have evolved in response to maximal microhabitat tempe ratures. Upper thermal tolerance limits increased during thermal acclimatio n at elevated temperatures, the amount of increase being greater for subtid al than for intertidal species. This result suggests that the upper thermal tolerance limits of some intertidal species may be near current habitat te mperature maxima, and global warming thus may affect the distribution limit s of intertidal species to a greater extent than for subtidal species.