Understanding the role of low tide habitat, thermal predictability, and food availability in shaping the thermal performance of the California mussel

Recent climate change models indicate that there will be an increase in thermal unpredictability and the frequency and intensity of hot days. How thermal unpredictability operates in intertidal low tide habitats with different habitat media, such as tidepool (submerged in water) or tidally exposed (circatidal exposure to air and water) environments is poorly understood, especially when coupled with other important determinants of physiological performance, such as food availability. We examined how acclimation to different levels of thermal predictability with either high or low food availability shapes performance during an acute thermal ramp in the California mussel Mytilus californianus in 2 low tide habitat treatments: tidepool and tidally exposed. Mussels were warmed at a rate of 6.5°C h -1 for 6 h in their respective habitat medium (water or air). Cardiac performance, glycogen, and Hsp/Hsc70 were determined during the heat ramp. We found that low tide habitat was the largest driver for shaping thermal performance. Tidepool mussels exhibited higher maximum heart rates, but also multiple breakpoints and lower upper thermal tolerance in cardiac performance, coupled with an increase in Hsp/Hsc70 levels and glycogen usage in comparison to tidally exposed mussels. Mussels exposed to unpredictable thermal regimes exhibited elevated cardiac performance and thermal tolerance, supported by elevated initial glycogen stores. Food availability had a minimal effect on mussel thermal performance. Our results suggest that intertidal organisms that experience thermal stress in a tidepool habitat may be more susceptible to climate-induced increases in temperature, but thermal unpredictability may aid in increasing thermal tolerance.