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 off the Persian Gulf 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 kids 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, have been collected for their shells in such numbers that many are now endangered. The turtle was hunted for use in “tortoise shell” 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, 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.

reef
This coral reef in Papua New Guinea is part of the island nation's 40,000 square kilometers (15,444 square miles) of coral reefs, sea grass beds, and mangrove forests.

Tropical Paradise
The glittering white sand beaches so popular with tourists are actually the remains of dead animals. Coral sand is the limestone exoskeletons of millions of coral, smashed by relentless ocean waves. Sand on continental coasts is mostly eroded sedimentary rock. Coral sand is lighter, finer, and softer than sedimentary sand.

Shipwrecked
Natural reefs have stranded many ships. Reefs sit below the water, are often hidden by tides, and have unpredictable shapes. Even experienced sailors fail to spot them.

Sometimes, the wrecked ships sink and become part of the reef. In 1824, the royal yacht of the King of Hawaii, the Haaheo o Hawaii (Pride of Hawaii), got stuck on a coral reef off the island of Kauai. The yacht was stranded in less than 1.5 meters (5 feet) of water. The remaining pieces of the ship were identified in 1995.

Having a large, modern ship is no benefit. In 2009, the 9,600-ton missile cruiser USS Port Royal was stranded on a coral reef . . . off another Hawaiian island, Oahu. After unloading thousands of pounds of anchors and other equipment, the ship was saved. No one was injured, but the reef was badly scarred and the ship sustained $25 million worth of damage.

accrete
Verb

to build up or grow together.

adapt
Verb

to adjust to new surroundings or a new situation.

aircraft carrier
Noun

large ship with runways for aircraft to take off and land.

algae
Plural Noun

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

algal bloom
Noun

the rapid increase of algae in an aquatic environment.

ancient
Adjective

very old.

antler
Noun

horn-like bony outgrowth on deer and related animals.

aquaculture
Noun

the art and science of cultivating marine or freshwater life for food and industry.

aquarium
Noun

a container or tank where aquatic plants and animals are kept, or an institution that keeps such containers.

aquatic
Adjective

having to do with water.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

a coral reef or string of coral islands that surrounds a lagoon.

barrier reef
Noun

ridge of coral or rock found parallel to the coast of an island or continent, but separated from it by a deep lagoon.

Noun

narrow strip of land that lies along a body of water.

bioerosion
Noun

the process in which a living organism wears away at rock or another hard substance.

budding
Noun

a method of reproduction used by some animals.

calcium carbonate
Noun

chemical compound (CaCO3) found in most shells and many rocks.

camouflage
Verb

to hide or disguise by blending in to surroundings. Also called cryptic coloration.

carbon dioxide
Noun

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

Noun

waterway between two relatively close land masses.

Noun

gradual changes in all the interconnected weather elements on our planet.

cnidocyte
Noun

a venomous cell used by some sea creatures to sting prey.

coastline
Noun

outer boundary of a shore.

cold-water coral
Noun

tiny marine animal that thrives in deep, cold water. Also called deep-water coral.

colony
Noun

group of one species of organism living close together.

concrete
Noun

hard building material made from mixing cement with rock and water.

construction
Noun

arrangement of different parts.

Noun

tiny ocean animal, some of which secrete calcium carbonate to form reefs.

coral bleaching
Noun

loss of symbiotic algae (zooxanthellae) in corals, leading to a loss of pigmentation.

coral island
Noun

low-lying island whose land is made up of organic material associated with coral.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

corrosion
Noun

process of chemicals breaking down or wearing away a material.

Noun

agricultural produce.

cyclone
Noun

weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.

decontaminate
Verb

to remove harmful substances or materials.

delicate
Adjective

fragile or easily damaged.

detonate
Verb

to cause something to explode.

disastrous
Adjective

very bad.

diverse
Adjective

varied or having many different types.

Noun

shape that is half of a sphere.

Noun

community and interactions of living and nonliving things in an area.

eject
Verb

to get rid of or throw out.

electric charge
Noun

property of all matter, either positive, negative, or zero.

emit
Verb

to give off or send out.

encourage
Verb

to inspire or support a person or idea.

endanger
Verb

to put at risk.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

erode
Verb

to wear away.

Noun

act in which earth is worn away, often by water, wind, or ice.

exoskeleton
Noun

the hard external shell or covering of some animals.

explosive
Noun

material that can quickly and violently expand due to a chemical change.

fatal
Adjective

causing death.

Noun

material, usually of plant or animal origin, that living organisms use to obtain nutrients.

fringing reef
Noun

a type of reef that extends from a coastline.

galleon
Noun

type of large sailing ship used during the 16th-18th centuries, square-rigged on the foremast and lateen (triangle) rigged on the aftermast.

genetic
Adjective

having to do with genes, inherited characteristics or heredity.

Noun

increase in the average temperature of the Earth's air and oceans.

government
Noun

system or order of a nation, state, or other political unit.

Great Barrier Reef
Noun

large coral reef off the northeast coast of Australia.

