Effects of Variable and Constant Acclimation Regimes on the Upper Thermal Tolerance of Intertidal Barnacle, Balanus Glandula

As a unique habitat that encompasses steep environmental gradients, it is important to evaluate threats posed to the intertidal zone by rapid climate change. It is thought that intertidal ectotherms are living close to their physiological limit; therefore slight changes in temperature could result in high levels of mortality. Past studies on intertidal species measured thermal tolerance under constant temperatures, neglecting to consider the impacts of natural variation in field temperatures. I conducted a study on the barnacle, Balanus glandula, to assess if a variable thermal environment would alter thermal tolerance. Barnacles were acclimated in an intertidal mesocosm to either daily cold (maximum 20.4◦C), daily warm (maximum 26.5◦C), or variable (two days cold, two days warm) low-tide temperatures. I measured each barnacle’s critical thermal maximum (CTmax) by increasing air temperature 6◦C/hour and identifying the point at which the barnacle ceased to function. Barnacles exposed to any warm temperatures demonstrated an increased thermal tolerance, suggesting that this population of barnacles is capable of shifting their thermal maximum. Furthermore, acclimation to thermal heterogeneity raised thermal maximum, reinforcing the need for future thermal tolerance studies to incorporate biologically-relevant thermal regimes in laboratory experiments. These results demonstrate that B. glandula in the field are well-adapted for increasing air temperatures