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What is a Coral Reef?

A coral reef is a marine ecosystem characterized by corals that create reefs. Reefs are made up of coral polyp colonies bound together by calcium carbonate. The majority of coral reefs are made up of stony corals, which have polyps that crowd together.

Shallow coral reefs, also known as sea rainforests, are home to some of the most diverse species on the planet. They cover less than 0.1 per cent of the world's ocean surface or about half the size of France, but they are home to at least 25% of all marine animals, including fish, molluscs, worms, crustaceans, echinoderms, sponges, tunicates, and other cnidarians. Coral reefs thrive in nutrient-depleted coastal waters. Coral reefs are most often seen at deeper depths in tropical oceans, although deep water and cold water coral reefs can be found on smaller scales in other regions. Let us take a look at different types of reefs.


Types of Reefs

It is classified into three types of coral: fringing, barrier, and atoll.

  1. Fringing Coral Reefs

  2. Barrier Coral Reefs

  3. Atoll Coral Reefs

Let us take a look at them in detail.

  1. Fringing Coral Reefs: One of the three major types of coral reef is a fringing reef. It differs from the other major groups, barrier reefs and atolls, in that it has either a completely shallow backreef zone (lagoon) or none at all. If a fringing coral rises straight from the shoreline, the reef flat stretches all the way to the beach, with no backreef. This is the most widespread kind of coral reef found in the Caribbean and the Red Sea. In a long-term reef development pattern, fringing reefs, according to Charles Darwin, are the first kind of reefs to develop around a landmass.

  2. Barrier Coral Reefs: Barrier reefs are isolated from the shores of a peninsula or island by a deep channel or lagoon.  Formal paraphrase They are similar to the later stages of a fringing reef with its lagoon, but they vary mostly in size and origin. Their lagoons can span many kilometres and range in depth from 30 to 70 meters. Over everything, the offshore outer reef edge formed in the open sea rather than along a shoreline. These reefs, including atolls, are thought to have evolved when the seabed lowered or the sea level rises. Barrier reefs are much rarer than fringing reefs and they take much longer to form.

  3. Atoll Coral Reefs: It is a ring-shaped coral reef with a coral surface that partly or entirely encircles a lagoon. Around the bottom, there may be coral reefs or cays. The creation of an atoll reef is described by Charles Darwin's subsidence model as the subsidence of a volcanic island around which a coral fringing reef has formed. The volcanic island became extinct and eroded over geologic time, eventually sinking entirely under the ocean's crust. When the volcanic island subsides, the coral fringing reef separates from the island, becoming a barrier reef. There are around 440 coral atolls on the planet.


Zones of Coral Reefs

Coral reef environments are divided into zones that house various types of habitats. 

The three main areas are usually recognized: 

  • Fore coral,

  • Reef crest, and

  • Back reef 

The three zones are connected both physically and ecologically. Oceanic cycles and reef life facilitate the exchange of seawater, sediments, nutrients, and aquatic life. The majority of coral reefs live in seas less than 50 meters deep. Some live on tropical continental shelves, such as the Great Barrier Reef, where cool, nutrient-rich upwelling does not occur. Others, such as the Maldives, are located in the deep ocean around islands or as atolls. Reefs form around islands as they sink into the water, and atolls form when an island sinks below the sea's level.


Reproduction in Coral Reefs

Corals reproduce sexually as well as asexually. During the course of its life, a polyp can use all reproductive types. Internal or external fertilization is used by corals to replicate sexually. The reproductive cells are located on the mesenteries, which are membranes that radiate inward from the stomach cavity's layer of tissue. Any mature adult corals are hermaphroditic, while others are either male or female. Only a few, change sexes as they mature.

Spawning happens like a mass orchestrated process along several reefs, where all coral species in an environment release their eggs and sperm at about the same time. Since male and female corals cannot come into reproductive interaction with each other, the timing of a broadcast spawning event is critical. Since colonies can be separated by large distances, this release must be specifically and widely timed, and it typically occurs in response to several environmental cues.

The less water that moves, the greater the chance of fertilization. The best time is in the spring. The release of eggs or planula is normally at night and is often in sync with the lunar cycle (three to six days after a full moon). The time between release and settlement is just a few days, but some planulae will float for several weeks.


Benefits of Coral Reefs

  • Coral reefs shield coastlines from hurricanes and flooding, provide employment for local residents and provide recreational opportunities.

  • They are source medicines and food. Reefs provide fuel, jobs, and security to over half a billion people.

  • On and around beaches, fishing, swimming, and snorkelling generate hundreds of millions of dollars for local companies. The annual net economic value of the world's coral reefs is estimated to be in the tens of billions of dollars.


Did You Know?

  • Coral reefs make up a small percentage of the ocean – less than 1% – but they are home to about 25% of all aquatic animals on the planet. Coral reefs are home to over 4,000 different species of fish!

  • Corals and algae have a symbiotic bond, so as the ocean temperature rises, corals remove their algae, causing them to white in colour and this process is bleaching and the coral is known as bleached coral. Many corals have been seen to emit bright colours in a vain effort to withstand rising ocean temperatures.

  • The Great Barrier Reef is the largest coral reef which is the only animal structure visible from space, stretching for 2300 kilometres.

FAQs (Frequently Asked Questions)

1. What is the Composition of a Coral Reef?

Ans: Coral reefs are massive aquatic habitats made up of the skeletons of coral, which are colonial marine invertebrates. A coral reef is a marine ecosystem characterized by corals that create reefs. Reefs are made up of coral polyp colonies bound together by calcium carbonate. The majority of coral reefs are made up of stony corals, which have polyps that crowd together. 

2. What Features Do Coral Reefs Have?

Ans: Coral reefs are shallow-ocean ecosystems teeming with marine life.

Corals love warm, clear, shallow water, and coral reefs can be found all over the tropics. Since various coral species live in different ocean basins, reefs in the tropical western Atlantic Ocean reefs can vary from reefs in the Indo-Pacific Ocean.


Coral reefs are formed by the calcium carbonate skeletons of corals, which are small immobile organisms similar to jellyfish. Individual corals can be very small, but they exist in communities of millions of other people, and over hundreds of thousands of years, they create reefs that are completely massive.

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