A reef is a ridge of material at or near the surface of the ocean. Reefs can occur naturally. Natural reefs are made of rocks or the skeletons of small animals called corals. Reefs can also be artificial—created by human beings.

Artificial Reefs

People create reefs for three chief reasons. The first is to protect the coastline. Reefs act as barriers between the coast and powerful ocean storms. In this way, reefs also protect coastlines from erosion. The Maldives, an island nation in the Indian Ocean, has constructed reefs to protect its low-lying coral islands from cyclones and other factors that may lead to beach erosion.

The second reason people construct reefs is to promote sea life for recreation and aquaculture. A reef ecosystem is very diverse. Plants, plankton, algae, sponges, eels, fish, crabs, and sea turtles are just some of the organisms that thrive in healthy reefs. The wide variety of fish (including sharks) make recreational fishing popular in reef ecosystems. Brightly colored fish, sea anemones, and sea stars also make reefs popular with scuba divers and snorkelers. Artificial reefs off the Atlantic coast of the U.S. states of Florida, Georgia, and South Carolina have contributed to the area’s wildlife and encouraged tourism.

Aquaculture is the art and science of cultivating ocean life for food and industry. The reef’s size and shape provide shelter for different kinds of fish, so fish farmers can increase their catch by investing in reefs. Japan creates artificial reefs to encourage the growth of schools of snapper, for instance. Artificial reefs can also prepare sedentary creatures, such as clams and oysters, for harvest. Japan is also a leader in creating artificial reefs for pearl-producing oysters.

A third reason for building reefs is to create a wave pattern that promotes the sport of surfing. Surfers ride boards on top of waves. Engineers have experimented with reef shapes to improve surfing conditions. These reefs are usually located far offshore and have the benefit of creating a larger, safer swimming area near the coast. El Segundo, California, was the first area to try an artificial reef to improve surfing. Australian beaches at Perth and the Gold Coast have also constructed artificial reefs for this purpose.

People have constructed underwater barriers for thousands of years. Coastal communities relied on artificial reefs for protection. Two thousand years ago, the Greek historian Strabo reported that Persians built reef barriers across the Tigris River to prevent pirates from India from crossing it.

Ancient civilizations like the Persians built artificial reefs out of organic material like trees and inorganic material like rocks. These days, people use a much wider—and much weirder—variety of materials for reef-building.

Sunken ships have provided reef structures for hundreds of years. Now, these galleons are joined by retired aircraft carriers, oil rigs, and even New York City subway cars. These vehicles are not simply sunk—careful attention is paid to the chemicals they may emit to the surrounding ocean and seabed. The structures are therefore decontaminated: all plastic and toxic materials are stripped from them. (Most toxic chemicals are found in insulating material that kept the vehicles from getting too cold or too hot when in use.) Carefully placed explosive devices detonate and sink the structures in a precise location. The aircraft carrier USS Oriskany is now a reef in the Gulf of Mexico. Subway cars have been sunk for reefs along the East Coast of the U.S. from New Jersey to Georgia.

In warm waters, life on these types of metallic artificial reefs is encouraged by a process called mineral accretion. An electric charge is applied to the structure. This charge causes a mineral, limestone, to build up (accrete) directly on the surface of the metal. Corals attach themselves to the limestone and add to it.

Reef balls are another type of artificial reef structure. Reef balls are circular structures with holes and nodules for organisms to swim through and live in. They are constructed using concrete and a special type of chemical compound called microsilica. Microsilica helps prevent corrosion and adds strength to the structure. Reef balls are used for completely artificial reefs, reef restoration, and erosion control. Reef balls have been used in places from Prince William Sound, Alaska, to the waters of the Persian Gulf off the coast of the United Arab Emirates.

Natural Reefs

There are several types of natural reefs. “Live-bottom” reefs are ledges or outcroppings of rock. Organisms such as sea anemones and seaweeds attach themselves directly to this rock, forming a live-bottom reef for fish and plants. The jagged rocks provide overhangs and protection for fish and other marine life, such as seals.

