Coral Reef Facts

By Pacific Coastal and Marine Science Center

These facts about coral reefs are presented in conjunction with the USGS Coral Reef Project.

What is a reef?

Shallow, crystal-clear coastal water with rounded, black rocks jutting out of a white sandy bottom. — Stromatolites, such as the ones shown above, are living fossils. They built the first reefs on Earth. Today, stromatolites live in stressed marine environments. These stromatolites are growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia. Photo by Paul Harrison, Reading, UK, CC BY-SA 3.0

The word “reef” holds different meanings for everyone. To a mariner, the term reef applies to shallowly submerged navigational hazards. To a surfer, a reef is an undersea obstruction that can make waves (and surfboards) break. Scientists generally restrict the definition of a reef to rigid biological constructions.

The Earth's first reef-building organisms were photosynthesizing cyanobacteria living about 3.5 billion years ago. From fossil remains, it is known that a variety of organisms have constructed reefs, including bivalves (clams and oysters), bryozoans (coral-like animals), and sponges. Corals have been found in fossil reefs as old as 500 million years, but corals similar to the modern colonial varieties have constructed reefs only during the last 60 million years.

What is a coral reef?

Simple illustration of coral anatomy showing tentacles, mouth, polyps, corallium, nenatocysts, zooxanthellae. — The hard skeleton of coral is formed by the secretion of calcium carbonate by the polyp. The cup-like skeleton deposited by an individual polyp is called a corallite. Polyps gather food particles with the nematocysts (stinging, venomous cells) in their tentacles, and feed from sugars produced by photosynthesizing zooxanthellae, a type of algae. The coral tissue protects these algae from herbivorous grazers, and the algae in turn use many of the polyps' waste products such as carbon dioxide, nitrogen, and phosphorus. Illustration by Laura Torresan, USGS

Corals are animals related to jellyfish and anemones. Solitary and colonial corals catch plankton and suspended food particles with arm-like tentacles, which feed a centrally located mouth. Coral reefs are formed by huge colonies of corals that secrete hard calcareous (aragonite) exoskeletons that give them structural rigidity. These colonial hard corals may form elaborate finger-shaped, branching, or mound-shaped structures, and can create masses of limestone that stretch for tens or even hundreds of miles.

Most hard corals also host symbiotic algae, a long-standing and successful partnership. These algae provide them with an additional food source through photosynthesis.

When corals are stressed, they expel these algal symbionts through a process known as coral bleaching. Corals also face serious risk of diseases; black band, white band, and yellow band diseases have been reported from many localities. Hawaiian corals, however, have been relatively free from disease, but the first case of black band disease was reported in 1994.

How do coral reefs form?

A series of 4 cartoon to illustrate how we go from an active volcanic island, to fringing and barrier reef, to atoll. — Coral atolls develop from reefs fringing volcanic islands. Reefs fringing volcanic islands build vertically to sea level, forming steep-walled barrier reefs. As a volcanic island subsides, or sinks, with time, the growing reef keeps pace with the rising water level. When the island eventually submerges, the ring-shaped reef forms an atoll with a central lagoon.

Corals have a wide distribution in the world's oceans, but the varieties that form reefs typically are restricted to relatively shallow, warm tropical waters between 30° north and south latitudes. Clean, clear water with the right amount of nutrients is essential to their health. After initial colonization of a hard substrate and given suitable conditions for coral growth, an individual larval “spatfall” gives rise to a colony.

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 along tropical coastlines, on the margins of volcanic islands, and as isolated coral atolls.

Coral reefs are dynamic, evolving through time into different forms. During his voyages on the HMS Beagle, Charles Darwin first recognized the progressive development of coral reefs on volcanic islands. Volcanic islands subside; that is, they have a tendency to cool, condense, and sink through time. As they subside, tropical coral reefs grow upward along their margins.

The Hawaiian Islands-Emperor Seamounts chain is a classic example of this process. Active volcanic islands are found at the southeast end of the chain. Beyond Kauaʻi, however, islands are subsiding slowly and coral reefs have developed around the volcanic cores of islands. French Frigate Shoals, Midway, and Necker Islands are dominantly coral limestone accumulations, but deep sediment cores have revealed the volcanic origins of these islands.

Why are coral reefs in peril?

Aerial satellite view of land and ocean with visible sediment runoff patterns and brightly colored features in the water. — The Great Barrier Reef arches over 2000 kilometers along the northeast coast of Australia. The white calcium carbonate that coats the coral reflects light, making the water above the reef appear bright blue from space. This phenomenon allows the reef to be visible in satellite images, such as this one.

Coral reefs are sensitive indicators of the health of marine environments. Yet coral reefs are in decline in many parts of he world. It is estimated that 30% will be destroyed or seriously degraded in the next ten years.

The causes of reef degradation are many. They are being stressed and killed by a variety of local human activities such as grounding of ships, improperly placed anchorages, destructive fishing practices, such as dynamiting or cyanide poisoning, and simply overfishing, which disrupts the balance of these fragile ecosystems.

Pollution and sediment runoff from land are major causes of stress, and even human activities conducted at great distance through warming and pollution can affect coral sustainability.

As coral reefs become stressed, they also are more susceptible to viral and bacterial infections, such as black, white, and yellow band diseases. It is critically important to better understand the role of natural processes and the impact that human activities may have on coral reef health.