What is the main body structure that reef-building corals secrete to form Great Barrier Reef habitats?

Reef-building corals secrete calcium carbonate skeletons that form the physical foundation of Great Barrier Reef habitats. The mineral is deposited mainly in the form of aragonite, a crystal form of calcium carbonate.

Individual coral polyps build tiny cups around themselves. Over generations, the accumulated skeletons of living and dead colonies create reef frameworks, ledges, crevices and sheltered spaces used by fish and invertebrates. The living coral occupies only a thin, active layer over much older material.

This process differs from shell-building by many molluscs, although some mollusc shells also contain calcium carbonate. Corals are animals, not plants; their reef-building ability depends on the polyps’ biology and their relationship with photosynthetic algae.