Pteropods are small swimming snails with thin, delicate calcium carbonate shells. These animals are thought to be especially vulnerable to ocean acidification due to calcium carbonate’s susceptibility to dissolution at lower pH values. Victoria L. Peck and colleagues recently found that the periostracum (a protective coating on pteropods’ outer shells) prevents this dissolution when intact. Moreover, the team was surprised to discover that even when the periostracum is breached, pteropods can still mitigate damage by rebuilding the inner shell wall.