Transcriptomic, and Metabolomic Insights into Ammonia-Nitrogen Stress in the Hepatopancreas of Litopenaeus vannamei Acclimated to Low Salinity
High ammonia nitrogen stress markedly decreases the survival rate of Litopenaeus vannamei under low-salinity conditions. Nevertheless, most available studies have primarily focused on the ammonia nitrogen response of small-sized shrimp under single-stressor exposure or normal seawater salinity. The molecular regulatory mechanisms, metabolic reprogramming characteristics, and crosstalk between key signaling pathways in L. vannamei exposed to high ammonia nitrogen stress at low salinity remain unclear. To elucidate the stress response and its underlying molecular mechanisms of L. vannamei following low-salinity acclimation and subsequent high ammonia nitrogen challenge, shrimp with an initial body weight of approximately 15 g were gradually acclimated from 30ppt to 5ppt salinity and cultured for one week. Subsequently, the shrimp were subjected to high ammonia nitrogen stress at a concentration of 42.32 mg/L for 96 h. Dynamic monitoring of physiological responses to high ammonia nitrogen stress was performed, and the antioxidant and detoxification capacities of the hepatopancreas were further assessed. Three key time points (12 h, 48 h, and 96 h) after acute ammonia nitrogen exposure were selected for subsequent transcriptomic and metabolomic analyses. Based on the reference genome, transcriptomic analysis annotated a total of 25,305 genes, among which 1,239 were novel genes. Metabolomic profiling detected remarkable alterations in lipid metabolites and amino acid derivatives, represented by 2'-Deoxyguanosine, Guanosine, and L-pyroglutamic acid. Integrated transcriptome and metabolome analysis screened out three core functional pathways, namely secretion pathway, basal metabolism pathway, and immune-related pathway. Furthermore, 11 pivotal genes including solute carrier family 4, member 2 (SLC4A3), nicotinamide/nicotinate riboside kinase (NMRK1), gamma-glutamyltranspeptidase (Ggt1), chitinase (CHIA) and mitogen-activated protein kinase (bsk), together with 17 significantly correlated differential metabolites, were finally identified. Considering the actual frequently occurring concentration of acute high ammonia nitrogen stress in intensive estuarine aquaculture, the present study focused on large-sized L. vannamei. Such large-specimen shrimp are not only more sensitive to ammonia nitrogen stress but also have higher economic value. The findings of this study provide practical and targeted theoretical guidance for health regulation and ammonia nitrogen stress mitigation during the middle and late culture stages of high-density farming in estuarine regions.