How do crustaceans obtain calcium carbonate?

Crustaceans obtain calcium carbonate primarily from their diet, which includes various marine organisms such as algae, mollusks, and detritus that contain calcium in the form of calcium carbonate. They also absorb dissolved calcium ions from seawater through their gills and other tissues. Once ingested or absorbed, crustaceans utilize these calcium sources to build and maintain their exoskeletons and other structures through biological processes such as calcification.