Prized as a luxury treat in the best restaurants and a staple food in the human diet for thousands of years, oysters and mussels are now being threatened by rising levels of carbon dioxide. By 2100 some waters are expected to be corrosive enough to cause the shells of mussels to dissolve. Scientists have found that the buildup of carbon dioxide in the atmosphere is causing the oceans to grow more acidic as increasing amounts of the gas dissolve in sea water. This change is reducing the ability of shellfish to make their protective shells. Marine biologists warn that this could have a devastating effect on the ocean environment, as other creatures that eat shellfish will find food increasingly scarce while corals, which make reefs, will also be unable to build their hard external skeletons. Ocean acidification is also expected to affect the ability of fish to fertilize eggs.