Low-Dose H₂O₂ Priming Improves Performance Under Simulated Marine Heatwave Conditions in a Coastal Bivalve

Marine heatwaves are intensifying under climate change, imposing thermal extremes on marine ecosystems. Priming, a process in which prior sublethal exposure shapes responses, is increasingly recognized as a key mechanism underlying tolerance to environmental variability. While widely studied in plants, chemical priming remains unexplored in animals. Here, we tested whether hydrogen peroxide (H₂O₂), acting as a redox signal, can induce a primed physiological state modulating response to simulated marine heatwaves in the Manila clam (Ruditapes philippinarum), an ecologically and economically important bivalve species. Clams were exposed to chemical, thermal, or combined priming treatments and subsequently challenged with a controlled heatwave. H₂O₂ priming emerged as an effective and well-tolerated treatment, with primed clams exhibiting faster burrowing during heatwave exposure and minimal transcriptomic perturbation. Microbiota analyses revealed transient shifts and reduced relative abundance of opportunistic taxa (including Vibrio and Tenacibaculum) during and after heatwave exposure. A long-term field trial under natural summer conditions did not reveal detectable adverse effects on the measured traits over several months following priming. Overall, these findings identify low-dose H₂O₂ priming as a potential mechanism shaping stress responses in marine bivalves under simulated heatwave conditions.