Hot Days and Heatwaves: How Environmental Complexity Shapes the Physiological Responses of Mussels to Climate Change

Rocky intertidal systems have been highlighted as potential early indicators for the impacts of climate change due to the simultaneous fluctuation of multiple environmental parameters caused by the predictable movement of the tide. A central prediction of recent climate change models is that there will be an increase in temperature variation and unpredictability, which will increase the probability of extreme hot days and multi-day heatwave events. Sessile intertidal organisms already experience a high degree of thermal unpredictability due to the combined effects of solar radiation and tidal movement. Most of our understanding of thermal physiology of intertidal organisms is based on experiments that have typically focused on exposing intertidal organisms to constant or predictable fluctuations in temperature when organisms are submerged in water, rather than unpredictable fluctuations in temperature that are more representative of the natural environment. How thermal unpredictability operates in intertidal habitats with different media, such as tidepool (submerged in water) or tidally-exposed (circatidal exposure of air and water) habitats to shape organismal performance is poorly understood, especially when coupled with other important determinants of performance, such as food availability. The overall goal of my dissertation was to understand how key aspects of environmental variation in the rocky intertidal interact to influence thermal performance in the California mussel (Mytilus californianus). I examined how intertidal habitat, thermal predictability and food availability shaped the performance of M. californianus to both a single, lethal thermal stress event (extreme hot day), as well as a sublethal multi-day thermal stress event (heatwave) and measured physiological performance through metabolic and biochemical analyses. I found that intertidal habitat, thermal predictability, food availability and the degree of thermal stress (lethal or sublethal; single or repeated thermal stress events) all interacted in nuanced ways to shape the thermal performance of M. californianus. It also became apparent that mussels tailor their physiological responses to predictable signals in their environment. In rocky intertidal systems, the most predictable environmental signal is the transition between immersion and emersion due to the movement of the tide. Here, I found that acclimation to a tidal cycle of immersion and emersion played a fundamental role in shaping physiological responses in mussels and was the predominant driver for regulating thermal performance to both lethal and sublethal thermal stress. When the predictable signal of air exposure was absent, as occurs in tidepool habitats, mussels became more dependent on thermal predictability and food availability to modulate thermal performance. Overall, my PhD research indicates that incorporating realistic environmental conditions into experimental design is crucial for understanding how future climate change scenarios will influence the physiological performance of intertidal mussels. By having a better understanding of how the complexity of the environmental signal impacts mussel performance, we can provide much more ecologically realistic predictions of the vulnerability of mussel populations to climate change.