How Do Coral Polyps Build Reefs?

Each coral polyp is a soft-bodied animal, typically just millimetres across, related to jellyfish. It extracts calcium and carbonate ions from seawater to build a hard limestone cup (calyx) around its base. Colonies grow as polyps bud and divide, laying down layer upon layer of skeleton — a large reef may grow only 1–3 cm per year. Most reef-building corals host symbiotic algae called zooxanthellae in their tissue; these algae photosynthesise, providing up to 90% of the coral's energy and enhancing calcification. The Great Barrier Reef, the largest reef system, stretches over 2,300 km and is built from billions of these tiny organisms over roughly 500,000 years.