And they are one of the biggest carbon sinks rainbow-6-patriots-system-requirements on earth. They cannot survive in acidic water, because they can't form their alkaline shells. And besides that, they are the very bottom of the food chain that all other sea life depends rainbow-6-patriots-system-requirements on. Except these little guys are critical to a balance in the carbon cycle, that has supported life as we know it for hundreds of millions of years.
Calcium carbonate is alkaline, the opposite of acid. Shellfish and coral need rainbow-6-patriots-system-requirements calcium carbonite to make their shells. When CO2 disolves in seawater, carbonic acid is formed. Acidification of the oceans, as a result rainbow-6-patriots-system-requirements of the excess CO2 emitted by humans, is the evil twin sister of global warming.