Combined impacts of a lipid lowering drug, gemfibrozil, and temperature on bioenergetics and digestive gland function of a marine bivalve Mytilus edulis

Pharmaceutical fibrates are increasingly detected in coastal waters, yet their organism-level effects under realistic thermal regimes remain poorly resolved. We tested how environmentally relevant gemfibrozil (GFB) exposure interacts with temperature to alter bioenergetics and digestive gland condition in the blue mussel Mytilus edulis spp. Adult mussels were exposed for 21 days to 15 °C or 20 °C with or without GFB (25 µg L⁻¹). We quantified clearance rate, assimilation efficiency, energy ingestion, oxygen consumption, ammonium excretion, and scope for growth (SFG), and assessed lysosomal membrane stability and structure, cathepsin D activity, intracellular neutral lipids, and histopathology of the digestive gland. Warming to 20 °C increased clearance rate (∼2.8-fold) and energy ingestion but reduced assimilation efficiency (from 93% to 57%); SFG was consequently higher at 20 °C. GFB and temperature interacted on respiration: at 15 °C GFB depressed oxygen consumption, whereas at 20 °C GFB elevated it; ammonium excretion was unchanged. GFB destabilized lysosomal membranes and led to an increase in the lysosomal volume and surface area, particularly at 20 °C. Histologically, GFB induced epithelial thinning and digestive gland atrophy, accompanied by loss of diverticula integrity and depletion of adipogranular cells at 20 °C. Overall, GFB acts as a metabolic disruptor in the mussels, impairing digestive gland health. Moderate warming partly offsets energetic costs by enhancing filtration and energy acquisition but does not reverse cellular and tissue-level damage in the digestive gland. These results indicate that warming coastal seas may buffer some functional consequences of fibrate exposure while allowing persistent sub-lethal pathology, with implications for long-term ecological risk to marine ecosystems from lipid-lowering drugs.