Coral Reefs: Characteristics, Types, and Major Threats

Reef refers to a bar made of coral, rock, sand, or other biological or geological material in relatively shallow sea water. The best-known reefs, called coral reefs, are formed by colonies of sessile coral animals, called polyps, that secrete exoskeletons made of calcium carbonate. These reefs, which are located mostly in tropical regions, are also home to a wide variety of other animals, as well as algae, making their ecosystems among the most biologically diverse on Earth. Many coral reefs are threatened ecosystems, partly because their polyps are extremely sensitive and susceptible to increases in water temperature attributed to climate change. Oyster reefs are another kind of biological reef ecosystem.

CHARACTERISTICS OF CORAL REEFS. Although coral reefs make up less than 1 percent of Earth’s surface area, as much as one- quarter of the planet’s marine species use these reefs for food or shelter. The biological foundation of coral reefs is the large colonies of small coral polyps, which are related to sea anemones and jellyfish (in the phylum Cnidaria). The polyps feed by using their stinging tentacles to capture tiny organisms. In stony, or hard, corals (the most common kind of corals in reefs), the polyps filter calcium and carbonate ions out of seawater and use them to secrete a hard calcium carbonate (limestone) skeleton that surrounds their soft tissues. The combined exoskeletons of the many polyps within coral colonies form different shapes depending on the species—shapes that are often reflected in their common names, such as elkhorn coral, brain coral, and lettuce coral.

The exoskeletons of coral colonies develop into a strong, stony reef structure on which other animals make their home or visit for feeding or reproduction activities. In addition to corals’ Cnidarian relatives, such as sea anemones and jellyfish, reef animals include fish of many kinds: sea turtles; crabs, shrimp, and other crustaceans; clams, snails, and other mollusks; starfish, sea urchins, and other echinoderms; marine worms; and sponges. Not all coral polyps build stony skeletons around them. There are also many species of soft corals, including sea fans, sea feathers, sea fingers, and sea whips. The soft corals typically live on the same reefs with the stony corals.

The corals in colonies may reproduce sexually (through the union of sperm and eggs) or asexually (such as through budding). As the coral polyps reproduce, the colony and the reef gradually grow in size. New corals grow on top of old dead corals. A typical reef is made up of many colonies of various coral species. Colonies can live for hundreds or thousands of years and continue expanding during that entire time. The largest coral reef is the Great Barrier Reef off the eastern coast of Australia. It stretches for about 1,600 miles (2,600 kilometers) and has been growing for an estimated 20,000 years Barrier reefs make up a category of coral reef in which the reef parallels the coastline and is separated from the coast by a deep, wide lagoon. Other reef categories include fringing reefs (which grow near coasts, from which they are separated by a shallow, narrow lagoon), patch reefs (which are small and isolated, typically near barrier or fringing reefs), and atolls (which form rings, generally in the middle of the sea).

Most coral reefs are found in shallow tropical or subtropical ocean waters around the world, where temperatures are normally about 70° F to 85° F (22° C to 29° C). Coral reefs cover a total of about 110,000 square miles (284,300 square kilometers).

OTHER KINDS OF REEFS. Unusual types of coral reefs known as cold-water reefs are found in cold waters along continental shelves, as well as in deep-sea areas on seamounts and ridges, where temperatures may be as low as 39° F (4° C). The largest-known cold-water reef—measuring about 25 miles (40 kilometers) in length—is off the coast of Norway’s Rost Island. These kinds of reefs typically have only a few coral species, unlike the hundreds that may populate large tropical reefs. But like tropical reefs, cold-water reefs are also home to a wide variety of other marine life, including fish, crustaceans, mollusks, echinoderms, worms, and sponges.

Oyster reefs, or oyster bars, are submerged structures built primarily by oysters in warm near-shore areas, such as along the southern US coast. The reefs develop and grow as new oyster larvae settle and spread on the shells of old or dead oysters. These kinds of reefs are especially common in brackish estuaries near the mouths of rivers, such as the Apalachicola River in northern Florida. Oyster reefs are home to commercially valuable species of oysters—such as the Eastern oyster in Florida—as well as other marine species, such as clams, scallops, mussels, barnacles, conchs, snails, shrimps, isopods, sponges, marine worms, and fish.

HUMAN USES AND THREATS. In addition to their natural value as diverse ecosystems, coral reefs serve practical purposes for humans. They are sources of food and medicines, help protect shorelines from erosion by ocean waves and currents, and provide popular destinations for tourists.

Coral reefs are highly sensitive to changes in water conditions, such as temperature increases, agricultural and industrial pollution, and sedimentation. Such water changes can damage or kill coral polyps by causing them to expel the zooxanthellae algae that live inside their tissues in a mutually beneficial relationship. In this relationship, the polyps give the algae a safe place to live, and the algae share some of the food they produce through photosynthesis with the polyps. When the pigmented algae are expelled, the polyps turn whitish (a condition called bleaching), and they may weaken and die if the bleaching lasts long enough. Marine biologists have attributed many cases of coral bleaching to rising water temperatures and water acidification related to carbon dioxide emissions from the burning of fossil fuels—emissions that are also blamed for global warming.

Other threats to coral reefs include overfishing; destructive fishing practices (such as bottom trawling and blast fishing); harvesting of various species for the aquarium trade; coral mining; and the introduction of foreign, invasive species to certain reefs. Deforestation and coastal development can harm coral reefs as a result of increased erosion of soil, which flows into the sea and blocks sunlight from reaching the corals’ zooxanthellae.

FURTHER READING: Coral Reef Alliance. 2014a. “Coral Reef Ecology.” http://coral.org/coral-reefs-101/coral-reef -ecology/. Accessed November 16, 2016.

Coral Reef Alliance. 2014b. “Coral Reef Ecology: Coral Polyps—Tiny Builders.” http://coral.org/ coral-reefs-101/coral-reef-ecology/coral-polyps/. Accessed November 16, 2016.

Coral Reef Alliance. 2014c. “Coral Reef Ecology: Types of Coral Reef Formations.” http://coral .org/coral-reefs-101/coral-reef-ecology/types-of-coral-reef-formations. Accessed November 16, 2016.

Coral Reef Alliance. 2014d. “Coral Reef Ecology: Where Are Coral Reefs Located?” http://coral .org/coral-reefs-101/coral-reef-ecology/geography/. Accessed November 16, 2016.

National Oceanic and Atmospheric Administration, Habitat Conservation. “Oyster Reef Ecology.” http://www.habitat.noaa.gov/pdf/ecology_of_oysters.pdf. Accessed November 16, 2016.

Smithsonian Marine Station at Fort Pierce. 2002. “Indian River Lagoon Species Inventory: Oyster Reef Habitats.” http://www.sms.si.edu/irlspec/Oyster_reef.htm. Accessed November 16, 2016.

Smithsonian National Museum of Natural History, Ocean Portal. 2016. “Corals and Coral Reefs.” http://ocean.si.edu/corals-and-coral-reefs. Accessed November 16, 2016.

World Wildlife Fund. 2016. “Coral Reefs: Cold-Water Corals.” http://wwf.panda.org/about_our _earth/blue_planet/coasts/coral_reefs/coldwater_corals. Accessed November 16, 2016.