Molecular and physiological responses of black rockfish (Sebastes schlegelii) to short- and medium-term ocean acidification

Ocean acidification (OA) is one of the greatest threats to marine species, with widespread impacts on their physiological functions. However, the adaptive capacities of many marine species to OA and the underlying mechanisms remain unclear. In this study, we investigated the effects of short-term (4 days) and medium-term (30 days) CO₂ exposure (pH 8.0, 7.6, and 7.3) on black rockfish (Sebastes schlegelii), focusing on histopathological changes in gill tissues, ion transport biomarkers, oxidative stress indicators, and transcriptomic responses. The results showed that both short-term and medium-term OA induced significant morphological changes in gill tissues, including epithelial lifting, hyperplasia, hypertrophy, and lamellar clubbing, which are likely adaptive mechanisms for maintaining homeostasis. Both Na⁺/K⁺-ATPase and carbonic anhydrase (CA) activities increased significantly in both short- and medium-term exposure, while Ca²⁺-ATPase activity was elevated only in the short-term, suggesting differential enzyme regulation over time to sustain ionic balance. Additionally, oxidative stress indicators (superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA), reduced glutathione (GSH) and glutathione peroxidase (GPx)) were significantly elevated after both exposure durations, indicating that the antioxidant defense system was activated. Moreover, the integrated biomarker response (IBR) index further indicated that the stress response was more pronounced during short-term exposure. Transcriptomic analysis reveals significant alterations in pathways related to calcium signaling, cytoskeletal structure, energy metabolism, and oxidative stress following short-term exposure. In contrast, medium-term exposure leads to significant enrichment of pathways associated with cell-environment interactions, highlighting the molecular adaptations of S. schlegelii to OA-induced stress. These findings provide valuable insights into the mechanisms of OA tolerance in S. schlegelii and contribute to understanding the adaptability of marine species in future ocean environments.