Coral reef

A coral reef is an underwater ecosystem made by reef-building corals. Reefs are formed from colonies of tiny animals called coral polyps, stuck together with calcium carbonate. Most coral reefs are built by stony corals, whose polyps grow in groups.

Coral belongs to the class Anthozoa in the animal phylum Cnidaria. This group also includes sea anemones and jellyfish. Unlike sea anemones, corals make hard shells that protect them. Most reefs grow best in warm, clear, shallow water that gets plenty of sunlight.

People often call shallow coral reefs the “rainforests of the sea.” Even though they cover only a tiny part of the ocean—about half the size of France—they are home to a huge variety of sea life. Many types of fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians all live on coral reefs. These reefs grow in ocean water that does not have many nutrients.

Formation

Further information: Atoll, Fringing reef, and The Structure and Distribution of Coral Reefs

Most coral reefs grew after the Last Glacial Period. When ice melted, sea levels rose and covered continental shelves. These reefs are usually less than 10,000 years old. As they grew, they kept up with the rising sea levels. Coral reefs can also be found deep in the ocean near oceanic islands and atolls.

In his book The Structure and Distribution of Coral Reefs, Charles Darwin described how atoll reefs form. He suggested that the shape of atolls comes from the movement of Earth's oceanic crust. Darwin explained that a fringing reef starts around a volcanic island. Over time, if the island sinks, the reef grows and changes into a barrier reef, and finally into an atoll.

Types

Main article: Fringing reef

Main article: Atoll

Coral reefs come in many shapes and sizes. One common type is the fringing reef, which grows right next to the shore. These reefs can stretch for many kilometers along coastlines. Another type is the barrier reef, which lies far from the shore, separated by a deep channel or lagoon. The largest example is the Australian Great Barrier Reef.

Atolls are ring-shaped reefs that form around a lagoon. Other types include platform reefs, which can form in the open ocean, and patch reefs, small reefs found within lagoons. Each type of reef provides a unique home for many sea creatures.

Zones

Coral reefs have different parts called zones. Each zone has its own special plants and animals. The three main zones are the fore reef, the reef crest, and the back reef, also called the reef lagoon. These zones are connected. Water, nutrients, and sea creatures can move between them.

Coral reefs are usually found in shallow water, less than 50 meters deep. They can be near continents, like the Great Barrier Reef, or around islands, like in the Maldives. Each part of a reef, such as the reef surface or reef face, gives sea creatures a place to live and find food.

Locations

Coral reefs grow in the ocean, mostly in the Indo-Pacific region. This includes places like the Red Sea, Indian Ocean, Southeast Asia, and the Pacific. Reefs grow best in warm water, but some can live in colder places like the Persian Gulf or near Norway.

Famous reefs include the large Great Barrier Reef near Australia, the Mesoamerican Barrier Reef System close to Mexico and Honduras, and the reefs in the Red Sea. Other important reefs are around the Maldives, the Philippines, and the Raja Ampat Islands in Indonesia.

Coral

Main article: Coral

Corals are groups of tiny animals called polyps that live inside calcium carbonate shells. These polyps work together to build reefs, and some can grow as large as 12 inches across. Reef-building corals can only live in shallow, sunny waters where they can get enough light.

Coral polyps have a special friendship with tiny algae called zooxanthellae that live inside them. These algae give the coral food and help it grow, while the coral gives the algae a safe home and carbon dioxide. If a coral loses its algae, it turns white in a process called bleaching, which can be harmful to the coral.

Gallery of reef-building corals

Coral reefs are beautiful underwater places made by tiny animals called coral polyps. These polyps live together in groups. They build hard homes from a material called calcium carbonate. This material forms the shape of the reef. Stony corals are the main builders of the reefs we see today.

Other reef builders

Corals are the main builders of reefs, but many other sea creatures help too. These include coralline algae, some sponges, and bivalves. Together, these animals create reefs over many years.

Coralline algae are tiny plants that add strong limestone to reefs, especially in rough waters. Sea sponges can also form reefs, like those found off the coast of British Columbia. Long ago, ancient sponges were the first animals to build reefs. Oyster reefs are groups of oysters that grow together, adding layers over time. These reefs were once common along coastlines but have changed over the years. Some snails also help by gluing themselves to rocks and adding to the reef structure.

Darwin's paradox

In The Structure and Distribution of Coral Reefs, Charles Darwin noticed that coral reefs grew in some tropical places but not others. He wondered why this happened.

Coral reefs can seem like a busy oasis in a quiet desert. This is called "Darwin's paradox." It asks how so much life can grow where there are few nutrients.

Coral reefs are home to many sea creatures. This is because of complex food webs. Big predator fish eat small forage fish, which eat tiny zooplankton. All of this depends on plants, which are primary producers. Coral reefs make about 5–10 grams of carbon each day for every square meter.

Tropical waters have few nutrients, but coral reefs still grow well. The sun shines all year in the tropics, warming the top layer of water. This warm water stays on top of cooler water because of a thermocline. In most oceans, these layers don’t mix much. When creatures die, they sink and break down, adding nutrients like nitrogen (N), phosphorus (P), and potassium (K). But in the tropics, these nutrients don’t usually come back up to the surface.

