What Are Coral Reefs? - Facts & Types

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Coral reefs are some of the most complex biological communities on earth. Teeming with life, these economically and environmentally important ecosystems provide a home for many unique marine organisms.


Big cities are teeming with life. There is a wide variety of people to meet and places to go. Like underwater cities, coral reefs are bustling communities filled with many different kinds of plants and animals. They are complex structures that are home to some of the world's most beautiful and colorful marine life.

The 'buildings' that make up these reefs are called corals. But unlike the buildings in a city, corals are living animals. They are not just the structure of the reef, but are also living members of their communities. This is what makes reefs such diverse and unique habitats.

Coral reefs are generally found in warm, clear, shallow waters, such as tropical seas. Because corals are living organisms, the surrounding environment not only determines which species of coral can live in a reef but also where on the reef they can live. Different areas of the reef will support different species of coral and other marine organisms.

The water around a reef is not productive - it does not provide many nutrients. However, the reef itself is quite productive. Small organisms called zooxanthellae (pronounced zo-a-zan-thel-ee) provide corals with the nutrients they need to grow and build the reefs. The zooxanthellae do this through photosynthesis - turning sunlight energy into nutrients that can be used by the corals.

But this relationship between the corals and zooxanthellae is not a one-way street. Corals also produce waste products that are nutritious to zooxanthellae, mainly carbon dioxide and ammonia. This sharing of resources between the zooxanthellae and corals is called a symbiotic relationship. This special type of relationship is beneficial to each organism involved because both gain something and lose nothing. This symbiotic relationship is the key to the high productivity of coral reefs.


There are three categories of coral reefs. Reefs are divided into these categories based on their structure and their relationship to the surrounding geologic features.

Barrier reefs are reefs that are separated from the nearby land by a lagoon. The largest barrier reef in the world is the Great Barrier Reef off the coast of Australia. This reef is so large that it can be seen from space!

Atolls are circular reefs with a central lagoon. These reefs are formed on top of submerged volcano cones. Even though these reefs are circular, there is usually at least one break in the reef that allows the lagoon water to connect with the sea.

Fringing reefs are the most common type of reef. These are found close to newer volcanic islands. They are called fringing because they are found directly offshore from continents, providing a 'fringe' around the land.

reef types


Because coral reefs are so productive, they are home to an incredible variety of marine organisms. Hundreds of species of fish, plants, and other marine organisms live on coral reefs. The reefs provide an abundance of nutrients as well as shelter and protection from predators. Some organisms that you are likely to find in a coral reef include sea anemones, sea stars, sea sponges, giant clams, octopi, eels, sea urchins, barracudas, and clownfish.

Coral reefs not only provide safety for marine life but also help protect coastlines and shores from flooding. Reefs are economically beneficial, too. The clear waters, beautiful colors, and unique marine life attract tourists and divers from all over the world.

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