Coral Reefs are the “Rainforests” of the ocean

Coral Reefs are the “Rainforests” of the ocean. Reefs are ecologically important ecosystems and have a high biodiversity that serves as a storage bank of rich genetic resources. They are a source of food and medicine, and they protect the coast from wave erosion.

Corals are marine animals related to jellyfish and anemones. Both colonial and solitary corals catch plankton (microscopic plants and animals) and other suspended food particles with arm-like tentacles, which feed a centrally located mouth. Most hard corals also host symbiotic algae, a long-standing and successful partnership. These algae provide them with an additional food source through photosynthesis. Coral reefs are formed by corals that secrete hard calcareous (aragonite) exoskeletons, giving them structural rigidity. These colonial “hard corals” form elaborate finger-shaped, branching, or moundshaped structures and can create masses of limestone that stretch for tens or even hundreds of miles.

Although corals have a wide distribution in the world’s oceans, the varieties that form reefs are typically restricted to relatively shallow, warm tropical waters between latitudes 30° north and 30° south. Clean, clear water is essential to their health. Once coral larvae settle on a hard substrate and become established, colonies can arise if conditions are suitable for growth. Given enough time, coral colonies become thickets. As coral thickets build upward on the skeletal remains of older colonies, a reef is established. Today, richly diverse coral reefs are found in the tropics along coastlines, on the margins of volcanic islands, and as isolated coral atolls.

coral reef map

There are two distinct regions in which coral reefs are primarily distributed: the Wider Caribbean (Atlantic Ocean) and the Indo-Pacific (from East Africa and the Red Sea to the Central Pacific Ocean).

coral reef coral reef coral reef coral reef

  • The diversity of coral is far greater in the Indo-Pacific, particularly around Indonesia, the Philippines, and Papua New Guinea. Many other groups of marine fauna show similar patterns, with a much greater diversity in the Indo-Pacific region.
  • Although they possess a smaller number of species the corals of the Atlantic are still unique, with few common species between the two regions .

coral reef map

The majority of reef building corals are found within tropical and subtropical waters. These typically occur between 300 north and 300 south latitudes. The red dots on this map show the location of major stony coral reefs of the world. Credit:NOAA

Coral reefs are found in about 100 countries. Coral Reefs are home to over 25 percent of all marine life and are among the world’s most fragile and endangered ecosystems. In the last few decades over 35 million acres of Coral Reefs have been obliterated. Reefs off of 93 countries have been damaged . When corals are stressed by high temperature, ultraviolet light or other environmental changes, they lose their symbiotic algal cells, and appear white (the white skeleton is actually visible through the transparent tissue). Depending on the intensity and duration of the stress, the corals may recover or die.

Tarawa, Kiribati

NOAA Image

If the present rate of destruction continues, 70% of the world’s coral reefs will be destroyed within the next few decades.

Coral Bleaching

Climate change will destroy the world’s great coral reefs within a century, according to a report by German and Australian marine scientists.Researchers say governments must take action now to reduce the emissions of gases such as carbon dioxide, which are thought to be behind a rise in average global temperatures.

bleached coral reef
A slight rise in temperature can bleach coral like this

The scientists combined their coral expertise with the latest climate models to project what is likely to happen to the world’s greatest reefs if global warming remains unchecked. Their study suggests the unique marine environments will increasingly become victim to a process known as coral bleaching.

A slight rise in maximum water temperatures – only one to two degrees – can stress the corals. This causes them to expel the microscopic organisms, known as zooxanthellae, which color their tissues and provide them with essential nutrients.

If the zooxanthellae do not return, the corals will die. In 1998 every reef system in the world’s tropical oceans were affected by some degree of bleaching. The report says the frequency and intensity of bleaching is set to rise.

The report’s lead author is Professor Ove Hoegh-Guldberg, an expert on coral bleaching at Sydney University. Coral reefs could be eliminated from most areas of the world by 2100, Even the world’s largest reef – the Great Barrier Reef off Australia – could be dead within 30 years unless measures are taken now to slow climate change.

