Acclimation capacity underlies susceptibility to climate change.

Recent reports have presented meta-analyses of global biological impacts of climate change (1, 2). However, there is debate as to the level of confidence ascribed to the certainty that global climate change has caused the observed biological changes (3). Two important considerations in the assessment of how climate change will impact organisms are (i) how close organisms are to their thermal limits in nature and (ii) an understanding of how organisms respond to increasing habitat temperatures, especially the degree to which organisms are able to adjust, or acclimatize, their thermal sensitivity. Here, I report the effects of thermal acclimation on thermal limits of cardiac function in four congeneric species of marine invertebrates (Porcelain crabs, genus Petrolisthes) from different thermal habitats. Crabs in this genus are distributed throughout the eastern Pacific, from Alaska to Chile, and at all sites in this latitudinal range sympatric species have discrete patterns of vertical intertidal zonation (4). During low tide, these crabs remain under stones, where temperatures during hot, low tide periods have been shown to change over 20°C in 6 hours (4,5). Two of the species (P. gracilis and P. hirtipes) are endemic to the Northern Gulf of California, and the other two (P. cinctipes and P. eriomerus) are distributed in the coldtemperate zone of the northeastern Pacific (4). P. gracilis and P. cinctipes are upper intertidal zone species, P. hirtipes is a middle intertidal zone species, and P. eriomerus is found in and below the low intertidal zone (4). P. gracilis and P. hirtipes are in one phylogenetic clade that diverged 6 million years ago (Ma), and P. cinctipes and P. eriomerus are in a different clade with a divergence time of 13 Ma (4). The upper and lower thermal limits of heart function (defined as CTmax and CTmin, respectively) (fig. S1) (5) were determined in specimens acclimated to 10°C temperature ranges (Fig. 1, A and B) (5). CTmax (Fig. 1A) and CTmin (Fig. 1B) values at either acclimation temperature were generally consistent with thermal microhabitat. CTmax values are at or near maximal habitat temperatures (Fig. 1, A and C) (4) in all species except for P. eriomerus. The striking result of this experiment is shown in Fig. 1C. The change in