Predicted pH decreases in ocean surface waters of ~0.3–0.5 and 0.7–0.8 pH units (for 2100 and 2300, respec- tively) are expected to negatively affect calcification processes and physiological performances of many marine organisms. Here we evaluated the response of important parameters such as growth, mortality, oxygen consumption, and mineralization of transplanted Pinna nobilis juveniles in the naturally acidi- fied waters of a CO2 vent system. Our field experiments show a general decrease of physiological responses of juveniles for the studied parameters along a decreasing pH gradient, even if significant effects are only observed under pH values of 7.6 units (“extreme” pH). In particular, the mortality rate in- creased from 10–30 % over the study period at control condi- tions to 60–70 % at extreme pH values. We conclude that near-future decreases in pH (decreases of 0.3–0.5 pH units) may not have a significant effect on performance of P. nobilis juveniles, while predicted longer-term decreases (decreases of​ 0.7–0.8 pH units) could affect the survival of the species. The combination of laboratory experiments with the assessment of naturally acidified environments can provide further insights into the threshold pH affecting the performance of vulnerable marine species.