Interesting Facts on Coral Reefs and Atolls

Corals are a kind of calcareous rock, chiefly made of the skeletons of minute sea organisms called 'polyps'. Coral reefs and atolls are formed due to the accumulation and compaction of the skeletons of these lime secreting organisms.
Created On: May 29, 2017 16:20 IST
Modified On: May 29, 2017 16:19 IST

Corals are a kind of calcareous rock, chiefly made of the skeletons of minute sea organisms called 'polyps'. Coral reefs and atolls are formed due to the accumulation and compaction of the skeletons of these lime secreting organisms.

Coral Reef

Conditions for Growth of Coral Reefs

1. The coral and the associated organisms and algae, which are the most common reef builders are confined to the tropical belt. The water temperature must not fall below 20°C and not exceed 35°C; the most favourable is 23°C to 25°C.

2. Corals can live only in saline water, and for their proper growth the average salinity should be between 27 to 40%.

3. For the growth of coral the depth of the water should not exceed 200m. Most vigorous growth is confined to shallow water less than 50 m. deep.

4. Corals also require sediment-free, clean water, which is disturbed by ocean waves and currents is beneficial for the corals.

5. In the open seas it is necessary to have platforms which may act as foundations for the corals. These platforms should not be deeper than 90 m.

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Classification of Coral Reefs

The coral reefs are classified on the basis of nature shape and mode of occurrence into three categories which are discussed below:

 (I) Fringing Reef

 Fringing Reef

Coral reefs that develop along the continental margins or along the islands are called fringing reefs. The seaward slope is steep and vertical while the landward slope is gentle. Such reefs are found near Rameshwaram in the Gulf of Mannar. Occasionally the fringing reef is separated from the shore by a shallow lagoon known as "Boat Channel" as found in Madagascar and Red Sea. For Example: South Florida reef, Mehetia Island, Sakau Island in New Hebrides.

(II) Barrier Reef

 Barrier Reef

It is the largest, most extensive, highest and widest reef among all types of coral reefs. It is formed off the coastal platforms and parallel to them. There is an extensive but shallow lagoon between the coastal land and the barrier reef. Generally barrier reefs encircle islands in an irregular and broken ring. The Great Barrier Reef of Australia is the largest barrier reef in the world.

(III) Atoll


A ring of narrow growing chorus of horse shoe shape and crowned with palm trees is called an atoll. It is generally found around an island or in an elliptical form on a submarine platform. There is a lagoon in the middle of a coral ring. They are more common in the Pacific Ocean. The circular ring is broken in a few places to allow the free flow of water. The depth of the lagoon is only a few metres with sand and limestone debris at the bottom. Example of Atolls: Fiji Atoll, Trent Atoll of W-Carolinas, Suvadivo in Maldives, Funafoothis Atoll of Ellice.

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Geographical Distribution of Coral Reefs

 Coral reef locations


1. Coral reefs- are limited to the tropical seas and are found almost entirely between latitudes 30°N and 25°S.

2. Rich growth of reefs is found off the eastern coasts of North America and Australia.

3. On coasts where the rivers bring large quantities of sediments from the land, corals are not found e.g. coasts of South America.

4. The coral reefs are most common in the Pacific and the Indian Ocean, due to their shallow, warm and clean water.

5. The most important area of coral reef growth lies in the seas off the east coast of Australia and in the Philippines.

6. Only a few coral reefs lie outside the Indo-Pacific tropical area, those of the Gulf of Mexico and the Caribbean Sea have some reefs.

Fauna or Zoogeographic Regions of the World

Importance of Coral Reefs

1. It is a source of food for millions of marine species.

2. It protects coastlines from storms and erosion.

3. It is a source of new medicines.

4. It assists in carbon and nitrogen fixing as well as helping with nutrient recycling.

Coral Reefs are the most diverse ecosystems on the planet and also considered the medicine cabinets of the 21st century because several medicines are developed to treat cancer, arthritis, human bacterial infections, Alzheimer’s disease, heart disease, viruses, and other diseases. Hence, it is very necessary to protect the coral bleaching which is happening due to environmental degradation.

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