LAUSR

The pteropod Limacina helicina has become an important bioindicator species for the negative impacts of ocean acidification (OA) on marine ecosystems. However, pteropods diversified during earlier high CO2 periods in Earth history and currently inhabit regions that are naturally corrosive to their shells, suggesting that they possess mechanisms to survive unfavourable conditions. Recent work, which is still under considerable debate, has proposed that the periostracum, a thin organic coating on the outer shell, protects pteropods from shell dissolution. Here, we provide direct evidence that shows that damage to the L. helicina periostracum results in dissolution of the underlying shell when exposed to corrosive water for ∼8 d, while an intact periostracum protects the shell from dissolution under the same conditions. This important first line of defence suggests that pteropods are more resistant to OA-induced shell dissolution than is generally accepted.