Pteropods are tiny planktonic sea snails. They swim by flapping their wing-like feet and eat by straining phytoplankton and smaller zooplankton from the water. Many of them spend the daytime in deeper water and swim up to the surface at night to feed on other plankton when they are less likely to be eaten by fish. Pteropods are vulnerable to ocean acidification caused by increased carbon dioxide in the atmosphere because the acidic water can dissolve their calcium carbonate shells.