What substance forms the hard skeletons of most reef-building corals on the Great Barrier Reef?

Most reef-building corals on the Great Barrier Reef form hard skeletons from calcium carbonate. Individual coral polyps extract dissolved minerals from seawater and deposit the material as aragonite, a crystal form of calcium carbonate.

As colonies grow, successive generations add new layers to the old skeleton. Over very long periods, these accumulated structures create reefs, platforms, and other carbonate formations. The living coral occupies only a thin surface layer; much of a mature reef consists of older skeletal material and sediments.

Calcium carbonate is also the main material in shells made by many molluscs. It can dissolve more readily as seawater becomes more acidic, which is one reason ocean acidification is a concern for coral ecosystems. Coral skeletons are not made of bone, and the living animals themselves are soft-bodied polyps.