Science
How does a coral reef form?
A coral reef works as a living structure built by tiny animals called coral polyps, which secrete hard skeletons that pile up over centuries. Living coral relies on algae inside it for food, creating a vibrant ecosystem that shelters a quarter of marine life.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how a coral reef works.
Step by step
- 1Coral polyps are tiny animals that build hard limestone skeletons.
- 2Over time these skeletons accumulate into massive reefs.
- 3Algae living inside the coral feed it through photosynthesis.
- 4Reefs shelter about a quarter of all ocean species.
- 5Heat stress makes coral expel its algae and 'bleach,' which can kill it.
Frequently asked questions
- Are corals animals or plants?
- Corals are animals — tiny polyps — though they host algae inside that make food by photosynthesis, blurring the line.
- What is coral bleaching?
- When stressed by heat, coral expels the colorful algae it depends on, turning white and starving; prolonged bleaching can kill the reef.
- Why are coral reefs important?
- They harbor a quarter of marine species, protect coastlines from waves, and support fishing and tourism for millions of people.