Noun

phenomenon where gases allow sunlight to enter Earth's atmosphere but make it difficult for heat to escape.

greenhouse gas
Noun

gas in the atmosphere, such as carbon dioxide, methane, water vapor, and ozone, that absorbs solar heat reflected by the surface of the Earth, warming the atmosphere.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

hard coral
Noun

type of coral that creates a hard shell or exoskeleton around itself.

harmful algal bloom (HAB)
Noun

rapid growth of algae, bacteria, or other plankton that can threaten an aquatic environment by reducing the amount of oxygen in the water, blocking sunlight, or releasing toxic chemicals.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

hurricane
Noun

tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.

industry
Noun

activity that produces goods and services.

inorganic
Adjective

composed of material that is not living, and never was, such as rock.

insulate
Verb

to cover with material to prevent the escape of energy (such as heat) or sound.

Noun

body of land surrounded by water.

islet
Noun

a very small island.

jagged
Adjective

having an uneven edge.

Noun

shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

larva
Noun

a new or immature insect or other type of invertebrate.

limestone
Noun

type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.

live-bottom reef
Noun

a type of reef formed by rocks. Also called a hard-bottom reef.

low tide
Noun

water level that has dropped as a result of the moon's gravitational pull on the Earth.

marine
Adjective

having to do with the ocean.

metal
Noun

category of elements that are usually solid and shiny at room temperature.

microsilica
Noun

chemical compound made of very find-grained quartz.

mineral accretion
Noun

process in which an electric charge is applied to a piece of metal in seawater. The charge causes minerals to build up on the surface of the metal.

Noun

process of extracting ore from the Earth.

nocturnal
Adjective

active at night.

nodule
Noun

a small lump.

Noun

large body of salt water that covers most of the Earth.

offshore
Adjective

having to do with facilities or resources located underwater, usually miles from the coast.

oil drilling
Noun

process of digging below the surface of the Earth for oil.

oil rig
Noun

complex series of machinery and systems used to drill for oil on land.

open ocean
Noun

area of the ocean that does not border land.

organic
Adjective

composed of living or once-living material.

outcropping
Noun

layer of rock visible above the surface of the Earth.

overfish
Verb

to harvest aquatic life to the point where species become rare in the area.

oyster bed
Noun

place where oysters live.

penetrate
Verb

to push through.

Persia
Noun

empire that dominated Mesopotamia from about 550 to 330 BCE. Most of the ancient Persian empire is in modern-day Iran.

pirate
Noun

thief who steals from ships or ships' crews while at sea.

Plural Noun

(singular: plankton) microscopic aquatic organisms.

plow
noun, verb

tool used for cutting, lifting, and turning the soil in preparation for planting.

Noun

introduction of harmful materials into the environment.

polyp
Noun

a type of animal with a fixed base, a tubelike body, and tentacles for catching prey.

precise
Adjective

exact.

predator
Noun

animal that hunts other animals for food.

promote
Verb

to encourage or help.

Noun

liquid precipitation.

recreational
Adjective

having to do with activities done for enjoyment.

Noun

a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.

reef ball
Noun

manmade structure for creating artificial reefs.

reef flat
Noun

part of a fringing reef connected to the coast.

reef slope
Noun

part of a fringing reef facing the open ocean.

remains
Noun

materials left from a dead or absent organism.

Noun

large stream of flowing fresh water.

rock
Noun

natural substance composed of solid mineral matter.

sand
Noun

small, loose grains of disintegrated rocks.

scuba
noun, adjective

(self-contained underwater breathing apparatus) portable device for breathing underwater.

sea anemone
Noun

type of marine animal related to corals and jellies.

seabed
Noun

the floor of the ocean.

seafood
Noun

fish and shellfish consumed by humans.

sea star
Noun

marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.

seaweed
Noun

marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

sedentary
Adjective

staying in one place.

Noun

solid material transported and deposited by water, ice, and wind.

sewage
Noun

liquid and solid waste material from homes and businesses.

shellfish reef
Noun

reef made of oysters or blue mussels.

shelter
Noun

structure that protects people or other organisms from weather and other dangers.

Noun

small sediment particles.

skeleton
Noun

bones of a body.

soft coral
Noun

type of coral that does not create a hard exoskeleton.

soil
Noun

top layer of the Earth's surface where plants can grow.

species
Noun

group of similar organisms that can reproduce with each other.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

Noun

abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

Strabo
Noun

(63 BCE-21 CE) Greek geographer and historian.

subway
Noun

underground railway; a popular form of public transportation in large urban areas.

suffocate
Verb

to be unable to breathe.

Noun

the sport of riding down a breaking wave on a board.

technology
Noun

the science of using tools and complex machines to make human life easier or more profitable.

tentacle
Noun

a long, narrow, flexible body part extending from the bodies of some animals.

tourism
Noun

the industry (including food, hotels, and entertainment) of traveling for pleasure.

toxic
Adjective

poisonous.

trawl
Verb

to fish by dragging a large net along the bottom of the body of water.

Plural Noun

region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

venom
Noun

poison fluid made in the bodies of some organisms and secreted for hunting or protection.

vital
Adjective

necessary or very important.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

vulnerable
Adjective

capable of being hurt.

waterway
Noun

body of water that serves as a route for transportation.

wave
Noun

moving swell on the surface of water.

wildlife
Noun

organisms living in a natural environment.