Coral Reefs: Types, Coral Bleaching, Importance, Coral Reefs UPSC

By BYJU'S Exam Prep

Updated on: November 14th, 2023

Coral Reefs, also known as ‘the rainforest of the ocean’, contribute significantly to the planet’s biodiversity. They are considered significant ocean ecosystems. The Coral Reefs have specifically come to light since the initiation of global warming and climate change happening around the world. Oceanic Coral Reefs are made up of communities of tiny living things. They are the subaquatic coral polyp-based structures that calcium carbonate holds together.

Coral reefs are an important topic in the UPSC exam as they fall under the section of Environment and Ecology in the UPSC syllabus. Candidates are expected to have a good understanding of the ecological significance, threats, and conservation measures. Candidates should focus on topics such as coral reef formation, coral bleaching, coral reef conservation strategies, and the impact of climate change on coral reefs. Familiarity with case studies and current developments related to coral reefs will also be beneficial for answering questions in the exam.

Coral Reefs

Coral Reefs are elementary sedimentary rocks located above the sea and oceanic bed of continental shelves and mid-oceanic ridges. These organic sedimentary rocks are formed on the platform of shelves and ridges through a combined process that includes sedimentation, compaction, cementation, and solidification of the skeleton of coral polyps.

Coral Reefs, or massive undersea structures, comprise the exoskeleton of aquatic invertebrates called corals, hence the name coral reefs. The corals that lead to the formation of reefs are called “hermatypic” (hard) corals. The hard corals extract calcium carbonate from the seawater to build a robust exoskeleton to shield their fragile, sack-like bodies.

Soft corals are another distinct species that do not support the creation of reefs in any manner. Instead, these flexible corals, which include fishes like sea fans and sea whips, frequently imitate plants and trees.

Types of Coral Reefs

Charles Darwin gave the classification of coral reefs in the year 1831. This classification is based on its nature, composition, and mode of occurrence. The types of Coral Reefs are:

Fringing Reef

The fringing reefs are mainly found near the coast of a volcanic island.

  • This bordering reef may reach 200 meters into the sea from the coast.
  • The active proliferating zone of fringing reefs is the reef front or the reef edge, which faces the ocean.
  • There is a shallow water channel that is approximately 50-100 meters wide that runs between the coast and the reef front.
  • Calcium carbonate, coral sand, mud, living and dead coral clusters (also called polyps), or the other organisms that make up most of this reef.
  • Examples of fringing reefs include the reef in South Florida, Mehetia Islands, and the Sakau Islands in the New Hebrides.

Barrier Reef

The barrier reef is situated at some stretch away from the coast.

  • It spreads for up to 2000 kilometers and is 15 to 20 kilometers offshore.
  • A Lagoon, a deep body of water, divides the reef from the beach. It is safe to navigate across the lagoon.
  • The most suitable example of a barrier reef is the Great Barrier Reef in Australia. It is also the largest barrier reef in the world.


Atoll is often referred to as a coral island or lagoon island.

  • This reef encircles a lagoon in the shape of a horseshoe or ring.
  • It is highly deep and is also very apt for navigation purposes.
  • To allow for the free flow of water, the ring-shaped reef might be breached in a few areas.
  • The Pacific Ocean has more of these reefs than any other ocean.
  • Examples of atolls include the Atoll of Fiji, Trent Atoll in the Western Carolinas, Suvadiva in the Maldives (the largest atoll in the world), and Funafuti Atoll of Ellice.

Types of Coral Reefs

How are Coral Reefs Formed?

Theories on the evolution and development of coral reefs can be classified into three categories: Subsidence Theory, Glacial Control Theory, and Standstill Theory. However, two of these theories hold more significance:

  • Darwin’s Subsidence Theory:
    • Reefs begin as fringing reefs along the steep shores of islands.
    • Over time, as the reef flat inside the lagoon subsides, fringing reefs develop into barrier reefs.
    • Eventually, the sinking and disappearance of the island lead to the formation of atolls, with a central lagoon surrounded by a barrier reef.
  • Glacial Control Theory by Daly:
    • During the last glacial era, the development of ice caps caused a decrease in sea levels by 60-70 meters.
    • The cold temperatures prevalent at that time favored the growth of ice, while the subsequent melting and temperature increase created favorable conditions for reef formation.
    • This process contributed to the creation of atolls and barrier reefs.

