Coral Reefs 101: Everything You Need to Know


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Quick Key Facts

  • Roughly 20 to 25 percent of fish, molluscs and crustaceans in developing countries come from coral reefs.
  • Corals grow an estimated 0.39 inches per year — one of the slowest growth rates of any animal on Earth.
  • A few square kilometers of coral reef takes roughly a million years to grow.
  • At nearly 135,136 square miles, Australia’s Great Barrier Reef is the largest structure in the world made by living organisms, and can be seen from outer space.
  • The Great Barrier Reef is actually made up of nearly 3,000 separate reefs.
  • Corals like strong currents because they circulate seawater quickly, keeping the temperature — especially at the surface — cooler and more stable.
  • Scientists have predicted that by 2055, 90 percent of coral reefs worldwide will experience severe annual bleaching.

What Are ‘Coral Reefs’?

Coral reefs consist of hundreds of thousands of coral polyps — marine animal invertebrates with hard calcium carbonate exoskeletons. Different species grow to form a variety of measurements and shapes — from the size of a pinhead to as large as a foot in diameter.

Coral colonies are habitats and breeding grounds for many marine species, including sharks, manatees, dugongs, fish, sea urchins, molluscs and sea sponges.

Types of Coral Reefs


Aerial photo of Ningaloo Reef in Western Australia. RichardALock / iStock / Getty Images Plus

There are three main types of coral reefs: fringing, barrier and atoll, the most common of which is the fringing reef. A fringing reef extends from the shore out into the ocean, forming a border along the coastline and nearby islands.

Fringing reefs grow more successfully on rising or stable shorelines. They consist of the reef flat, also known as the “back reef,” and the reef slope, or “fore reef.” The fore reef is the part closest to the sea, while the back reef makes up the widest portion of the reef. Corals in a fringing reef grow upward toward the ocean’s surface or outward in the direction of the sea.

The planet’s largest fringing reef is the Ningaloo Reef, which extends 155 miles along Australia’s western coast. It is home to 300 coral species, 500 fish species, 600 species of mollusks and a host of other sea life and marine invertebrates.


The largest type of coral reef, barrier reefs border the continental shelf like fringing reefs, but grow further out in a linear fashion, separated by a frequently deep lagoon of water. Shallow parts of the reef sometimes touch the surface, forming a “barrier” that can impede boat traffic.

Barrier reefs can be hundreds of miles long and several miles wide, but are not nearly as common as the two other main types of reefs.

Probably the best known barrier reef in the world, the Great Barrier Reef is located off the coast of Australia in the Coral Sea. The Great Barrier Reef is Earth’s largest coral reef system — rather than one reef, it is actually made up of nearly 3,000 separate reefs. It extends 1,429 miles over an area of nearly 135,136 square miles. The Great Barrier Reef is so extensive, it can be spotted from space.


Small islands on the Three Brothers group on the western Great Chagos Bank. Brian W. Bowen

An atoll is actually a fringing reef that once surrounded a volcano, but as the volcano sank below sea-level, the corals continued to grow, becoming the only thing visible from the surface.

Atolls are typically circular, oval or shaped like a horseshoe and have a sandy, shallow lagoon in the middle. The coral rim of an atoll may or may not entirely surround the lagoon, but if it does, little or no sea water will move in and out.

The 4,881 square-mile Great Chagos Bank, located in the Indian Ocean, is the largest atoll on the planet. It consists of the protected Eagle Islands, Danger Island, Nelson Island and the Three Brothers islands.

Why Are Coral Reefs Important? Why Do They Matter?

Support One-Quarter of All Marine Species

Fish swim among corals near the island of Ambon, Indonesia. Velvetfish / iStock / Getty Images Plus

It is estimated that a quarter of marine species are supported by coral reef ecosystems, though reefs do not even comprise one percent of the seafloor.

These cradles of biodiversity act as habitat, feeding, reproduction and nursery grounds for more than a million marine species.

According to the International Union for Conservation of Nature (IUCN), many of the species living within the coral reef ecosystem have been listed under the IUCN Red List of Threatened Species, including one-third of reef-building coral species, whale sharks, the giant clam and the hawksbill sea turtle.

Iridescent mantle of a giant clam in Micronesia, Palau. Reinhard Dirscherl / ullstein bild via Getty Images

Protect Coastlines From Storms, Flooding and Erosion

Coral reefs act as natural barriers that protect coastlines from erosion. Corals grow laterally across the seabed and up between the seafloor and the water’s surface. This stabilizes the seabed and absorbs wave energy and elements from the open ocean. Some coral reefs are capable of absorbing more than 95 percent of wave energy.

The buffer of a reef also reduces storm damage from cyclones, as well as some of the energy of tsunamis.

Reefs not only protect the shoreline and its human developments, but the ecosystems — like lagoons containing seagrass meadows — that lie between the coast and the reefs themselves.

Clean the Water

A great number of corals and sponges consume nutrients from particulate matter through the process of filter feeding. This helps prevent harmful particles from settling on the ocean floor and keeps waters clear.

