Contrasting responses of

The widespread use of gadolinium‐based contrast agents (GBCAs) in medical imaging has raised concerns regarding their persistence in aquatic environments and potential ecotoxicological impacts. This study evaluated the effects of gadolinium (Gd) exposure, under environmentally relevant concentrations (10 µg/L), on two ecologically and economically important bivalves from the Ria de Aveiro, Ruditapes philippinarum and Mytilus galloprovincialis, while considering the role of salinity fluctuations (30 and 40) associated with climate change. Organisms were exposed for 14 days and a suite of biochemical markers was analysed, including antioxidant and biotransformation enzymes, energy metabolism, neurotoxicity, oxidative damage, and osmoregulation. Both species bioaccumulated Gd, with clams showing higher tissue concentrations at salinity 30 compared to 40, while mussels accumulated lower levels overall. Multivariate analyses revealed species‐specific biochemical responses. Clams primarily mobilised energy reserves and detoxification pathways under moderate salinity, while at hypersalinity they shifted towards antioxidant defenses (SOD, CAT), indicating oxidative stress management. In contrast, mussels showed an energy‐conserving profile: at salinity 30 Gd depressed SOD and induced CbEs, while at salinity 40 both antioxidant (SOD, CAT) and conjugation (GSTs) activities were reduced. Integrated Biomarker Response Index version 2 (IBRv2) peaked at salinity 40 (with and without Gd), confirming salinity as the dominant driver of response. These findings demonstrate that salinity modulates Gd toxicity and highlight interspecific differences in resilience strategies. Overall, the results underscore the importance of integrating emerging pollutants and climate‐related stressors in ecotoxicological assessments to better predict risks to marine ecosystems.