Why coral reefs hold the ocean together

A coral reef looks like rock, but it is built by animals. Each coral polyp is a small, soft-bodied creature that lays down a skeleton of calcium carbonate. Over centuries, millions of these skeletons stack into the structures we call reefs.

Most reef-building corals live in partnership with microscopic algae that sit inside their tissue. The algae photosynthesise and pass sugars to the coral; the coral offers shelter and nutrients in return. That exchange is why reefs grow in clear, sunlit, shallow water.

A city, not a garden

The value of a reef is its architecture. Caves, overhangs and branches create thousands of small spaces, and each space is a home for something: juvenile fish, crabs, shrimp, octopus, eels. Remove the structure and most of that life has nowhere to go.

Reefs also break waves before they reach the shore, protecting beaches, mangroves and the communities behind them. For coastal towns, a healthy reef is both a fishery and a sea wall.

What weakens them

Warm water stress, pollution running off the land, sediment and destructive fishing all reduce a reef's ability to recover. Reefs have always faced storms; the difference now is that the gaps between disturbances are getting shorter.