Ocean acidification
One of the largest potential threats of ocean acidification to marine invertebrates is the corrosive properties of undersaturated waters with respect to calcium carbonate skeletons/shells, and a theoretical inability to calcify under these conditions. Ocean acidification is a lowered pH of ocean waters caused by increased carbon dioxide emissions into the atmosphere, which results in more CO2 dissolving into the ocean. This poses a threat to corals and calcifying macroalgae, two main calcifying groups that occur in reefs, as well as other shelled organisms and the organisms that depend on them. Corals and calcifying macroalgae such as coralline red algae and calcifying green algae are extremely sensitive to ocean acidification because they build their hard structures out of calcium carbonate. The sinking pH of ocean waters makes it difficult for these shelled creatures to produce enough calcium carbonate to build and maintain their skeletal structures. Echinoderms, such as sea stars and sea urchins, and mollusks, including squid and clams, as well as many other species are also at risk because of thinning shells and weakened skeletal structures. By targeting calcifying organisms, ocean acidification threatens the health of reef ecosystems as a whole.