Size and position (sometimes) matter: small-scale patterns of heat stress associated with two co-occurring mussels with different thermoregulatory behaviour

Heat-related mass mortalities and local extinctions are expected to rise as the frequency, duration, and intensity of extreme heat events increase due to climate change, particularly in the case of sessile or sedentary species that cannot relocate. Little is known, however, of how biotic factors, such as the size and non-motile behaviour of individuals, contribute to small-scale variation in susceptibility to heat-related mortality during such events. We used infrared thermography to investigate how manipulated mussel bed size and the size structure of individuals within beds influence small-scale variability in the body temperatures of two intertidal mussel species with different thermoregulatory behaviours (gaping vs. non-gaping) during simulated extreme heat conditions. At times, body temperatures of small individuals reached higher temperatures than large individuals, irrespective of mussel bed size, though this was more apparent for the non-gaping species. Average body temperatures and heating rates of individuals within large mussel beds were generally greater than for individuals within small mussel beds, irrespective of species or individual size. This pattern seems to reflect an effect of the greater circumference/ area ratio for small mussel beds as individuals on the windward side of all beds displayed convective cooling and body temperatures 3-5 degrees C cooler than those on the leeward side. Such high levels of inter-individual variability in body temperatures at small spatial scales suggest the need for a greater appreciation and inclusion of biotic factors in assessing susceptibility of populations to climate change.