Oyster beds, also called shellfish reefs, are a unique type of live-bottom reef. Oyster larvae attach themselves to large adult oysters at the bottom of the reef, building layers into huge oyster columns. Oyster shells, not rocks, provide the hard surface on which reef organisms like sponges can grow. They provide protection for fish like gobies and a food source for animals like turtles. Oyster beds can be seen at low tide in the Chesapeake Bay in the U.S.

The most familiar type of natural reef, however, is the coral reef. These multicolored limestone ridges are built by tiny sea animals called corals. Their hard outer skeletons (exoskeletons) are what make up coral reefs. There are many different kinds of corals. The ones that build reefs are known as hard, or stony, corals. Corals that do not produce exoskeletons are known as soft corals.

Coral reefs grow slowly, usually at the rate of only a few centimeters each year. Some have formed over millions of years and measure hundreds of meters thick. The largest is the Great Barrier Reef off the northeast coast of Australia. It stretches for 3,000 kilometers (1,600 miles).

Coral reefs provide shelter for thousands of kinds of sea animals. As the reefs build up along coastlines and form new islands, they change the face of the Earth.

Coral Reef Formation

A coral is known as a polyp. It grows no larger than a human fingernail and is often only the size of a pinhead. It has a simple, tubelike body with tentacles at one end.

Most hard corals reproduce by budding, a process of forming small buds that develop into new polyps. The polyps build hard, cup-shaped skeletons around their soft bodies. Sometimes, corals reproduce from eggs. A larva hatches from an egg laid by an adult polyp. The larva drifts through the water until it reaches a spot it can attach itself to, usually part of an existing coral reef or other limestone structure. The young polyp produces a material called calcium carbonate, also known as limestone. This hardens around the polyp and joins it to the reef.

The exoskeletons protect corals from enemies. Some predators, however, can wear right through the hard material, a process known as bioerosion. The parrotfish, for example, has a beak-like mouth that tears through the exoskeleton to the coral within. While eating, the parrotfish chews and eats the limestone, which is then ejected as sand.

Corals usually live together in large groups, called colonies. Colonies are typically made of millions of genetically identical corals—natural clones produced by budding. Side by side, the polyps build their exoskeletons. As the animals die, more polyps build exoskeletons on top of the remains.

Different species of corals build formations in different shapes. Some look like branching trees or bushes. Others look like large domes, fans, or antlers. The bodies of living polyps are often vivid colors of pink, yellow, blue, purple, and green.

Coral colonies usually only grow in shallow water, often no deeper than 46 meters (150 feet). This is because tiny organisms called algae live inside most coral polyps. Algae are vital to these coral because they produce chemicals that help polyps make calcium carbonate. Algae need sunlight to survive, so the corals will not grow in water deeper than sunlight can penetrate. Algae are also what give corals their bright colors. Because the algae that live in coral thrive only in warm water, coral reefs grow mostly in the ocean waters of the tropics.

In addition to warm water, coral needs water that is clear. Water filled with silt or other sediment would suffocate the polyps.

Some corals, however, do not need algae to survive. These corals can live in much deeper, colder water. Cold-water coral reefs, also known as deep ocean reefs, are found from Norway to the Aleutian Islands.

There are three kinds of coral reefs: fringing reefs, barrier reefs, and atolls.

A fringing reef forms along the edge of a coast and is attached to land. It extends outward from shore like a shelf, just below the surface of the water. A fringing reef is composed of a reef flat and a reef slope. The reef flat is nearest to shore. Due to heavy sediment, few live corals live in the reef flat. It is mostly made of exoskeletons. The reef slope faces the open ocean. Most marine life is found on the reef slope. Fringing reefs are the most common type of coral reef.

A barrier reef is separated from the shore by a lagoon. The reef forms a barrier between the coast and the open ocean or sea. Some barrier reefs consist of chains of smaller reefs separated by narrow waterways. The Great Barrier Reef is constructed this way.

An atoll is a reef in the open sea that surrounds a lagoon. This kind of reef forms when a ring of coral builds up on the sides of an undersea volcano that has risen above the ocean surface. The volcanic peak gradually erodes and sinks below the water’s surface, and the reef continues to build. Over time, parts of the reef appear above the sea as a ring-shaped island or chain of islets.

Reefs are broken and eroded by fast-moving, powerful waves that crash into them. When waves break down the hard coral of a reef, they pound it into a fine sand. Such sand covers many tropical beaches and helps form new land.

