What is a coral reef?

A coral reef starts with corals: living animals that grow in warm tropical water. Most of them do best between 23 and 29 degrees Celsius. Together they build a massive calcareous structure. That structure is what we call a coral reef. As a result, it holds the greatest biodiversity of any community on earth.

The coral and its algae

Corals live in symbiosis with a single-celled alga called zooxanthellae. The alga photosynthesises inside the coral’s tentacles. In addition, it supplies the carbon the coral needs to build its own calcium skeleton. The corals themselves are modular animals called polyps.

What is a reef?

The word reef describes a three-dimensional structure that rises from the seabed. However, coral is not its only builder. Sponges, algae and marine worms shape it as well.

What is a coral reef?

In short, a coral reef is an underwater ecosystem that reef-building corals create.

The ecosystem is a group of corals that lay down rigid calcareous skeletons. They live in colonies of every shape and size. Moreover, they interact with other species as one functional unit.

Coral reefs rank among the most productive and complex ecosystems on earth. Only the top millimetre of a reef is alive. Everything below is skeleton, which other species use as habitat and food. As a result, no other marine system is this diverse. That is why people often compare a coral reef to a tropical rainforest.

How a coral reef forms

The reefs of the tropical seas form the largest living community in the ocean. Tiny, primitive animals from the group of cnidarians build them, and we know those animals as corals. Roughly 2,500 coral species can extract calcium carbonate from seawater and turn it into a limestone skeleton for their own protection. Polyps then multiply in one place and add layer upon layer of calcareous skeleton. Over time, therefore, those layers grow into large reefs where millions of plant and animal organisms live.

In other words, the coral reef is the richest living community we know. It is home to the greatest variety of plants and animals, ways of life and patterns of behaviour.

Put simply, the coral builds the reef. Coral is the engineering species: it creates an environment, and other species then move in.

NOAA Coral Reef Conservation Program

How you can help protect the coral reef

Life Below Water organises beach and river clean-ups, guest lectures and ocean events across the Netherlands. See what we do, find out how to get involved, or support the work through our eco shop.

Want the bigger picture? The United Nations explains what is at stake in Sustainable Development Goal 14: Life Below Water.