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  • 28 June, 2021

  • 20 Min Read

Everything about Coral Reefs

Coral Reefs

  • Coral Polyp is a living animal living in a symbiotic relationship with zooxanthallae algae.
  • Corals are actually white and they receive their colors and Energy from zooxanthallae algae.
  • Coral polyp provides Algae with protected Environment.
  • Productivity of Estuaries > Swamp, Marshes, Wetlands > Coral Reefs > Equatorial and Tropical rainforests > Savannah.
  • Coral reefs are CaCO3 (Limestone) material. Majority of corals are found in tropical and subtropical water, there are also deepwater corals in colder regions.
  • Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish.
  • Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral.
  • Most reefs grow best in warm, shallow, clear, sunny and agitated water.
  • Sometimes called rainforests of the sea, shallow coral reefs form some of Earth's most diverse ecosystems.
  • They are most commonly found at shallow depths in tropical waters, but deep water and cold water coral reefs exist on smaller scales in other areas.

Other terms

  • Cold Water Corals
    1. They inhabit deep, cold water. UNEP says that there are more cold water coral reefs worldwide than tropical reefs.
    2. The largest cold water coral reef is the Rost Reef off the coast of Norway.
  • Snowflake Coral: is a species of soft coral native to tropical western Atlantic Ocean. It is invasive species. It has emerged as a major threat to Gulf of Mannar, Gulf of Kutch and A&N Islands.
  • Retrotransposons: They are the specialized genes which may help Coral adapt more to heat stress.
  • Symbiodinium is a unicellular algae which gives Coral hosts photosynthetic products in return for nutrients and shelter.
  • According to UNEP, % of world area covered by each region: Indonesia 18%, Australia: 17% and Philippines: 9% so collectively that'll be more than 33%.
  • Staghorn Coral is Critically Endangered in IUCN. It's species declined by > 80% over last 30 years because of humans, disease and CC. It is vulnerable to bleaching.

  • Coral reefs deliver ecosystem services for tourism, fisheries and shoreline protection.
  • Coral reefs are fragile, partly because they are sensitive to water conditions.

Features of Coral Reefs

  • They occur in shallow tropical areas where the sea water is clean, clear and warm.
  • Coral reefs are among the world’s most diverse ecosystems, with more than 800 species of corals providing habitat and shelter for approximately 25% of global marine life.

Benefits of Coral Reefs

  • They protect coastlines from tropical storms, provide food and income for 1 billion people, and generate $9.6 billion in tourism and recreation each year.
  • Functions of Coral Reefs: Natural barriers against erosion and storm surge; captures nutrients; provide substrate for Mangroves and habitat for flora and fauna.

Growth conditions for Coral reefs

  • The temperature of the water should not be below 20°C. The most favourable temperature for the growth of the coral reefs is between 23°C to 25°C. The temperature should not exceed 35°C.
  • Corals can survive only under saline conditions with an average salinity between 27% to 40%.
  • Coral reefs grow better in shallow water having a depth less than 50 m. The depth of the water should not exceed 200m.

Types of Coral Reefs

Coral Reefs are differentiated into three categories based on their shape, nature and mode of occurrence.

  • Fringing Reef: The coral reefs that are found very close to the land and form a shallow lagoon known as Boat Channel are called Fringing Coral Reefs. The Fringing Reefs develop along the islands and the continental margins. They grow from the deep bottom of the sea and have their seaward side sloping steeply into the deep sea. Fringing Reefs are the most commonly found coral reefs among the three. For example Sakau Island in New Hebrides, South Florida Reef.
  • Barrier Reef: Barrier Reefs are considered as the largest, highest and widest reefs among the three coral reefs. They develop off the coast and parallel to the shore as a broken and irregular ring. Being the largest reef among the all, they run for 100kms and is several kilometres wide. Great Barrier Reef = It is World's largest coral reef system. Can be seen from space. World's biggest single structure made by living organisms. UNESCO WHS. It is in Queensland, Australia. Biggest coral reseeding project launched on Great Barrier Reef.
  • Atolls: An atoll can be defined as a reef that is roughly circular and surrounds a large central lagoon. This lagoon is mostly deep having a depth of 80-150 metres. The atolls are situated away from the deep sea platforms and are found around an island or on a submarine platform in an elliptical form. For example Fiji Atolls, Suvadivo in Maldives and Funafoothis Atoll of Ellice.

