Pteropods, or “sea butterflies,” are sea creatures about the size of a small pea. Pteropods are eaten by organisms ranging in size from tiny krill to whales, and are an important source of food for North Pacific juvenile salmon. The photos above show what happens to a pteropod’s shell when it encounters seawater that is too acidic. The left panel shows a shell collected from a live pteropod from a region in the Southern Ocean where acidity is not too high. The shell on the right is from a pteropod collected in a region with higher acidity (Photo credits: (left) Bednaršek et al. 2012;16 (right) Nina Bednaršek).