Limited post-stress recovery and constrained physiological resilience: Multi-biomarker evidence for metabolic trade-offs in a gastropod under ocean acidification.

Ocean acidification (OA), driven by increasing atmospheric CO2 absorption, represents a critical environmental stressor that disrupts hepatopancreas function in aquatic organisms, particularly affecting metabolic homeostasis and immune competence. This study employed an integrated multi-biomarker approach combining histopathology, integrated biomarker response (IBR) indices, and transcriptomics to systematically investigate the effects of short-term (14 d), medium-term (28 d) CO2-driven acidification exposure and a subsequent recovery period on the whelk Brunneifusus ternatanus. Hepatopancreatic cells suffered progressive structural and functional damage during 14-28 d of acidification, characterized by reduced hemocyte counts, disruption of rough endoplasmic reticulum membranes, and obscured cell boundaries. During recovery, damage was attenuated yet persistent, manifesting as intracellular matrix edema, hemocyte rebound, and restored membrane integrity-but cell boundaries remained altered relative to controls. The IBR index (integrating oxidative stress markers and hepatopancreas function enzyme indices) increased continuously during early and medium exposure, then decreased significantly during recovery yet remained elevated above control levels, indicating partial but incomplete physiological restoration. Transcriptomic analysis revealed time-specific molecular reprogramming: energy metabolism and mitochondrial function pathways were downregulated during early acidification and recovery stages, whereas medium-term exposure significantly activated immunity- and metabolism-associated pathways. These findings demonstrate that B. ternatanus mounts vigorous physiological regulatory responses to OA stress at a demonstrable energetic cost, characterized by immune-metabolic trade-offs and sustained metabolic reprogramming even after stress removal. This study provides evidence for assessing OA impacts on gastropod mollusks and contributes to predictive frameworks for evaluating marine ecosystem vulnerability under climate change.