Plants, including tiny phytoplankton, need sunlight and nutrients to grow. They live near the surface where sunlight reaches them. But they also need nutrients, which run out because of the thermocline.

Around coral reefs, lagoons fill with bits from the reef and nearby land. These areas shelter sea life from big waves.

Most importantly, reefs recycle nutrients better than open oceans. Small plants and algae on reefs share nutrients with corals. These tiny plants are the start of the food chain, eaten by fish and small animals. This recycling means reefs need fewer new nutrients.

Corals also take in nutrients from water, like nitrogen and phosphorus. At night, many corals catch zooplankton with their tentacles. This gives the coral nitrogen, which it shares with tiny plants inside it.

Sponges live in reef crevices. They eat tiny plants floating by and later release nutrients that corals can use.

The rough surfaces of corals help them survive rough waters. Waves break on these rough edges, mixing the water and bringing nutrients to the corals. This mixing helps reefs grow.

Sometimes, deep water with more nutrients moves up into reefs. This can change water temperature and nutrient levels. This mixing happens in places like the Bahamas and Florida.

Water moves through reefs in many ways, like tides and wind. These movements create waves inside the water. These internal waves can bring up nutrients and cool water. This helps sea creatures grow and can affect the whole reef.

The shape of reefs can mix water better, creating pockets of cooler water with more nutrients. This helps many sea creatures, including tiny plants and animals. It also helps explain why reefs can grow in places with few nutrients.

Cyanobacteria make nitrates through nitrogen fixation.

Coral reefs often need help from nearby areas like seagrass meadows and mangrove forests. These places give reefs dead plants and animals rich in nitrogen. In return, reefs protect these areas from waves and help them stay rooted with sediment.

Biodiversity

Coral reefs are some of the world's most productive ecosystems. They provide complex and varied marine habitats that support many different kinds of animals. Over 4,000 species of fish live in coral reefs. Other animals that live there include sponges, cnidarians, worms, crustaceans, mollusks, echinoderms, sea squirts, sea turtles, and sea snakes. These reefs offer many hiding places for animals, which helps support so many different kinds of life.

Reefs also support many seabirds, such as the Laysan albatross and black-footed albatross, especially in places like Midway Atoll in Hawaii. Sponges are important because they help recycle nutrients and provide homes for small creatures. Different animals use the reef at different times, with some hiding during the day and coming out at night.

Ecosystem services

Coral reefs help people and nature in many ways. They support tourism and fishing, and they protect coastlines from waves. Many people benefit from what coral reefs offer.

Coral reefs also protect shorelines by reducing wave energy. They can cut wave energy by up to 97%, helping to prevent damage to property. This protection is especially important for small islands. Many people living near reefs get safety benefits from them.

Threats

See also: Environmental issues with coral reefs, Coral bleaching, and Environmental threats to the Great Barrier Reef

Coral reefs have faced many threats over millions of years. Today, human actions add new dangers. Things like taking coral from the sea, fishing that hurts the ocean floor, and pollution can harm reefs. Some sunscreens can also hurt corals. A bigger danger comes from gases that warm the Earth. This raises sea temperatures and causes coral bleaching. Climate change can also bring stronger storms and change ocean currents, which can damage reefs. While these changes are serious, some corals can adapt and become stronger.

Protection

Marine protected areas (MPAs) are special zones in the ocean where we protect sea life and help habitats stay healthy, including coral reefs. They help control fishing and keep nature safe. MPAs can help different kinds of sea animals and plants live better and give jobs to people nearby.

To protect coral reefs, we need to stop overfishing, especially of fish like parrotfish. We also need to reduce pollution from sewage and not have too many visitors. Keeping reefs healthy needs good planning and help from everyone. Cutting down fewer trees and controlling pollution also helps reefs by stopping the ocean from getting too sour. Countries use rules and plans to keep these important places safe.

Restoration

See also: Aquaculture of coral, Artificial reef, and Restoration ecology

Coral reef restoration is very important today because reefs all over the world are in trouble. Things like pollution, hot ocean water, big storms, and too much fishing hurt reefs. Reefs help protect coastlines and give fish a home. To help reefs get better, scientists started coral restoration in the 1970s and 1980s.

One way to restore reefs is coral aquaculture, or coral farming. In this method, very small pieces of coral, called seeds, are grown in special nurseries. They stay there until they are strong enough to be moved to damaged reefs. This helps the young corals stay safe from dangers like animals that might eat them or rough water. Coral farming is now the most used way to restore reefs because it works well and is easier than moving big coral groups, which might hurt the reefs already there.

Another way to help reefs grow is by adding special surfaces where corals can stick. These surfaces can be made from old tires, ships, subway cars, or concrete shapes. Some scientists use a method called biorock. This uses tiny electric currents to make a substance that helps corals grow faster and stronger. By choosing the right places and ways to add these surfaces, scientists can make conditions that help new corals grow and keep reef ecosystems healthy.