Biogeographic Patterns and Drivers of Endolithic Symbiosis in Mussels Across Atlantic Europe and North Africa

ABSTRACT Aim Ecological interactions are central to ecosystem health. In intertidal habitats, mussels serve as foundational bioengineers. Microbial endoliths erode mussel shells and impair fitness but may also provide thermal benefits under stress. This relationship can be context‐dependent, shifting from parasitism to mutualism. Yet, environmental drivers and large‐scale patterns of this symbiosis remain poorly understood. We aim to investigate the prevalence, severity, spatiotemporal dynamics and environmental drivers of endolithic symbiosis in intertidal mussels. Location Atlantic coasts of Western Europe and North Africa (2011–2024). Taxon Mytilus spp. Methods Using a Random Forest machine‐learning framework, we assess endolithic symbiosis across mussel size classes and evaluate 10 environmental variables for their potential roles in mediating spatiotemporal patterns: eight dynamic (e.g., solar radiation, chlorophyll‐a concentration, air temperature) and two static (tidal range, proximity to sandy beaches). With this approach, we identify key predictors and develop two models of endolithic colonisation probability: a parsimonious model with minimal but highly informative predictors, and an inclusive model incorporating a broader set of relevant variables. Results Solar radiation and chlorophyll‐a concentration emerged as the strongest predictors, especially for small and medium mussels. Model predictions reveal distinct seasonal and latitudinal patterns of endolithic infestation: low probabilities between 45° N and 55° N from July to October (summer to early autumn); low probabilities between 30° N and 45° N from March to May (late winter to early spring); and high probabilities between 20° N and 30° N from July to January (summer to early winter). Main Conclusions Our findings highlight the complex, dynamic nature of mussel–endolith interactions and the critical role of environmental context in shaping symbiosis. Long‐term spatiotemporal predictions reveal clear seasonal bioerosion probability and underscore the need to monitor mussel beds under climate change. These findings contribute to a broader understanding of mussel–endolith interactions and offer context for interpreting potential ecological shifts.