Other than shells, marine organisms also need carbon and calcium to form coral skeletons, echinoderm endoskeletons (such as in sea stars, sea urchins, and brittle stars), crustacean exoskeletons, calcified cell walls in certain red algae (e.g., Corallinales), foraminifera and pteropods shells, and the calcareous skeletons of planktonic animals. These structures are primarily composed of calcium carbonate minerals like calcite and aragonite, which provide support, protection, and structural integrity essential for their survival and ecosystem functions. Additionally, some bacteria and cyanobacteria precipitate calcium carbonate as part of their biogenic calcification processes.