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Day 85 – Q 1. What are coral reefs and how do they get formed? Discuss the global distribution of coral reefs.

1. What are coral reefs and how do they get formed? Discuss the global distribution of coral reefs.

प्रवाल भित्तियाँ क्या हैं और वे कैसे बनती हैं? प्रवाल भित्तियों के वैश्विक वितरण पर चर्चा करें।


Coral reefs are the most diverse of all marine ecosystems. Coral reefs cover less than 1 percent of the ocean floor — all the reefs combined would equal an area of about 285,000 square km. About 25 percent of all known marine species rely on coral reefs for food, shelter and breeding. They are also referred to as “the rainforests of the sea” for their biodiversity.


  • Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral. The coral species that build reefs are known as hermatypic, or “hard,” corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies. Other species of corals that are not involved in reef building are known as “soft” corals. These types of corals are flexible organisms often resembling plants and trees and include species such as sea fans and sea whips.
  • Each individual coral is referred to as a polyp. Coral polyps live on the calcium carbonate exoskeletons of their ancestors, adding their own exoskeleton to the existing coral structure. As the centuries pass, the coral reef gradually grows, one tiny exoskeleton at a time, until they become massive features of the marine environment.
  • Most corals, however, depend on algae called zooxanthellae to provide energy via photosynthesis. The corals have a symbiotic, or mutually beneficial, relationship with the zooxanthellae. These algae live inside the coral polyp’s body where they photosynthesize to produce energy for themselves and the polyps. 
  • The polyps, in turn, provide a home and carbon dioxide for the algae. Additionally, the zooxanthellae provide the coral with their lively colors — most coral polyp bodies are clear and colorless without zooxanthellae.
  • Deep-sea corals live in much deeper or colder oceanic waters and lack zooxanthellae. Unlike their shallow water relatives, which rely heavily on photosynthesis to produce food, deep sea corals take in plankton and organic matter for much of their energy needs.
  • Coral reefs are typically divided into four categories, according to CORAL:
  • Fringing reefs are the most commonly seen reef and grow near coastlines. 
  • Barrier reefs differ from fringing reefs in that they are separated from the coastlines by deeper, wider lagoons. 
  • Patch reefs typically grow between fringing and barrier reefs on the island platform or continental shelf.
  • Corals are colonial, the size of a colony can be large. Reefs, which are usually made up of many colonies, are much bigger still. Reefs form when corals grow in shallow water close to the shore of continents or smaller islands. It takes a long time to grow a big coral colony or a coral reef, because each coral grows slowly. The fastest corals expand at more than 6 inches (15 cm) per year, but most grow less than an inch per year. Reefs themselves grow even more slowly.
  • Reef-building corals are restricted in their geographic distribution by their physiology. For instance, reef-building corals cannot tolerate water temperatures below 18° Celsius (C). But there is a presence of cold water corals.
  • Many grow optimally in water temperatures between 23° and 29° C, but some can tolerate temperatures as high as 40° C for short periods. Most also require very saline (salty) water ranging from 32 to 42 parts per thousand, which must also be clear so that a maximum amount of light penetrates it. 
  • Corals are found all over the world’s oceans, from the Aleutian Islands off the coast of Alaska to the warm tropical waters of the Caribbean Sea. The biggest coral reefs are found in the clear, shallow waters of the tropics and subtropics. The largest of these coral reef systems, the Great Barrier Reef in Australia, is more than 2,400 kilometers.
TLP Phase 1 – Day 85 Synopsis

TLP Phase 1 – Day 85 Synopsis


Hundreds of millions of people rely on coral reefs for essential nutrition, livelihoods, protection from life-threatening storms and crucial economic opportunity. At the same time, about half the world’s shallow water coral reefs are already gone, and without urgent action to address climate change, pollution, overfishing and destructive coastal development, these life-sustaining natural wonders could all but disappear.

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