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Water: Habitat Protection

Coral Reef Protection: What Are Coral Reefs?

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What Are Coral Reefs?

French angel fish swimming among coral and sponges on a Caribbean reef, close up of coral polyps with tentcales extended, brain coral and seafan on a Caribbean reef

Protection of coral reefs depends on careful vessel management to avoid hitting the reef, reducing nearby outfalls and runoff, and careful tourists who respect the delicate communities that give such beauty to tropical shorelines around the world.

The mention of coral reefs generally brings to mind warm climates, colorful fishes and clear waters. However, the reef itself is actually a component of a larger ecosystem. The coral community is really a system that includes a collection of biological communities, representing one of the most diverse ecosystems in the world. For this reason, coral reefs often are referred to as the "rainforests of the oceans."

Corals themselves are tiny animals which belong to the group cnidaria (the "c" is silent). Other cnidarians include hydras, jellyfish, and sea anemones. Corals are sessile animals, meaning they are not mobile but stay fixed in one place. They feed by reaching out with tentacles to catch prey such as small fish and planktonic animals. Corals live in colonies consisting of many individuals, each of which is called polyp. They secrete a hard calcium carbonate skeleton, which serves as a uniform base or substrate for the colony. The skeleton also provides protection, as the polyps can contract into the structure if predators approach. It is these hard skeletal structures that build up coral reefs over time. The calcium carbonate is secreted at the base of the polyps, so the living coral colony occurs at the surface of the skeletal structure, completely covering it. Calcium carbonate is continuously deposited by the living colony, adding to the size of the structure. Growth of these structures varies greatly, depending on the species of coral and environmental conditions-- ranging from 0.3 to 10 centimeters per year. Different species of coral build structures of various sizes and shapes ("brain corals," "fan corals," etc.), creating amazing diversity and complexity in the coral reef ecosystem. Various coral species tend to be segregated into characteristic zones on a reef, separated out by competition with other species and by environmental conditions.

Virtually all reef-dwelling corals have a symbiotic (mutually beneficial) relationship with algae called zooxanthellae. The plant-like algae live inside the coral polyps and perform photosynthesis, producing food which is shared with the coral. In exchange the coral provides the algae with protection and access to light, which is necessary for photosynthesis. The zooxanthellae also lend their color to their coral symbionts. Coral bleaching occurs when corals lose their zooxanthellae, exposing the white calcium carbonate skeletons of the coral colony. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.

Scientists descending to reef carrying underwater clipboards for notes

Because the zooxanthellae depend on light for photosynthesis, reef building corals are found in shallow, clear water where light can penetrate down to the coral polyps. Reef building coral communities also require tropical or sub-tropical temperatures, and exist globally in a band 30 degrees north to 30 degrees south of the equator. Reefs are generally classified in three types. Fringing reefs, the most common type, project seaward directly from the shores of islands or continents. Barrier reefs are platforms separated from the adjacent land by a bay or lagoon. The longest barrier reefs occur off the coasts of Australia and Belize. Atolls rest on the tops of submerged volcanoes. They are usually circular or oval with a central lagoon. Parts of the atoll may emerge as islands. Over 300 atolls are found in the south Pacific.

Coral reefs provide habitats for a large variety of organisms. These organisms rely on corals as a source of food and shelter. Besides the corals themselves and their symbiotic algae, other creatures that call coral reefs home include various sponges; molluscs such as sea slugs, nudibranchs, oysters, and clams; crustaceans like crabs and shrimp; many kinds of sea worms; echinoderms like star fish and sea urchins; other cnidarians such as jellyfish and sea anemones; various types of fungi; sea turtles; and many species of fish.

For more background on corals and coral reefs, see the section of the links page with General Coral Reef Information.


