DIFFERENCES IN PH REGULATION IN CO-OCCURRING BARNACLE SPECIES AT OPPOSITE LIMITS OF THEIR BIOGEOGRAPHIC RANGES.

1. As sea surface temperature increases around the south-west coast of England the intertidal barnacle Semibalanus balanoides, at the southern limit of its range, is decreasing in abundance, while its co-occurring competitor Chthamalus spp. is increasingly abundant and extending its range northward. The physiological mechanisms underlying the differing responses of the species to temperature increase are not understood. 2. Intertidal barnacles experience rapid acute temperature changes during emersion periods, and physiological regulation during these episodes is superimposed on seasonal temperature changes and global warming. As physiological responses are species specific, knowledge of the differences in regulatory capacity are necessary to understand the limits of thermal tolerance and thus predict biogeographic range changes with global warming. 3. This study measured the pH of whole-body tissue homogenate from S. balanoides and Chthamalus spp. at intervals during emersion and acute warming after acclimation to current and predicted sea surface temperatures. 4. The two species were found to show the same response pattern in pH to acute temperature increase with Chthamalus spp. maintaining a significantly higher pH than S. balanoides. Acclimation had no significant effect on regulatory response in either species. 5. Although more pH data are required to confirm the conclusions of this study, the results suggest than S. balanoides has a lower capacity for physiological regulation than Chthamalus spp. during acute high temperature stress. Finding no change in response to acute warming 3 after chronic increase in temperature indicates that S. balanoides adults have the capacity to tolerate the predicted increase in sea surface temperature, and supports the hypothesis that the larval life stages are most vulnerable to climate change and are responsible for the decline in abundance of this species at their southern range limit