Most species show intraspecific variation in thermal tolerances across their geographic range, but physiology is not always well-matched to location. Thermal mis-matches increase vulnerability to warming temperature under climate change. Marine intertidal species are unique in occupying a habitat with two distinct thermal environments. While extreme heat stress occurs primarily at low tide, most species are only active during high tide. Moreover, low and high tide temperatures are not spatially correlated along the west coast of the United States. We compared thermal performance among three populations of the intertidal barnacle Balanus glandula, spanning 1000 km: LA (warm water/warm air), BB (cold water/cold air) and FH (cold water/warm air). Barnacles were acclimated in the lab under a typical neap tidal cycle for each population with local water temperatures and a common 19 °C air temperature before being exposed to one of 8 low tide temperature trials (10, 15, 20, 25, 30, 35, or 38 °C). Oxygen consumption rates were measured in individual barnacles over a five-hour low tide exposure and for six hours of subsequent immersion. Oxygen consumption rates were calculated from oxygen concentrations measured at 10-second intervals in both air and water.
Results/Conclusions
Patterns of oxygen consumption during low tide in all 3 populations were consistent with a thermal performance curve. The two populations with the coldest water temperatures (FH, BB) showed peaks on oxygen demand at 23.3 and 25.6 °C. A quadratic equation showed a significantly better fit to the data from those populations than a linear fit (p < 0.01). The third population (LA) showed increased oxygen demand with temperature, without a clear peak. Overall, a site’s water temperature was a better predictor of its population’s thermal performance than low tide conditions, despite the fact that the temperature manipulation was run on the low tide phase. These results suggest that aquatic thermal environments may be the primary driver of thermal adaptation in these species. Populations in areas with cold water temperatures and elevated low tide temperatures, like FH, may be the most vulnerable to climate change, regardless of their latitude or position within the species’ range.