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Coral reefs UPSC | Coral bleaching UPSC| Oceanography | Physical Geography

Polyps vs Coral vs Coral Reefs?

  • Polyps are tiny microscopic marine animals. 
  • When polyps die, their skeletons are left out.
  • These left-out skeletons made up of Calcium carbonate are called simply coral or coral reef.
  • These coral reefs grow higher and higher, and later coral island forms.
Based on the shape and location of the coral reef, it can be categorized into the following three types:
Coral reef UPSC

Types of Coral reefs

The following are types of coral reefs:

  • Barrier reef
  • Fringing reef
  • Atolls

Barrier reef:

  • Barrier reefs are separated from land by the wide width of water.
  • For example, the Great barrier reef of Australia. It is more than 1500 miles in length.

Fringing reef:

It is widely found in tropical water. It grows close to shore and extensively under submerges shallow water sea.


  • Atolls are circular horseshoe-shaped coral reefs.

From where do Polyps get food?

  • Most of the Polyps give habitat to algae called "Zooxanthellae" on their tissues.
  • Polyps and Algae have a mutual symbiosis relationship.
  • Polyps provide nutrients and habitat to Algae and help in the photosynthesis process.
  • Zooxanthellae supply photosynthesis products such as carbohydrates to coral polyps.
  • Zooxanthellae also provide beautiful colors to coral.
  • Many polyps also get some food from floating ocean minerals directly.

Favorable conditions for Coral polys:

The following are the favorable condition:

  • Ocean water temperature is between 23 to 25 degrees centigrade with some tolerance variation.
  • Average salinity is between 27 to 40 ppm.
  • Shallow water, not more than 200 meters. Favorable 50 m to 200 m.
  • Ocean water must be Mud free or sedimentation-free.
  • Nearly stable water.

Distribution of Coral reefs in India:

  • Fringing reefs:
    • Andaman and Nicobars
    • Palk Bay
    • Gulf of Mannar
    • Gulf of Kutch
  • Atolls reefs:
    • Lakshadweep islands
Marine resources distribution

Importance of Coral reefs:

  • It is also called the rainforest of the ocean because of its biodiversity.
  • Areas covered by coral reef is 0.1 % of the ocean cover but 25 % of marine species live in these areas only.

Coral bleaching:

  • When coral polys are under stress then they expel algae( Zooxanthalle color provider). After leaving the algae, coral became white; this phenomenon is called coral bleaching.
  • In 2016, a large scale of coral bleaching is seen in the Barrier reef.
  • As per UN reports, 70 % of coral reefs on the earth are threatened categories.

Cause of Coral Bleaching:

The following are the reasons:

  • Rise in seawater temperature.
  • Ocean acidification
  • UV radiation due to Ozone depletion
  • Pollution in the ocean
  • Increase of sedimentation in the ocean water.
Try to solve the following questions:
  • Give an account of recent observations on coral bleaching with reference to Clive Wilkinson's report. (UPSC 2014, 200 words, 15 marks)

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