Environmental conditions necessary for the formation of coral reefs include:

  • Hard Surface: A solid underwater platform, such as the continental shelf or mid-oceanic ridges, is required for the compaction and solidification of coral polyps’ skeletons.
  • Depth: The submarine platform’s depth should not exceed 80 meters from sea level to ensure sufficient sunlight for photosynthesis by algae.
  • Warm Oceanic Water: Coral polyps thrive in warm water with temperatures around 20 degrees Celsius. The east coast of continents tends to have extensive coral reefs, except in the case of India, where sediment deposits hinder their growth.
  • Sediment-Free Water: Coral reefs prefer relatively sediment-free water, as excessive sediment can impede respiration and cause early death of coral polyps. The west coast of India is dominated by coral reefs due to lower sediment levels.
  • Nutrient Supply: Adequate nutrient supply is essential for the healthy development of coral polyps. Seaward sides of reefs benefit from oceanic waves that facilitate nutrient delivery.
  • Mild Salinity: Optimal salinity levels support the growth and development of coral polyp skeletons, as they extract calcium from seawater for protection.

Coral Reef Sites in India

Corals can be found in different regions in the oceans around the world. The clean, shallow waters in the tropical and subtropical regions or where one can find the richest Coral Reefs. The Great Barrier Reef in Australia, the largest of these coral reef systems, stretches for more than 2400 km (1500 miles). Below are the places where one can find major Coral Reefs in India.

  • Lakshadweep Islands: The island has developed a coral reef that encircles the coastline, which is extremely close to the coastline. There are about 78 different species of coral, as well as fishes, crabs, bivalves, brittle stars, sea stars, sea urchins, sea cucumbers, and sea green turtles.
  • Andaman and Nicobar Islands: One of the world‘s richest and most magnificent marine creatures calls it home. There are about 560 different coral species in this area. Shells, whales, dugongs, saltwater crocodiles, sea snakes, dolphins, sea turtles, and other marine species can also be seen here. Additionally, there are soft corals, wire corals, boulder corals, boomerang corals, and red and brown algae. This is also the largest of all the Coral Reefs in India.
  • Gulf of Kutch: In the Gulf of Kutch, a marine sanctuary has been established for Pirotan islands and 42 other nearby islands. It is 58 km in width and 170 km broad. The sanctuary covers a total area of 162.89 square kilometers. The Coral Reefs in Azad, Pirotana, Narara, and Positara are among the best reefs.
  • Netrani Islands in Karnataka: The Netrani Islands are located in the Arabian Sea, 18 kilometers from Murudeshwar. It boasts some of the best diving experiences on the west coast due to the rich corals. It is home to approximately 69 species of corals.
  • Gulf of Mannar and Palk Bay: On the Southeast coast of the state of Tamil Nadu are present the Gulf of Mannar and Palk Bay. The 140 km long region connecting Rameshwaram and Tuticorin is where the reef deposits in the Gulf of Mannar are located. The varieties of corals found here are of the shore, platform, patch, and fringing types, which include a total of 117 different types of coral species. Palk Bay, separated from the Gulf of Mannar by the Mandapam Peninsula, is located on India’s southeast coast.

Importance of Coral Reefs

Coral Reefs are supremely beneficial for our marine ecosystems. Some of its benefits are listed below:

  • Reefs are the most diversified and valuable ecosystems on the earth.
  • Coral Reefs’ ecosystem services include the protection of shorelines, fisheries, and eco-tourism.
  • Reefs are crucial for safeguarding the coastline from storm surges and flooding.
  • Coral Reefs are the building blocks for intricate food webs and our home to various fish.
  • This web’s foundation comprises a variety of invertebrates, soft corals, sponges, and algae.
  • Corals also act as water purifiers. Most corals and sponges are filter feeders, i.e., they eat suspended particles in the water.
  • Corals contribute to the ocean by catalyzing nutrient cycling, carbon and nitrogen fixation, and various other processes.