In India the types of Coral reefs are

  • Fringing Reefs in Andaman and Nicobar.
  • Barriers reefs in Gulf of Mannar and Gulf of Kachch (Kurumbar, Neora and Bhaidar Island).
  • Atolls are found in Lakshadweep and Nicobar.
  • A&N corals are very rich in species diversity followed by Lakshadweep Islands, Gulf of Mannar and Gulf of Kuchch.

Threats: They are under threat from excess nutrients (nitrogen and phosphorus), rising temperatures, oceanic acidification, overfishing (e.g., from blast fishing, cyanide fishing, spearfishing on scuba), sunscreen use, and harmful land-use practices, including runoff and seeps (e.g., from injection wells and cesspools)(PT).

Coral Bleaching

  • Coral Bleaching is the process when due to increased water temperature Coral hosts expels/ breaks the relation with Algae and thus it turns into white. It occurs when the zooxanthallae alge decline due to high temperature, solar irradiance, sedimentation etc.
  • When corals face stress by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae zooxanthellae living in their tissues, causing them to turn completely white. This phenomenon is called coral bleaching.
  • Xenobiotics: Zooxanthallae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants like Cu, herbicides and oil.
  • They can recolonize when the water temperature reduces on the basis of Ecosystem resilience.
  • Epizootics: Pathogen induced bleaching is Epizootics.
  • The pale white colour is of the translucent tissues of calcium carbonate which are visible due to the loss of pigment producing zooxanthellae.
  • Corals can recover if the stress-caused bleaching is not severe.
  • Coral bleaching has occurred in the Caribbean, Indian, and Pacific oceans on a regular basis.

The United Nations has reported that:

  • 70% of the Earth's coral reefs are threatened,
  • 20% have been destroyed with no hope for recovery,
  • 24% are under imminent risk of collapse, and
  • an additional 26% are at risk due to longer-term threats.
  • According to a recent report, if stern measures to bring down the greenhouse gas emission levels are not adopted urgently, then Australia’s Great Barrier Reef might be in danger of coral bleaching as frequently as every two years by 2034. Large scale coral bleaching in 2016 destroyed thousands of square kilometres of the Great Barrier Reef.
  • Hawaii became the first U.S. state to put curbs on the sale of sunscreens containing oxybenzone and octinoxate, which can cause coral bleaching.

Causes of Coral Bleaching?

  • Rise in Sea Temperature: Most coral species live in waters close to the warmest temperature they can tolerate i.e., a slight increase in ocean temperature can harm corals. El Nino elevates the sea temperature and destroys coral reefs.
  • Ocean Acidification: Due to rise in carbon dioxide levels, oceans absorb more carbon dioxide. This increases the acidity of ocean water and inhibits the corals ability to create calcareous skeletons, which is essential for their survival.
  • Solar radiation and ultraviolet radiation: Changes in tropical weather patterns result in less cloud cover and more radiations which induce coral bleaching.
  • Infectious Diseases: Penetration of bacterium like vibrio shiloi inhibits photosynthesis of zooxanthellae. These bacteria become more potent with elevated sea temperatures.
  • Chemical Pollution: Increased nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of organisms that compete with coral for space.
  • Increased Sedimentation: Land clearing and coastal construction result in high rates of erosion and a higher density of suspended silt particles which can
    1. smother corals when particles settle out (sedimentation),
    2. reducing light availability (turbidity) and
    3. potentially reducing coral photosynthesis and growth.
  • Human Induced Threats: Over-fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems also adversely impact corals.


  • Changes in coral communities can affect the species that depend on them, such as the fish and invertebrates that rely on live coral for food, shelter. Loss of such marine animals can disturb the entire food chain.
  • Declines in genetic and species diversity may occur when corals die as a result of bleaching.
  • Healthy coral reefs attract divers and other tourists. Bleached and degraded reefs can discourage tourism, which can affect the local economy.
  • Coral bleaching can cause large shifts in fish communities. This can translate into reduced catches for fishers, which in turn impacts food supply and associated economic activities.
  • Coral reefs protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion and flooding.