Coral Reef Bleaching

What are Corals?

NOAA Image

Coral is a general term used to describe a group of cnidarians, which indicates the presence of skeletal material that is embedded in the living tissue or encloses the animal altogether. -National Oceanic and Atmospheric Administration, U.S. Dept. of Commerce. “Glossary of Coral Reef Terminology.”

NOAA Image

Corals themselves are tiny animals which belong to the group cnidaria (the “c” is silent). Other cnidarians include hydras, jellyfish, and sea anemones. Corals are sessile animals, meaning they are not mobile but stay fixed in one place.They feed by reaching out with tentacles to catch prey such as small fish and planktonic animals.

Corals are anthozoans, the largest class of organisms within the phylum Cnidaria. Comprising over 6,000 known species, anthozoans also include sea fans, sea pansies and anemones. Stony corals (scleractinians) make up the largest order of anthozoans, and are the group primarily responsible for laying the foundations of, and building up, reef structures. For the most part, scleractinians are colonial organisms composed of hundreds to hundreds of thousands of individuals, called polyps.


Corals live in colonies consisting of many individuals, each of which is called polyp. They secrete a hard calcium carbonate skeleton, which serves as a uniform base or substrate for the colony. The skeleton also provides protection, as the polyps can contract into the structure if predators approach. It is these hard skeletal structures that build up coral reefs over time. The calcium carbonate is secreted at the base of the polyps, so the living coral colony occurs at the surface of the skeletal structure, completely covering it. Calcium carbonate is continuously deposited by the living colony, adding to the size of the structure. Growth of these structures varies greatly, depending on the species of coral and environmental conditions– ranging from 0.3 to 10 centimeters per year. Different species of coral build structures of various sizes and shapes (“brain corals,” “fan corals,” etc.), creating amazing diversity and complexity in the coral reef ecosystem. Various coral species tend to be segregated into characteristic zones on a reef, separated out by competition with other species and by environmental conditions.


coral reef polyps

Most corals are made up of hundreds of thousands individual polyps like this one. Many stony coral polyps range in size from one to three millimeters in diameter. Anatomically simple organisms, much of the polyp’s body is taken up by a stomach filled with digestive filaments. Open at only one end, the polyp takes in food and expels waste through its mouth. A ring of tentacles surrounding the mouth aids in capturing food, expelling waste and clearing away debris. Most food is captured with the help of special stinging cells called nematocysts which are inside the polyp’ outer tissues, which is called the epidermis. Calcium carbonate is secreted by reef-building polyps and forms a protective cup called a calyx within which the polyps sits. The base of the calyx upon which the polyp sits is called the basal plate. The walls surrounding the calyx are called the theca. The coenosarc is a thin band of living tissue that connect individual polyps to one another and help make it a colonial organism.

As members of the phylum Cnidaria, corals have only a limited degree of organ development. Each polyp consists of three basic tissue layers: an outer epidermis, an inner layer of cells lining the gastrovascular cavity which acts as an internal space for digestion, and a layer called the mesoglea in between

coral reef


The diagram above shows the anatomy of a nematocyst cell and its “firing” sequence, from left to right. On the far left is a nematocyst inside its cellular capsule. The cell’s thread is coiled under pressure and wrapped around a stinging barb. When potential prey makes contact with the tentacles of a polyp, the nematocyst cell is stimulated. This causes a flap of tissue covering the nematocyst—the operculum—to fly open. The middle image shows the open operculum, the rapidly uncoiling thread and the emerging barb. On the far right is the fully extended cell. The barbs at the end of the nematocyst are designed to stick into the polyp’s victim and inject a poisonous liquid. When subdued, the polyp’s tentacles move the prey toward its mouth and the nematocysts recoil back into their capsules.


