Weather and topography regulate the benefit of a conditionally helpful parasite

Heat-induced mass mortalities involving ecosystem engineers may have long-lasting detrimental effects at the community level, eliminating the ecosystem services they provide. Intertidal mussels are ecologically and economically valuable with some populations facing unprecedented heat-induced mass mortalities. Critically, mussels are also frequently infested by endolithic parasites that modify shell albedo, hence reducing overheating and mortality rates under heat stress. Using a biophysical model, we explored the topographical and meteorological conditions under which endolithically-driven thermal buffering becomes critical to survival. Based on meteorological data from a global climate analysis, we modelled body temperatures of infested and non-infested mussels over the last decade (2010-2020) at nine sites spread across ca. 20° of latitude. We show that thermal buffering is enhanced where and when heat stress is greatest, that is, on sun-exposed surfaces under high solar radiation and...