What is an example of mutualism in the Great Barrier Reef?

The Great Barrier Reef itself is an example of mutualism. The corals which create coral reefs such as the Great Barrier Reef have a symbiotic relationship with algae known as zooxanthellae. This algae, which lives within the tissues of the coral, causes a photosynthetic reaction which enables the polyps to make skeletons about three times faster in the light than in the dark. This means the corals deposit skeletons faster than the environment can remove it, and these skeletons build upon each other over time, creating a coral reef.