Act as Cultural Heritage Sites

The indigenous Jarawas traditionally go in small groups to catch fish among the coral of the Andaman and Nicobar Islands in India. Thierry Falise / LightRocket via Getty Images

Corals have cultural importance for many of the world’s coastal communities, such as the Torres Strait Islanders with the Great Barrier Reef, and Hawaiians, for whom the coral polyp holds significance in traditional stories.

Even for those who don’t live close to the sea, coral reefs are associated with the vast and colorful scope of marine biodiversity on our planet. Visits to coral reefs result in millions of trips each year and billions in tourism dollars for local communities.

Provide Food, Income and Recreation for Humans

Roughly six million fishers count on coral reefs for the economic goods and services they provide. In the U.S., their annual commercial value is estimated to be more than $100 million, while the estimated economic value globally is $375 billion.

Hundreds of millions of people rely on coral reefs for their resources, protection and tourism. Many of the people who depend on the reefs are from island nations and developing countries and take food directly from the reef’s waters.

Roughly 20 to 25 percent of fish, molluscs and crustaceans in developing countries come from coral reefs, 70 to 90 percent in Southeast Asian nations and 10 percent worldwide.

Snorkelers and divers flock to coral reefs to admire the stunning array of colorful species, with more than 100 countries reaping the benefits of reef tourism.

A snorkeler with corals and tropical fish in Rarotonga, Cook Islands. chameleonseye / iStock / Getty Images Plus

For many small island nations, coral reefs are responsible for most of their new economic development.

Responsibly managed reefs that limit pollution and harm from tourists can help provide sources of income for developing coastal countries.

A Source of Medicine

Corals spend their lives fixed to one spot. In order to protect themselves, they have evolved chemicals to defend against other organisms threatening their space. Some of the chemicals have been studied by scientists for their medicinal benefits related to cell aging and certain types of cancer. The skeleton of a coral is similar to human bones, and has been used for bone grafts for decades.

Only a small number of reef organisms have been tested and analyzed, so there may be undiscovered pharmaceutical remedies hiding in coral reef waters.

Challenges Facing Coral Reefs

Ocean Warming & Acidification

The greatest threats to coral reefs worldwide are increasing ocean temperatures due to climate change and the resulting changes in ocean chemistry. Warmer temperatures are causing oceans to heat up, while atmospheric carbon gas dissolves into the water, causing acidification.

Atmospheric and oceanic carbon dioxide are in equilibrium with each other, so when carbon concentrations in the atmosphere increase, so do those in seawater. When carbon enters the ocean, it forms carbonic acid — a compound of carbon, hydrogen and oxygen — increasing the water’s acidity.

Roughly a quarter of the carbon emitted from humans burning fossil fuels is absorbed by the ocean. The acidity of the ocean has increased about 30 percent since the Industrial Revolution — the fastest rate in millions of years. By the end of the century, ocean acidity levels are predicted to be 40 percent higher than they are now.

When ocean acidity increases, the amount of ions and salts corals need to form calcium carbonate are reduced, leading to slower coral growth. The decrease in coral growth affects species to differing degrees, but severe acidification can cause some coral skeletons to dissolve.

Runoff can lead to nutrient enrichment, increasing the acidity of coastal waters in local areas and worsening ocean acidification.


Microscopic algae grow on corals in a symbiotic relationship, providing the corals with food. The algae are expelled by their coral hosts when they become stressed due to increased ocean temperatures. This puts further strain on the corals and exposes their white calcium carbonate structure — a process called coral bleaching.

Prolonged or severe bleaching can leave coral colonies vulnerable to additional threats like infectious diseases.

Research has shown that when corals are exposed to high carbon dioxide levels, their bleaching risk increases by as much as 50 percent.

Scientists have predicted that by 2055, 90 percent of coral reefs worldwide will experience severe annual bleaching.

A diver observes major bleaching on the coral reefs of the Society Islands in Moorea, French Polynesia on May 9, 2019. Alexis Rosenfeld / Getty Images

Human Activities

Since most coral reefs are near the shoreline in shallow water, they face numerous threats and are especially vulnerable to human activities. Many human actions that degrade corals are indirect — like pollutants, nutrients and microplastics — while others such as overfishing and coral harvesting are more direct.


Nature is about balance, so anything that involves the adding or subtracting of too many nutrients or resources disrupts its equilibrium — not just in the present, but it can lead to cascading effects and change the structure of the ecosystem.

Overfishing depletes populations of grazing fish who help keep corals from becoming overgrown with algae. An overabundance of algae is food for microbes and encourages their growth. The microbes then deplete corals’ oxygen and introduce diseases into their environment. Less corals mean more algae, which further endanger the remaining corals.

The use of explosives to kill fish — blast fishing — is an especially destructive practice that can physically damage not only fish, but corals too.


Stormwater and sewage that has not been treated properly, as well as livestock runoff, can result in pathogens ending up in the ocean. Parasites and bacteria from fecal contamination — though rare — can infect corals, particularly if they are already experiencing environmental stressors.

Healthy ecosystems do sometimes experience outbreaks of coral disease, but the addition of pollution containing pathogens can make the intensity and frequency worse.


