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What are Coral Reefs and Coral Bleaching ?

What are Coral Reefs and Coral Bleaching ?


  • Coral reefs are calcium carbonate CaCO3 (limestone) structures. The builders of coral reefs are tiny animals called coral polyps.
  • On their death, the coral polyps leave calcium carbonate skeletons. These CaCO3 skeletons are arranged into layers by the living polyps.
  • A coral reef is built up of layers of these skeletons. These layers are covered ultimately by living polyps.

Requirements for the Survival of Coral Organisms

  • Coral organisms survive in warm waters of tropical regions. Some species may survive in temperate cold waters as well.
  • Corals live in water which is free from sediments and thus their habitat is not near estuaries.
  • Corals do not exist below 180 feet, because the microscopic algae which share symbiotic relationship with coral organisms require sunlight to carry out photosynthesis.

Symbiotic Relationship of Coral Organisms

  • A symbiotic relationship is a relationship in which both the species living together derive mutual benefit.
  • Coral organisms share a symbiotic relationship with Zooxanthellae algae which are microscopic in nature and live in the tissues of corals.
  • Zooxanthellae produce carbohydrates by photosynthesis.
  • These carbohydrates are consumed by coral organisms. On the other hand, Zooxanthellae algae living in corals receive more sunlight as corals live nearer to sea surface (rather than living on the sea floor).
  • This sunlight enables algae to carry out photosynthesis. Secondly, coral organisms provide safe environment for algae.
  • Thirdly, coral organisms release CO, which is consumed by algae to carry out photosynthesis.

Advantages of Coral Reefs

  • Coral reefs provide refuge to many marine species. Thus, coral reefs facilitate marine biodiversity. Due to high biodiversity, coral reef ecosystem is also called ‘Tropical Rain Forest of the Oceans’.
  • Coral reefs provide calm refuge to marine species from wave action.
  • Coral reefs trap nutrients dissolved in water bodies.

Classification and Location of Coral Reefs in India

Coral reefs are classified as follows:

  1. Fringing reef: A coral reef that lies close to the shore or is contiguous with the shore.
  2. Patch reef: A coral reef in the form of isolated patches.
  3. Barrier reef: A coral reef which is continuous and runs parallel to the coastline. Water body formed between reef and coast is called lagoon.
  4. Atoll: A coral reef which is circular or semi-circular in shape.

Coral reefs can be found in India in the following regions:

  • Andaman and Nicobar Islands
  • Lakshadweep Islands
  • Gulf of Mannar
  • Gulf of Kutch.

Coral Bleaching

  • Coral organisms are naturally white in colour. They acquire different colours as a result of algae living within their tissues.
  • Coral bleaching refers to the emergence of white colour or fading of colour of the corals due to the discharge of algae from their tissues.
  • Coral bleaching occurs when corals discharge algae from their bodies or when the rate of photosynthesis carried by algae decreases.
  • Corals are sensitive to changes in their environment. They can discharge algae due to various reasons such as:
  • Rise in temperature of water body
  • Sedimentation of water bodies
  • Water pollution
  • Eutrophication
  • Threat from invasive species
  • Disturbances in their habitat, etc.
  • If algae loss is prolonged, then eventually the coral host dies.
  • Death of corals affect marine ecosystem because corals constitute significant part of marine food chain and coral reefs created by corals provide safe refuge to many marine animals.
  • At present, coral bleaching takes place at a very fast pace. Coral bleaching is considered by scientists as evidence of climate change.
  • It has been spotted not just among corals of Indian waters, but also in Great Barrier Reef, the largest coral reef near Queensland, Australia.

Deep-water Corals

  • The habitat of deep-water corals, also known as cold-water corals, extends to the deeper, darker parts of the oceans, ranging from near the surface to beyond 2,000 m (6,600 ft).
  • Deep-water corals are most often stony corals, but they also include soft corals.
  • Like tropical corals, they provide habitat to other species, but deep-water corals do not require Zooxanthellae to survive.
  • While there are nearly as many species of deep-water corals as shallow-water species, only a few deep-water species develop traditional reefs.
  • Instead, they form ‘reefs, in the shape of ‘mounds, which more accurately describes the large calcium carbonate skeleton that is left behind as a reef grows and corals below die off, rather than as a living habitat and refuge. Thus, mounds may or may not contain living deep-sea reefs.
  • Unlike shallow-water corals, however, deep-sea corals do not need sunlight.
  • They obtain the energy and nutrients they need to survive by trapping tiny organisms in the passing currents.
  • They are far more widely distributed than scientists previously imagined.                      What are Coral Reefs and Coral Bleaching ?
  • They grow in all the world’s ocean basins. Scientists have even found deep-sea corals off the coast of Antarctica.


Environment & Biodiversity

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