Efforts to save Coral reefs and other programmes

  • Coral Rehab Programme, 2002 implemented by Gulf of Mannar Marine NP: To recover coral reefs. It has recovered 8 sq. km area in GoM region, where coral reefs suffered bleaching and degradation due to climate change and high temperature.
  • Australia has started a campaign to save great barrier reef from coral eating crown of thorns star fish. 2 reasons: Agriculture runoff & pollution. Thus money will be spent as an incentive to farmers to reduce runoffs and vessels & divers targeting that fish.
  • STAPCOR 2018: International Conference on Status and Protection of Coral Reefs held at Bangaram coral Island of Lakshadweep. Theme: Reef for Life. 1st international year of Reef in 1997. Lakshdweep to establish International Atoll research center.
  • Kelps and coral reefs are composed of algae that grow in the shallow parts of the ocean in warm and sunny waters. However, kelp forest grows in nutrient-rich waters while corals can develop in low nutrient waters.
  • A bid to restore Coral reefs in Gulf of Kutch using Biorocks or Mineral Accretion technology
    1. Kurumbar, Neora and Bhaidar Island of Kutch has Coral reefs.
    2. Biorock is the substance formed by electro accumulation of minerals dissolved in seawater on steel stuctures that are lowered onto the sea bed and are connected to a power source. Biorock is kept within HTL - LTL of 8 m to 4 m respectively..
    3. When anode (positive) and cathode (negative) are placed on seafloor and current is flowing, Calcium ions combine with Carbonate ions to form Calcium Carbonate.
    4. In 2015, the same group restored bleaching coral species (staghorn corals) belonging to family Acroporidae that had gone extinct about 10000 years ago.

What is the news?

  • As a fabled wonder of the natural world, Australia’s Great Barrier Reef and its diversity of marine life ranging from corals to whales found a place on UNESCO’s World Heritage List in 1981.
  • Made up of a couple of thousand individual reefs off the continent’s northeastern coast, it has a geological history going back an estimated 23 million years to the Miocene epoch, and has survived many challenges.
  • The GBR is about 2,300 km long and extends across a breathtaking 346,000 sq. km. area, hosting an assemblage of fishes and invertebrates in the reefs, dugongs, green turtles and other species in seagrass meadows, and sharks, rays, anemones, sponges, worms and myriad other forms all of which need a delicate ecological balance to thrive.
  • Idyllic as it appears, the reef system faces severe environmental threats, and this year, the World Heritage Committee has sounded a warning by drawing up a resolution to inscribe the reef on the 'List of World Heritage in Danger'.
  • The Committee took note of the 2019 Outlook Report of the Great Barrier Reef Marine Park Authority, which says in no uncertain terms that the long-term state of the ecosystem has further deteriorated from poor to very poor.
  • At the heart of the crisis is climate change, which has led to three big events of coral bleaching in 2016, 2017 and 2020. UNESCO’s move to list the GBR as ‘in danger’ brings pressure on Australia’s government to review its record on responding to climate change.
  • As a continent that has recorded a rise in its average temperature by 1.4 degrees C since 1910, the devastating fires of 2019-20 were another wake-up call on climate change aggravating extreme events.
  • The World Heritage Committee resolution calls upon Prime Minister Scott Morrison’s government to heed the conclusions of the Outlook Report, particularly on accelerated action needed to address climate change with the Paris Agreement goals in focus.
  • The updated Reef 2050 Plan that the country is pursuing for conservation should incorporate this.
  • Further, the government should stop destructive impacts of human activity such as land-based and farm run-off that has polluted waters, coastal development and other commercial uses, it adds.
  • The World Heritage Centre, the administrative body, had sent a letter to Australia in 2019 raising concerns “about the approval of the Carmichael Coal Mine”, a controversial project with impacts for the reef and the climate, to which it got a response noting “that the project’s approval is subject to over 180 regulatory conditions and that compliance with these conditions will be monitored.”
  • The Outlook Report records harm to “the abundance and health of many species groups, including corals, invertebrates, some bony fishes, marine turtles and seabirds” from the rising sea temperatures and thermal extremes due to global warming.
  • It adds that since 2014, there has been widespread and significant declines in many coral species. In 2018, coral larvae declined by 89% averaged across the region, arising from consecutive bleaching events, as the adult broodstock was reduced.
  • Warmer temperatures led to “feminisation of green turtles originating from nesting beaches in the northern Region, potentially leading to significant scarcity or absence of adult males in the future”. Coral growth is also endangered by the proliferation of crown-of-thorns starfish, which consumes them.
  • Some relief is available from the reported recovery of humpback whales, and slow gain in southern populations of green turtles. Urban coastal dugong populations also show an improved breeding rate. But overall, the reef is under threat.
  • A magnet for eco-tourists, the vast expanse coloured by algal hues draws thousands annually. The report says that in 2015-16, tourism, fishing, recreational uses and scientific activities contributed an estimated $6.4 billion to the Australian economy, a rise of about 14% since 2011-12. Without resolute action on climate and pollution threats, though, all sectors stand to lose.

Source: TH

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