Coral Reefs

A coral reef is an offshore ridge which is composed mainly of calcium carbonate. It is formed from the secretions of small marine animals called coral polyps. Coral polyps thrive in shallow, warm, nutrient poor, tropical waters. These tiny animals attach themselves to objects such as rocks or existing reefs and build tiny shells of calcium carbonate around themselves. They sit in these shells and use their tiny tentacles to catch food. When coral polyps die, new coral polyps build their shells on top of the old shells of the previous generation. In this way the reef expands over time. Take a look at the cross section of a coral polyp below.

coral polyp

Diagram showing a cross section of a coral polyp. The polyp is shown sitting in its calcium carbonate shell with its  tentacles outstretched.

There are many different types of coral polyps. They form coral in a variety of shapes and colours. A coral reef which is made up of many different types of coral is a spectacular sight. Click here to view the coral gallery.

Watch this time lapse video of coral polyps emerging to feed.

Types of Reef

The CSEC/CXC syllabus requires students to study three types of coral reef; fringing reefs, barrier reefs and atolls.

Fringing Reefs

This type of reef forms along the shoreline in many tropical areas. The waters just offshore in tropical areas are often ideal for coral growth. Coral reefs may form in the water close to shore. There is usually a shallow lagoon between the reef and the shore. Many Caribbean islands have fringing reefs including Barbados, Antigua, Tobago and St. Lucia.

This fringing reef is found off the coast of Eilat, Israel

This fringing reef is found off the coast of Eilat, Israel

Barrier Reefs

This type of reef may be several kilometers from shore. Barrier reefs start off as fringing reefs. However, as sea level rises, the low lying areas near the coast are submerged. As the shoreline moves back, the distance between the reef and the shore increases. If the reef grows upward quickly enough to keep up with the rising sea level, it becomes a barrier reef. There is a wide lagoon or shallow sea between the reef and the shore. The world’s largest barrier reef is known as the Great Barrier Reef. It is found 50 to 250 kilometers off the coast of Australia and stretches for over 2,600 kilometers. It is made up of over 2,900 individual reefs and about 900 islands. The photo below shows a small section of this reef.

A small part of the Great Barrier Reef

A small part of the Great Barrier Reef

The world’s second largest barrier reef (the largest in the northern hemisphere) is found off the coast of Belize. It is known as the Belize Barrier Reef and it is just over 300 kilometers long. Parts of this reef are located about 40 kilometers from the shore. The picture below shows a section of this reef.

A small part of the Belize Barrier Reef

A small part of the Belize Barrier Reef


The National Geographic online resource library defines the term atoll as ” a ring-shaped coral reef, islet or series of islets”. This type of reef is found most often in the Pacific Ocean. There are about four known atolls in the Caribbean Sea, three of which are found off the coast of Belize.

Usually, the process of atoll formation begins when an undersea volcano rises above the surface of the sea forming a volcanic island. If conditions are favourable, a fringing reef will form around this island. Over millions of years, the island may recede into the ocean and disappear completely, leaving behind a ring shaped coral reef ( an atoll) surrounding a shallow body of water called a lagoon. The photo below shows Tikehau atoll in the South Pacific.

Tikehau Atoll (image by NASA)

Conditions which favour Coral Growth

Most coral polyps can only thrive in the following conditions:

  • The water should be warm. Most types of coral thrive in water temperatures between 21 and 30 degrees Celcius.
  • The water should be clear so that sunlight will be able to penetrate to where the coral is growing. Thus most coral grows in relatively shallow water.
  • The water should contain the right amount of salt. Coral will not grow in areas where a large amount of fresh water pours into the sea such as near the mouth of a river.
  • The water should be clean. Corals are sensitive to sediment and pollution. Muddy water damages coral. Organic pollution, such as sewage, promotes the growth of other organisms (such as seaweed or other algae) which may outcompete coral for space or other resources.

The Importance of Coral Reefs

A thriving coral reef is a colorful and spectacular sight. Reefs provide a home for many species of marine life including sponges, algae, sea urchins, shellfish and fish. Many tourists visit tropical areas hoping to observe and enjoy the beauty of the coral reefs and the creatures that live there. Therefore, a thriving coral reef is an asset to the tourist industry in many tropical areas.

The abundance of marine creatures that inhabit coral reefs make them very important to our fishing industry. There may be a hundred times more life on or near the reefs than out in the open ocean.

A coral reef can also help to protect the coast by acting as a natural breakwater. Waves encounter the coral reef before reaching the shore and are weakened as a result. Therefore, waves are less likely to erode beaches which are protected by coral reefs. The World Research Institute has determined that over 40% of St. Lucia’s coastline is protected by its coral reefs. Furthermore, coral is slowly broken down by wave action into tiny fragments. These tiny fragments make up much of the material on many of our beaches.

Coral reefs are very important natural resources. Therefore, it is very important that we protect our coral reefs and ensure that they continue to thrive.

The video below shows scuba divers exploring coral reefs just off the coast of St. Lucia. Notice the different types of coral and the many marine creatures which make the coral reefs their home.

Take the Coral Reefs Quiz!




Atoll or island. What is it?

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