The negative response of corals and mollusks to ocean acidification stemming from increasing atmospheric carbon dioxide levels and warming temperatures varies widely, which has perplexed scientists because they have not been able to pin down general patterns for how the shells and skeletons of calcifying sea creatures grow and dissolve under lower pH conditions. Studies led by Riccardo Rodolfo-Metalpa of the University of Plymouth, in England, now provide clearer results by showing that the animals’ tissues and organic skinlike layers play a major role in protecting their calcium carbonate structures (Nat. Clim. Change, DOI: 10.1038/nclimate1200). The researchers transplanted corals, limpets, and mussels in the ocean off the coast of Italy near Mount Vesuvius, where CO2 bubbles up from the seabed as a result of volcanic activity. The test area serves as a natural laboratory to see how the animals respond to acidifying conditions. They also studied animals exposed to 45Ca-labeled water in the lab, which allowed them to measure CaCO3 formation and dissolution rates under varying conditions. The team observed that the creatures generally calcify more quickly at higher CO2 levels, but their unprotected surfaces dissolve away faster at lower pH, especially in combination with warmer water temperature. The net detrimental decline in calcification depends on how much protection individual species have.