Sediment from the agriculture sector flows with the rivers onto the Great Barrier Reef where the nutrients harm the corals in Queensland, Australia on Oct. 10, 2019. Jonas Gratzer / LightRocket via Getty Images

Excessive amounts of nutrients — such as phosphorus and nitrogen from sewage discharge, animal waste and residential and agricultural fertilizer and pesticides — can cause algal growth that takes away from corals’ oxygen supply, occludes sunlight and can lead to bleaching. This can cause an imbalance in the ecosystem as well as the growth of fungi and bacteria that can be unhealthy for corals.

Toxic Substances

Toxic substances like chemicals and metals — such as lead, mercury, oxybenzone, dioxin and polychlorobiphenyls (PCBs) — that come from sunscreens, industrial discharge and mining can affect the growth rate, feeding, reproduction and defense responses of corals.

Trash and Microplastics

A sea turtle swims by a plastic bottle on a coral reef in Australia. Philip Thurston / E+ / Getty Images

Microplastics can end up in waterways and the ocean from many sources, including old fishing gear, plastic bags, plastic bottles and the microbeads used in personal care products and cosmetics.

This marine debris can get caught on corals and damage or break them, block sunlight and kill or entangle reef organisms. Microplastics and beads can be eaten by corals, sea turtles, fish and other marine life, poisoning them with toxins and blocking their digestive tracts.

Habitat Destruction

Coastal development, ship groundings and anchors, quarrying, dredging, harmful fishing practices and the touching or removing of corals during recreation or harvesting can all lead to physical damage and destruction of coral habitat.


Agriculture, coastal development, forestry and urban stormwater runoff can cause the deposit of sediments onto reefs. This sedimentation can smother corals, as well as disrupt their ability to grow, feed and reproduce.

Coral Harvesting

Collecting live corals to be used to make jewelry and curios and for use in aquariums can result in the overharvesting of certain species, as well as the destruction of coral reef habitat.

What Can We Do to Support Coral Reefs?

As a Society?

The most important thing society can do to help corals and reef ecosystems is to reduce the use of fossil fuels, as they are the cause of global heating and ocean warming, which leads to ocean acidification and coral bleaching.

The U.S. Environmental Protection Agency is in charge of implementing Clean Water Act programs to reduce the pollution that degrades coastal waters and coral habitat. The natural resilience of sensitive reef systems will be enhanced by people reducing stressors to reefs in their local communities.

Another societal practice that is essential to coral reefs is to strictly manage coastal development, as it can erode the shoreline, lead to sediment deposits and encourage the overvisitation of coral reefs. It is also important that mining and agricultural runoff be limited or prohibited, as metals and chemicals from their processes can make their way into waterways and the ocean, causing damage to corals.

The pristine marine ecosystem of Hanauma Bay in Oahu, Hawaii. Mary Baratto / iStock / Getty Images Plus

Overfishing must cease and sustainable fisheries be supported to ensure marine life and the balance of the coral reef ecosystem are not disrupted. Managing tourism is necessary to limit overexposure and damage to reefs. At the same time, laws prohibiting sunscreens that are toxic to corals and reef animals need to be passed and enforced.

In Our Own Lives?

There are many things individuals can do to support the health of coral reefs. When visiting reefs, be sure not to touch corals or any marine life and don’t take anything away from the reef system when diving or snorkeling in the area. Reduce your use of sunscreen by wearing a rash guard or long-sleeved shirt to prevent sunburn — if you do use sunscreen, always make sure it’s reef safe.

In your everyday life, save as much energy as possible by turning off lights and using energy-efficient appliances; eat sustainable seafood; use environmentally-friendly forms of transportation; reduce stormwater runoff and use green alternatives to pesticides and toxic fertilizers, since they can be washed into the sewer system and end up in rivers and the ocean; reduce your use of plastic — microplastics have become ubiquitous in every environment on Earth and can damage corals and other reef life; when buying fish for an aquarium, be sure they have been collected sustainably and don’t purchase living coral; and recycle as much as possible and dispose of trash properly so it does not make its way into waterways and the ocean.


Great biodiversity of a coral reef in the Banda Sea, Indonesia. ifish / iStock / Getty Images Plus

Coral reefs are some of the most beautiful ecosystems on Earth. Their vast array of marine life of all colors, shapes and sizes is an incredible testament to our planet’s biodiversity. Global heating has led to ocean warming, which affects sensitive corals and has increased detrimental bleaching. Coastal development, sedimentation and toxic chemicals are all threats to reef ecosystems and must be limited or prohibited to protect them.

Everyone has a part to play in the protection of coral reefs — the decisions we make in how we travel, eat, buy, garden and conduct ourselves when visiting reef habitats all affect these unique ecosystems. Working together to protect coral reefs is the best chance they have of surviving the global climate crisis.

An octopus on a coral reef in the Cap de Creus, Costa Brava, Spain. ullstein bild / Getty Images

This article originally appeared on EcoWatch and was syndicated by MediaFeed.

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This article originally appeared on EcoWatch and was syndicated by MediaFeed.

Like MediaFeed's content? Be sure to follow us.