Molecular responses and recovery patterns of the sea urchin Strongylocentrotus intermedius under individual and combined thermal and hypoxic stress.

Due to intensifying global climate change and coastal eutrophication, dissolved oxygen (DO) and temperature constitute crucial environmental factors modulating physiological and biochemical processes of echinoderms. Strongylocentrotus intermedius, a commercially valuable mariculture sea urchin, is increasingly threatened by high temperature and hypoxia. This study established four groups: control (NC), high temperature (HT), hypoxia (LO), and high temperature - hypoxia combined stress (OT), followed by 48-h temperature recovery and reoxygenation. We assayed time to first death (TFD) and median lethal time (LT50), conducted transcriptome sequencing of tube feet, and validated dynamic expression of genes after stress exposure and during the recovery phase via real-time quantitative polymerase chain reaction (RT-qPCR). Survival time under combined stress was significantly shorter than under individual stress. A total of 2391, 21552, and 2973 differentially expressed genes (DEGs) were identified in hypoxia, high temperature and combined stress groups, respectively. DEGs in LO group were enriched in hypoxia-inducible factor-1 (HIF-1) signal pathway and glycolysis/gluconeogenesis; DEGs in HT group were mainly enriched in antigen processing and presentation, and protein processing in endoplasmic reticulum; the apoptosis pathway was uniquely enriched in OT group. Genes such as L-lactate dehydrogenase gene (ldh), mitogen-activated protein kinase 15 gene (mapk15) and phosphatidate phosphatase gene (plpp1_2_3) were selected as key DEGs. Key DEGs were significantly upregulated under stress and reversibly downregulated during recovery. These findings clarify the molecular mechanisms of S. intermedius responding to complex stresses, providing potential molecular targets for breeding stress-resistant sea urchin strains.