Acidification of oceans is a serious threat to shellfish, including mussels, as it directly affects their ability to build and maintain their shells. Mussels use calcium carbonate to create their shells, but as ocean water becomes more acidic, there is less carbonate available for them to use. This makes it difficult for mussels to grow and repair their shells, leading to thinner, weaker shells that are more prone to damage and predation. The continued acidification of oceans presents a significant challenge to the survival and reproduction of mussels and other marine calcifying organisms.