Acute ammonia nitrogen stress induces hepatopancreatic oxidative injury and multi-omics detoxification responses in hard clam Meretrix meretrix

Total ammonia nitrogen (TAN) is a major water-quality stressor in intensive shellfish aquaculture and can accumulate in low-exchange benthic culture environments, yet the hepatopancreatic mechanisms by which marine bivalves respond to acute TAN stress remain unclear. The hard clam Meretrix meretrix was exposed to a TAN concentration gradient to evaluate acute survival and to an LC₅₀-level TAN challenge designed as a short-term aquaculture water-quality stress model for antioxidant, histopathological, transcriptomic and untargeted metabolomic analyses. Survival decreased with increasing TAN concentration and exposure duration, and the 96 h LC₅₀ was 11.686 mg L⁻¹ TAN. In the hepatopancreas, TAN induced a rapid but transient increase in antioxidant enzymes followed by reduced antioxidant capacity and increased lipid peroxidation. Histology showed loosened digestive tubules, enlarged lumina, increased vacuolization and gill injury. Transcriptomic and metabolomic analyses of surviving clams indicated that sulfur metabolism, cysteine and methionine metabolism, glutathione metabolism, peroxisome-related processes and ABC transporters were major response pathways. Integrated analysis suggested that sulfur-containing substrate turnover, glutathione-dependent redox buffering and transporter-mediated detoxification formed a coordinated defense network under acute TAN stress. These findings provide mechanistic evidence for understanding ammonia-related health risks in hard clam aquaculture and support early-warning assessment and water-quality management in shellfish culture systems.