With a Little Help From Their Friends: Endolithic Microbial Communities Modulate Mussel Survival, Behaviour and Molecular Responses Under Thermal Stress.

As thermal stress intensifies under global climate change, ecosystems worldwide are undergoing profound transformations that disrupt biological processes, species distributions and biodiversity, with coastal marine systems particularly vulnerable. Organisms have evolved behavioural, physiological and morphological strategies to cope with extreme conditions, while species interactions, especially symbioses, can further shape responses to stress. These symbioses often shift along a continuum from mutualism to parasitism depending on environmental context. One notable example is the dynamic relationship between bivalves and their endolithic microbial communities. Here, we investigated how variation in visible endolithic shell corrosion relates to differences in mussel survival, behaviour and molecular responses during repeated aerial heat stress. Mussels with endolithic shell corrosion consistently showed enhanced survival. Behaviourally, heat stress impaired mobility (i.e., net and gross distance travelled) and byssal thread production (i.e., number of byssal threads), but mussels with endoliths exhibited comparatively buffered responses to these effects. At the molecular level, mussels with endoliths showed lower induction of HSP24 and HSP70 after thermal stress, but higher baseline HSP60 and HSP90 expression, suggesting distinct stress-response pathways. Together, our findings reveal consistent, multi-level differences in host responses associated with endolithic shell condition across behavioural and molecular traits and survival, suggesting that endoliths may contribute to enhanced thermal resilience and, consequently, could play a role in shaping the resilience of mussel populations under future climate-driven heat stress. More generally, these results highlight the critical role of interspecific interactions, including host-associated microbial assemblages, in shaping adaptive responses in hosts.