Laws for the Protection of Coral Reefs

The marine reserves of India are safeguarded under the Coastal Regulation Zone (CRZ) of 1991. Building hotels or resorts on Coral Reefs is strictly prohibited under section 7(2) of the CRZ. In addition, some states forbid coral reef mining, other than the exception of scientific research.

The standards and legislation safeguarding coral reefs are made and appropriately enforced by the Ministry of Environment, Forest, and Climate Change. If the coral reef region is included in a protected area, the State Wildlife Department is responsible for maintaining the corals.

Coral Bleaching

Coral Bleaching is when the white calcium carbonate exoskeleton is visible through the transparent tissue. The loss of symbiotic algae called zooxanthellae, which distributes 90% of the nutrients synthesized by it to the coral, is a result of extreme environmental stress. This leads to coral bleaching. Corals will eject the algae (zooxanthellae) dwelling in their tissues if the water is too warm, turning the coral white.

Factors Responsible for Coral Bleaching

Numerous dangers to coral reefs are caused directly or indirectly by human activity. By 2050, all coral reefs are expected to be threatened, with 75% of those hazards being of high risk. Some of these dangers include:

  • Global Warming: Rising temperature of oceanic water due to global warming.
  • Ocean Acidification: Adverse changes in the pH value of oceanic water due to acid rain and atmospheric absorption of CO2 by oceans.
  • Marine Pollution: rising marine pollution and oil spills.
  • Use of explosives for the mining.
  • Ozone depletion results in an increase in solar radiation.
  • Algal bloom.
  • Increase in sedimentation in seas and oceans from rivers due to deforestation.
  • Disease outbreaks in Coral Reefs also affect the chances of their survival.

Threats on Coral Reefs

Ecosystems of coral reefs are in grave danger. Natural dangers include illnesses, predators, and storms. Human activity is also a concern, as seen by pollution, sedimentation, destructive fishing methods, and climate change, which are increasing ocean temperatures and generating ocean acidification.

  • Corals cannot generate the calcium carbonate exoskeletons that they need for their protection due to the sea’s growing acidity brought upon by the oceans’ absorption of massive amounts of carbon dioxide emitted into the atmosphere due to the burning of fossil fuels.
  • Coral Reefs are also suffering from water contamination. The growth of corals is hampered by agricultural fertilizers and pesticides, petroleum products, sewage effluent, and eroded soil.
  • Both on land and in marine ecosystems, invasive alien species pose a threat. Ballast water is one of the primary sources via which invasive alien species are disseminated in aquatic ecosystems.

Coral Reefs UPSC

The topic of coral reefs is covered under the Environment and Ecology section of the UPSC Syllabus. Candidates must study and understand the basics of coral reef formation, the ecological functions they serve, coral reef conservation strategies, and the impact of human activities and climate change on coral reefs.

To prepare the topic of coral reefs for the UPSC Exam, candidates must focus on its conservation techniques, as it has been recently in the news. For the prelims part, they can focus more on the types of coral reefs, along with the famous coral sites in India. Referring to the Environment and Ecology Notes for UPSC will help a lot during the revision time.

Coral Reefs UPSC Questions

To excel in the topic of Coral Reefs, candidates should practice UPSC previous year’s question papers to familiarize themselves with the question patterns. This will help them follow the trend and efficiently answer questions within the given time limit. It is crucial to note that questions related to coral reefs can be expected in both the UPSC Prelims and mains exams.

Question: Which of the following factors is crucial for the growth and survival of coral reefs? (A) Low water temperature, (B) High salinity, (C) High nutrient content, (D) High turbidity

Answer: (B) High salinity

Question: Which of the following types of coral reefs is located closest to the shore, parallel to the coastline? (A) Barrier reef, (B) Atoll reef, (C) Fringing reef, (D) Patch reef

Answer: (C) Fringing reef

Question for Mains: How do coral reefs work? Discuss their origin. Additionally, describe the ecological benefits that coral reefs provide.

Question for Mains: Analyze the major threats to coral reefs and their associated ecosystems, and suggest measures for their conservation and sustainable management.

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