Persistence under ocean acidification is often framed as long-term adaptation, yet on the timescales over which seawater chemistry is changing, organismal persistence may depend on rapid physiological adjustment. Distinguishing such plasticity requires manipulative approaches that natural CO₂ vents make possible. At the Levante Bay vent system (Vulcano Island, Mediterranean Sea), where the symbiotic sea anemone Anemonia viridis thrives under chronically low pH, we tested this hypothesis using a reciprocal transplant experiment. Anemones from an ambient pH site (~8.1) and a low pH site (~7.5) were transplanted in both directions, with within-site controls, and assessed before transplantation and after 24 h and 5 d. Net and gross oxygen production and respiration were quantified from O₂ concentration changes in closed benthic chambers, while symbiont photosynthetic efficiency (Fv/Fm) was measured by PAM fluorometry. Performance tracked the pH conditions experienced after transplantation. Anemones moved to low pH increased net and gross oxygen production within 24 h relative to controls (p = 0.030), and this pattern persisted after 5 d (among-group Kruskal-Wallis p = 0.024), whereas those moved to ambient pH showed reduced net production (p = 0.029 at 24 h). Respiration was unaffected, indicating that the response involved production rather than respiration. PAM fluorometry after 5 d corroborated this pattern, with photosynthetic efficiency significantly higher at the low pH site (p = 0.004) and, among ambient-origin individuals, in those transplanted to low pH than in their controls (p = 0.016), supporting a rapid adjustment of symbiont performance to local pH conditions. These rapid responses indicate that A. viridis adjusts its performance to ambient carbonate chemistry through phenotypic plasticity, consistent with relief of carbon limitation in the symbionts at low pH. Rapid adjustment of a key symbiotic function may therefore contribute to the persistence of Mediterranean anemone-zooxanthellae symbioses as the ocean continues to acidify.