Unveiling the Role of Endolithic Symbionts in Enhancing Mussel Resilience to Thermal StressExtreme Events

Sarah Bollina¹, Gerardo Zardi^2,3,4, Katy Nicastro^1,4, Virginie Cuvillier⁵, Laurent Seuront^1,4,6

¹LOG – Laboratoire d’Océanologie et de Géosciences (Univ. Lille, CNRS, Univ. Littoral Côte d’Opale), France - ²Normandie Université, France - ³BOREA (CNRS, MNHN, UPMC, UCBN, IRD) - ⁴Rhodes University, South Africa - ⁵Evo-Eco-Paleo (Univ. Lille, CNRS), France - ⁶Tokyo University of Marine Science and Technology, Japan

Temperature has long been recognized as a critical determinant of organismal performance and ecosystem dynamics in marine intertidal systems. Recent climate change models predict an increase in thermal unpredictability alongside more frequent and intense hot days, posing significant challenges to coastal marine life. Symbiotic relations are widespread in nature and play a crucial role in modulating the host’s response to a variety of stressors, enabling adaptations and survival in challenging conditions. In coastal marine ecosystems, one example of such symbiosis is the relationship between shell-corroding microbial endoliths and intertidal mussels. These endolithic symbionts have been shown to reduce mussels body temperature on hot, sunny days, through shell-whitening, thereby mitigating thermal stress. Here, we investigate behavioral and molecular responses of the bioengineering blue mussel Mytilus edulis in the presence and absence of endoliths, under both thermally stressful and stress-free conditions. A key mechanism for thermal tolerance, the heat shock response, was analysed by measuring heat shock protein (HSP24, HSP60, HSP90) expression in individuals with and without endoliths, showing generally higher expression after heat stress and in the absence of the buffering effects of endolithic symbionts. Similarly, behavioral traits such as attachment strength, aggregation, and movement patterns, as well as survival rates, were examined, with the presence of symbionts significantly enhancing survival under thermal stress. These findings highlight the significance of symbiotic relationships, deepening our understanding of their importance in stress resilience and paving the way for future conservation and monitoring efforts to include symbiotic relationships.