03 Coral Reefs

Chapter -1



Coral is a cold rock type that mainly consists of the skeletons of small marine organisms known as “polyps”. Coral reefs and atolls are formed from the accumulation and pressure of the skeletons of these lime-secreting organisms. Coral reefs are vibrant, colorful, and multifaceted underwater ecosystems, and are home to many fish, invertebrates, algae (crabs, shrimps, starfish), and many more. These are underwater communities that never rest. As a diver or diver, you will dive into this dynamic underwater world. Corals first appeared more than 400 million years ago, and coral reefs as we know them today appeared less than 25 million years ago.

Coral reefs are ocean ecosystems that are the residence of living animals. These colonies are groups of individual animals called polyps. Polyps actually secrete a substance called calcium carbonate, which makes up the structure of the corals they live on. Corals live in an ecosystem with warm waters ranging from 21 to 29 degrees Celsius or 70 to 85 degrees Fahrenheit. They need a lot of light to grow; It grows best in water less than 23 meters or 75 feet deep. Light is essential for coral animals, especially with regard to photosynthesis. Within each coral polyp, there are single-celled algae called zooxanthellae.

Zooxanthellae use sunlight for photosynthesis, a process in which algae use sunlight to produce food and oxygen for a tumor. In turn, zooxanthellae receive compounds needed to maintain life and photosynthesis. Thus, the polyp and the zooxanthellae live in a relationship that is beneficial to both. Oceans cover 70 percent of the Earth’s surface, but less than 0.1 percent of the world’s oceans are covered with coral reefs. If all the corals were put together, the equivalent area would be half the size of France. 25 percent of all ocean species live on coral reefs. It is estimated that at least 100,000 species of nearly all species live on tropical coral reefs. (Critter type is the primary subdivision in the plant or animal kingdom) There are currently 4000 recorded species from 100 families of fish, and more have yet to be discovered.


Coral are huge underwater structures made up of colonial marine invertebrates skeletons known as corals reefs. The species of coral that build reefs are known as hermetic or “hard” corals because they extract calcium carbonate from seawater to create a hard and durable exoskeleton that protects their soft, sac-like bodies. Other types of corals that are not involved in reef-building are known as “soft” corals. According to the Coral Reef Alliance (CORAL), a nonprofit environmental organization, these types of corals are flexible organisms that often resemble plants and trees, and include species such as sea fans and sea whips.

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 favorable is 23°C to 25°C.

2. Corals can live only in saline water, and for their proper growth the average salinity are 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 need clean water without sediment, which is disrupted by ocean waves and currents, which is beneficial for corals.

5. On the high seas, platforms are needed to serve as a foundation for corals. These sites should not be deeper than 90 m.

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:

1. Fringing Reef: Coral reefs that develop along the outskirts of continents or along islands are called fringing reefs. The slope towards the sea is steep and vertical, and the slope towards the coast is gentle. These reefs are located near Rameshwaram in Mannar Bay. Occasionally the fringing reef is separated from the shore by a shallow lagoon known as the “Boat Canal” that occurs in Madagascar and the Red Sea. For example South Florida Reef, Mechetia Island, Sakau Island in the New Hebrides.

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2. Barrier Reef: It is the largest, widest, highest, and widest reef of any coral reef. It forms at and parallels to the offshore platforms. There is a vast but shallow lagoon between the foreshore and the barrier reef. Usually, barrier reefs surround islands in an irregular and broken ring. Australia’s Great Barrier Reef is the largest barrier reef in the world.

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3. Atoll: The ring of a narrow, growing horseshoe-shaped choir topped with palm trees is called an atoll. It can usually be found around an island or in an elliptical shape on a submarine platform. In the center of the coral ring is the lagoon. They are more common in the Pacific Ocean. The circular ring is broken in several places to allow water to flow freely. The lagoon is only a few meters deep with sandy and limestone debris at the bottom. Examples of atolls: Fiji Atoll, Trent Atoll in West Carolina, Suvadivo in the Maldives, Funafutis Atoll in Ellis.

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Regional distribution of plant kingdoms in the world

Geographical Distribution of Coral Reefs

Coral Reefs

1. Coral reefs are bounded by tropical seas and are almost entirely between 30 ° N and 25 ° S.

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

3. On the shores where rivers bring a large amount of sediment from land, corals are not found, for example. coast of South America.

4. Coral reefs are most common in the Pacific and Indian Oceans because of their shallow, warm and clear waters.

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

6. Only a few coral reefs are located outside the tropical Indo-Pacific region, there are some reefs in the Gulf of Mexico and the Caribbean.

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 ( ANWAR.S et al. Coral reefs, MAY 29, 2017).

Coral Reef Mapping and Reef Health Monitoring

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Figure: Distribution Of Corals along the Indian Coast.

Coral reefs are one of the most biologically diverse marine environments in the world. Coral walls grow in a geographical period and are about 200 million years old. Coral reefs reached their current level of diversity 5 million years ago. Coral walls play an important role in marine ecosystems and help plants and animals live in the ocean. From an environmental point of view, coral reefs are important because they are similar to tropical rain forests in terms of marine species diversity and biological productivity. The wide variety of animal and plant species that represents its system and genetic heritage that it represents has been at increasing risk over the past few decades. The coral wall enables the formation of corresponding ecosystems that allow for the necessary habitat, fisheries, and livelihoods. Also, coral reefs are important from a climatic point of view because they provide accurate long-term records of climate change and help us expand our knowledge of the season-to-climate change in many remote tropical oceans. Coral reefs exist in the Glass Bay, Gulf of Manna, Andaman and Nicobar Islands, Lakshadweep Islands, and Malwan. Coral eco-morphological maps were created using satellite images of the Andaman and Nicobar Islands, the Gulf of Glass, the Gulf of Manna, and the Malwan region: produced on 1: 25,000 scales.

These maps indicate the density, density, etc. of the corals and this will be the baseline information. It is proposed to bring such maps in periodic frequency to compare conditions and amounts of corals and to take appropriate measures to protect the coral ecosystem.

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Common clownfish

Coral reefs in India are one of the most ancient and dynamic ecosystems of India. The coral reefs not only provide a sanctuary to a myriad of marine life but also play a key role in protecting the coastline from erosion. India has about 7517 km of coastline including islands but the mainland coast is 6100 km. The Coral reefs in India are mainly restricted to the Andaman and Nicobar Islands, Gulf of Mannar, Gulf of Kutch, Palk Strait, and the Lakshadweep islands. All of these reefs are Fringing reefs, except Lakshadweep which are Atolls. Spotted corals are found along with the intertidal areas of the central west coast such as the Ratnagiri intertidal areas, the Gaveshani coast, etc. Hermetic corals are also found along the coastline from Kollam in Kerala to Enayam Putenturai in Tamil Nadu  (Vineeta Hoon. “Coral Reefs of India: Review of Their Extent, Condition, Research, and Management Status by Vineeta Hoon”. Food and Agriculture Organization. Retrieved 4 August 2020).

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Source :- https://media.springernature.com

Video Credit – pixabay.com/videos/coral-ocean-underwater-diving-5270/


1. ANWAR.S et al. Coral reefs, MAY 29, 2017.

2. (Vineeta Hoon. “Coral Reefs of India: Review of Their Extent, Condition, Research, and Management Status by Vineeta Hoon”. Food and Agriculture Organization. Retrieved 4 August 2020).

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