The possible effects of ocean acidification on growth, organic tissue and protein profile in the bryozoan Myriapora truncata (Pallas, 1766) were studied in samples transplanted along a gradient of different pH conditions in an area of natural volcanic CO2 vents at Ischia Island (Tyrrhenian Sea, Italy). Living colonies from normal (mean pH 8.10), intermediate (pH 7.83) and low (pH 7.32) pH sites were investigated after intervals of 34, 57 and 87 d of exposure. M. truncata formed new and complete zooids at the normal site, whereas at the intermediate and low pH sites, neither partial nor complete zooids were produced. After 34 d at intermediate and low pH conditions, the organic cuticle which envelops the skeleton increased in thickness when compared to normal colonies, suggesting a protective role against dissolution of the high-Mg calcite skeleton. Significant changes in the protein profile and expression displayed by samples from intermediate and low pH conditions suggest that M. truncata makes an initial attempt to overcome the decrease in pH by up-regulating protein production but eventually, especially in the lowest pH condition, exhausts biochemical energy to maintain this rate of protein production, leading to eventual death.