Coral skeletons are the other key structures that need both carbon and calcium to form, in addition to shells.

What they are made of

  • Many marine organisms build hard parts from calcium carbonate, a mineral that uses calcium and carbon from seawater.
  • Corals use this calcium carbonate to create rigid skeletons that support the living coral polyps and form reefs over time.

Why corals need carbon and calcium

  • Reef‑building corals take in dissolved calcium ions and carbonate (a carbon‑containing ion) from seawater to precipitate calcium carbonate skeletons.
  • These skeletons provide long‑term structure and protection for the colony, similar to how shells protect mollusks.

Quick classroom-style answer

  • Question: As well as shells, what other parts of marine organisms need carbon and calcium to form?
  • Answer: Coral skeletons , which are made of calcium carbonate using calcium and carbon from seawater.

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