Integrating multi-level approaches to assess blue mussel (Mytilus spp.) responses to short-term temperature and salinity changes.

Climate change is exacerbating temperature and salinity variations in marine ecosystems, thereby challenging the physiological resilience of bivalves like blue mussels (Mytilus spp.). Despite comprehensive studies on these organisms' physiological and molecular responses to such environmental stress, our understanding of tissue-specific adaptations and the role of lipid metabolism is limited. This study investigated the short-term effects of warming (+3.5 °C and +6.0 °C) and reduced salinity (-6 and -12 PSU) on Mytilus spp. using an integrative approach that included physiological indicators, lipidomic, biochemical, and molecular analyses across several tissues. The mussel clearance rate increased under thermal stress and decreased under hyposaline stress. Both stressors resulted in a lower condition index, suggesting energy depletion and specific metabolic changes. Lipidomic analysis revealed significant fatty acid and lipid composition alterations, indicating membrane remodelling to maintain cellular integrity. Gene expression analyses demonstrated distinct responses across tissues: the gills activated protective mechanisms (i.e., up-regulation of heat shock protein-encoding genes (hsp)) in response to both stressors, whereas the digestive gland prioritised energy conservation and damage mitigation (i.e., down-regulation of hsp, metabolic, and apoptotic marker genes). These divergent responses indicate that mussels use tissue-specific strategies to balance stress resistance and metabolic trade-offs. Despite demonstrating notable physiological plasticity, our findings highlight the significant metabolic costs associated with environmental stress adaptation. This study provides new insights into the complex, multi-level strategies that Mytilus spp. employ to maintain homeostasis in the face of multiple stressors, with implications for predicting their resilience under future climate conditions.