As it grows, a coral reef provides homes for a vast number of living creatures. Coral reefs are among the richest, most varied communities of life found anywhere in the ocean.

Many reef creatures, including the coral polyps themselves, are nocturnal. They are active only at night. During the day, the corals close up inside their skeletons to hide from predators such as sea stars. After dark, the corals open and extend tentacles, which are covered with stinging cells. (These cells, called cnidocytes, manufacture a venom and inject it into the body of the victim as the coral’s tentacles grab it.) Waving in the water, a coral’s tentacles catch and sting plankton. Plankton are the tiny organisms (plants, animals, and algae) that the coral polyps eat.

Other reef animals survive by blending in with their surroundings. Many tropical reef fish, for example, are brilliantly colored. They match the bright colors of warm-water coral.

Natural Reefs and People

For centuries, reefs have supplied people with fish and other seafood. Today, some human activities threaten the health of reefs. As people along coasts plow the earth to plant crops or bulldoze it to build homes and roads, they loosen the soil. Rain washes much of it into rivers, which carry it to the ocean. There, the soil forms sediment that can suffocate and bury coral reefs.

Construction and agricultural industries also release harmful chemicals into rivers and drains that empty into oceans. Many coastal cities dump sewage and other wastes into the ocean. Such pollution causes certain types of algae to grow so rapidly that they form thick mats on top of the ocean. These harmful algal blooms block sunlight. This can be fatal to a living coral reef.

Reefs have been harmed by underwater mining and oil drilling. Some have been damaged by explosives used to clear out channels in the seabed for ships to pass through.

People have overfished some reefs. Cold-water coral reefs are especially vulnerable to overfishing because of the technology used. Huge trawling nets drag along the bottom of the ocean, destroying vulnerable coral habitats. Overfishing and habitat destruction have made shellfish reefs among the most endangered aquatic habitats on Earth.

Killing tropical fish for sport or collecting them live to sell to aquarium dealers puts tropical coral reefs at risk. Other reef creatures, such as the Hawksbill sea turtle (Eretmochelys imbricata), have been collected for their shells in such numbers that many are now endangered. The turtle was hunted for use in tortoiseshell jewelry. Red and orange species of coral are also valuable material for jewelry.

Some artificial reefs have proven disastrous for the environment. Osborne Reef, near Fort Lauderdale, Florida, United States, was constructed of more than a million used car and truck tires. Few animals adapted to life around the rubber reef. The tires were not fastened properly and many broke loose. Heavy tires crashed into natural coral reefs nearby, damaging the ecosystem. Hurricanes and storm surges carry tires from South Florida to beaches as far away as North Carolina.

Climate change is also a threat to coral reefs. Global warming is the current period of climate change, owing to an increased greenhouse effect. Carbon dioxide and other greenhouse gases trap sunlight in the Earth’s atmosphere. As more greenhouse gases are released through industry and vehicle emissions, more sunlight is trapped. Global warming and the greenhouse effect have been associated with rising ocean temperatures. Since corals thrive in warm water, the changes it brings about may be surprising.

Rising ocean temperatures do not encourage more coral growth. In fact, warmer waters cause corals to expel the algae that lives inside them. Without the algae, corals lose their color. This is called coral bleaching. Coral bleaching threatens the entire reef ecosystem. Fish and shellfish that were camouflaged by the brightly colored algae can no longer hide from predators. Without their algae, corals will die. The Seychelles, an island nation in the Indian Ocean, has lost almost all of its living coral reefs and atolls due to coral bleaching.

Coral bleaching does not kill coral immediately, however. As ocean warmth returns to more comfortable temperatures for the coral, they may regain their algae, and their color. Bleached coral usually takes weeks or months to regain its color. The Great Barrier Reef experienced a bleaching event in 2002, but most corals recovered.

Protecting delicate reef ecosystems is a global task. Scientists are studying types of algae that can survive warmer ocean temperatures. Chemicals or other qualities that allow these corals to resist bleaching may help damaged reefs survive. Many countries place limits on the amount of pollutants released into streams and drains. Governments, companies, and individuals are working to reduce the effects of global warming.