All coral polyps share two basic structural features with other members of their phylum. The first is a gastrovascular cavity that opens at only one end. At the opening to this cavity, commonly called the mouth, food is consumed and some waste products are expelled. A second feature all corals possess is a circle of tentacles, extensions of the body wall that surround the mouth. Tentacles help the coral to capture and ingest plankton for food, clear away debris from the mouth, and act as the animal’s primary means of defense.

coral polyps

coral polyps

Credit:University of Texas

While coral polyps have structurally simple body plans, they possess several distinctive cellular structures. One of these is called a cnidocyte—a type of cell unique to, and characteristic of, all cnidarians. Found throughout the tentacles and epidermis, cnidocytes contain organelles called cnidae, which include nematocysts, a type of stinging cell. Because nematocytes are capable of delivering powerful, often lethal toxins, they are essential to capturing prey, and facilitate coralline agonistic interactions

Most corals, like other cnidarians, contain a symbiotic algae called zooxanthellae, within their gastrodermal cells. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis. These include carbon dioxide, produced by coral respiration, and inorganic nutrients such as nitrates, and phosphates, which are metabolic waste products of the coral. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, they supply the coral with organic products of photosynthesis. These compounds, including glucose, glycerol, and amino acids, are utilized by the coral as building blocks in the manufacture of proteins, fats, and carbohydrates, as well as the synthesis of calcium carbonate (CaCO3). The mutual exchange of algal photosynthates and cnidarian metabolites is the key to the prodigious biological productivity and limestone-secreting capacity of reef building corals.


(Courtesy Scott R. Santos, of the State University of New York at Buffalo)

Zooxanthellae often are critical elements in the continuing health of reef-building corals. As much as 90% of the organic material they manufacture photosynthetically is transferred to the host coral tissue . If these algal cells are expelled by the polyps, which can occur if the colony undergoes prolonged physiological stress, the host may die shortly afterwards. The symbiotic zooxanthellae also confers its color to the polyp. If the zooxanthellae are expelled, the colony takes on a stark white appearance, which is commonly described as “coral bleaching”

Coral Reef Facts

  • Fact: Coral reefs are among the oldest ecosystems on Earth.
  • Fact: Coral reefs are the largest living structure on the planet.
  • Fact: Although coral reefs cover less than 1% of the Earth’s surface, they are home to 25% of all marine fish species.
  • Fact: 500 million people rely on coral reefs for their food and livelihoods.
  • Fact: Coral reefs form natural barriers that protect nearby shorelines from the eroding forces of the sea, thereby protecting coastal dwellings, agricultural land and beaches.
  • Fact: Without the existence of coral reefs, parts of Florida would be under water.
  • Fact: Coral reefs have been used in the treatment of cancer, HIV, cardiovascular diseases and ulcers.
  • Fact: Corals’ porous limestone skeletons have been used for human bone grafts.
  • Fact: It is estimated that coral reefs provide $375 billion per year around the world in goods and services.
  • Fact: If the present rate of destruction continues, 70% of the world’s coral reefs will be destroyed by the year 2050.



How Can You Help Save Coral Reefs?

This year, help us rescue the coral reefs of Indonesia — located in the heart of the Coral Triangle. The seas surrounding this archipelago contain the world’s highest fish and coral diversity. Indonesia’s waters are also home to whales, dolphins, dugongs and sea turtles.

However, threats such as destructive fishing practices and coastal development are straining marine resources beyond sustainable levels. With your help, we can protect Indonesia’s beautiful seascapes, have a lasting impact on conservation and Rescue the Reef!


Related Readings:

Types of Coral Reefs-Click Here



About Rashid Faridi

I am Rashid Aziz Faridi ,Writer, Teacher and a Voracious Reader.
This entry was posted in oceans and tagged , , . Bookmark the permalink.

7 Responses to Coral Reefs are the “Rainforests” of the ocean

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  3. joanna rose says:

    this is abt CORAL REEFS


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