How Do Coral Reefs Form?

Coral reefs begin when free-swimming coral larvae attach to hard substrate in shallow, warm, clear water. Each tiny polyp (typically 1-3 mm) secretes a cup-shaped calcium carbonate (limestone) skeleton around its base. As polyps reproduce by budding, new skeletons are built on top of old ones, gradually forming massive reef structures. The process is powered by zooxanthellae — photosynthetic dinoflagellate algae that live within coral tissue in a mutualistic relationship. The algae provide up to 90% of the coral energy through photosynthesis (which is why reefs need clear, sunlit water), and the coral provides the algae with shelter and nutrients. Reef growth rates average 1-25 cm vertically per year. The Great Barrier Reef has been building for about 20,000 years on older reef foundations. Coral bleaching occurs when heat stress causes polyps to expel their zooxanthellae — without the algae, the coral turns white and can die if conditions persist.