Bryozoan colonies were grown at a site in the Tyrrhenian Sea (Italy) where natural emissions of carbon dioxide associated with volcanic activity lower seawater pH to 7.76, simulating levels of ocean acidification predicted for the end of the 21st century. The colonies showed impaired growth and had fewer defensive polymorphs. In addition, corrosion of the skeleton was observed in both new and old zooids, and feeding zooids were slightly smaller but had larger orifices. This study corroborates that ocean acidification may promote the dissolving of carbonate skeletons, and it suggests that bryozoan colonies may respond to ocean acidification by adjusting the proportions of different types of zooids that they produce.