The effects of thermal stress on the morphology, physiological functions, gut microbiota, and metabolism of Penaeus vannamei under seawater and low-salinity conditions

Global climate change is increasing heat accumulation on the Earth's surface and raising temperatures in oceans and lakes, threatening aquatic ecosystem health. As a euryhaline species, Penaeus vannamei is temperature-sensitive, with its temperature adaptation mechanisms under varying salinities and temperature-salinity effects in complex environments remaining unclear. This study investigated the adaptability of P. vannamei to thermal stress under two salinity conditions (30 ‰ and 2 ‰). Results showed low salinity was less conducive to high-temperature adaptation, with heat stress decreasing low-salinity shrimp survival by 15.83 %. It also induced fluctuations in oxidative stress markers (SOD, MDA, ROMO1, and HSP90), osmotic regulation indicators (NKA-α, NKA-β, CLC, and CA), and digestive enzymes (AMS and chymotrypsin) under both salinity conditions. Moreover, due to temperature-salinity interaction, heat stress more readily induces physiological dysfunction and tissue damage in low-salinity shrimp, with 47 % reduced intestinal villus height. Furthermore, temperature and salinity interacted, with thermal stress more likely to cause physiological dysfunction and tissue damage in shrimp maintained at low salinity. Metabolomic analysis revealed that thermal stress induced changes in hepatopancreatic metabolic phenotypes at different salinities, with enriched differential pathways primarily related to amino acid and lipid metabolism. A total of 18 biomarker metabolites were identified. Additionally, thermal stress altered the richness, diversity, and composition of the intestinal microbiota, leading to the identification of 19 genus-level microbial markers. A correlation was observed between hepatopancreatic metabolites and intestinal microbiota composition. The study further identified a correlation between hepatopancreatic metabolites and intestinal flora composition. Our results indicate that low salinity may exacerbate high-temperature adverse effects on shrimp by impacting digestion, energy metabolism, oxidative stress, and intestinal physical-biological barriers.