When larvae of Mediterranean pteropods were cultured in pH 7.82 water, they had malformations and less shell growth. At pH 7.51, the larvae did not make shells but survived and showed normal development. In a natural setting, the smaller shells or lack of shells would have both ecological and biogeochemical consequences. These results suggest that pteropod larvae, as well as the species dependent upon them or upon adults as a food resource, might be significantly impacted by ocean acidification. (Laboratory study)