Short-term heat stress adaptation in intertidal oysters (Crassostrea sikamea): Integrative biochemical,...

Marine heatwaves threaten intertidal bivalves, including oysters, which can tolerate moderate temperature increases but are vulnerable to extreme thermal events. The ability to mount a rapid molecular and physiological response is crucial for surviving sudden environmental changes. However, the molecular mechanisms underlying the immediate response upon the initiation of temperature change, including transcriptional and metabolic processes remain poorly understood. This study employed an integrated approach to investigate the response of Kumamoto oysters to sublethal high temperatures. We conducted a survival assay, biochemical analyses, and transcriptomic and metabolomic profiling on gill tissues collected at intervals from 0.5 h up to the onset of shell gaping behavior (3-4 h). Despite exhibiting minimal mortality below 45 °C, the oysters showed a survival rate of only 14 % after 1-h exposure at 45 °C, with any temperature beyond this proving lethal (0 % survival). Biochemical analyses revealed that oxidative stress was elevated in all temperature stress treatments, but antioxidant activity was only significantly induced after exposure to 39 °C. Transcriptomic analysis identified 4442 heat shock responsive genes with functions in energy, immune, and antioxidant processes. Metabolomic assessments identified 420 differentially abundant metabolites, among which associated with glycolysis and fatty acid oxidation pathways were particularly notable. The ten most upregulated metabolites were amino acids and their derivatives, such as N-acetyl-L-glutamic acid, acetylvaline, and N-acetyl-leucine. Conversely, the ten downregulated metabolites primarily pertained to fatty acid metabolism, including 11β-Prostaglandin F2α and Carnitine. Integrative analysis of transcriptomic and metabolomic revealed links between gene and metabolite expression, particularly in pathways related to amino acid biosynthesis, cysteine and methionine metabolism, and galactose metabolism. This study identifies candidate genes and metabolites for future research on marine heatwave adaptation, elucidates adaptive strategies of oysters to temperature fluctuations, and provides valuable data for the future of oyster aquaculture amid global warming challenges.