  • Ruppert, EE and Barnes, RD, 1994, Invertebrate Zoology, 6th Edition, Saunders College Publishing, Philadelphia
  • An Introduction to Coral Reefs, University of the Virgin Islands 

Photo credits:

Top photo: French angel fish swimming among corals and sponges on a Caribbean reef.
Photo credit: Charles Lobue, EPA Region 2, New York, New York

Second photo: Close-up of coral polyps with tentacles extended.
Photo credit: Charles Lobue, EPA Region 2, New York, New York

Third photo: Brain coral and seafan on a Caribbean reef.
Photo credit: Charles Lobue, EPA Region 2, New York, New York

Bottom photo: Scientists descending to a coral reef carrying underwater clipboards for note taking.

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Why Are Coral Communities Important?

Coral reefs and their associated communities of seagrasses, mangroves and mudflats are sensitive indicators of water quality and the ecological integrity of the ecosystem. They tolerate relatively narrow ranges of temperature, salinity, water clarity, and other chemical and water quality characteristics. Reefs thus are excellent sentinels of the quality of their environment. Proper monitoring of reefs can identify changes in water quality or impacts from land-based activities. Monitoring changes in water quality can help local resource managers understand the implications of actions occurring in watersheds that are associated with particular coral communities. These connections will help in development of sound management plans for coral reefs and other coastal and marine resources.

Man has had a long association with reefs. They are important fishery and nursery areas, and more recently have proved to be very important economically as tourist attractions. Reefs provide protection from erosion to coastlines and sand for beaches. However, reefs located near coastal populations are showing increasing signs of stress and are not faring as well as reefs which are more distant from centers of human population.

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What Problems Exist?

There are two types of stresses associated with reef systems: natural and human-induced. The effects of these stresses can range from negligible to catastrophic. Reefs display a surprising adaptation to short-term natural catastrophic events, such as hurricanes, and usually recover to normal community structure. These natural events can even be considered beneficial in regards to biological diversity. Severe storm events on land can topple large trees. This opens up the forest to recolonization and results in a greater diversity of plants. This same process occurs with storm impacts to reefs. The damaged area of the reef is often recolonized by a greater diversity of organisms than existed before the storm. In the long term this event benefits the ecological integrity of the reef.

However, reefs are not well adapted to survive exposure to long-term stress. Some examples include agricultural and industrial runoff, increased sedimentation from land clearing, human sewage and toxic discharges. Many land-based activities have important implications for reefs. Agricultural activities can introduce herbicides, pesticides, fertilizers and runoff from animal feed lots. Sewage discharges can introduce nitrogen and phosphate compounds along with pathogens and mixtures of toxics. Uncontrolled land clearing can result in erosion, with the resultant increase in sediment loads to surface waters. Roadways, parking lots and buildings consist of impervious surfaces. These surfaces increase runoff rates and carry with those waters mixtures of dissolved substances to surface waters. The surface waters in any watershed eventually discharge into coastal or near-coastal waters. These waters can then impact coral communities associated with these discharge points. Thus, activities occurring in distant locations have impacts to reefs which are far away from these activities.

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Are There Solutions?

There have been increasing efforts to establish better management and conservation measures to protect the diversity of these biologically rich areas. Management practices have historically focused on the coral reef proper and not considered associated communities, such as seagrasses, mangroves, mudflats or defined watersheds (which transport complex mixtures in their waters), in a meaningful manner. This attempted to manage the reef in isolation, like an island.

Current management efforts recognize the importance of including reefs as part of a larger system, where integrated coastal zone management tools and watershed concepts can be used in the development of comprehensive management and conservation plans. One example is where EPA has joined with NOAA and the state of Florida in the establishment of the Florida Keys National Marine Sanctuary (FKNMS) Exit EPA Disclaimer . The Marine Sanctuary operates under a broad-based management plan intended to enhance the sustainability of the Florida Keys reefs. Other important efforts in coral reef conservation and management include the U.S. Coral Reef Task Force and the International and the U.S. Coral Reef Initiatives.

When reefs are considered as part of a larger watershed, the recognization of the complexity of environmental stressors can be understood. Management plans can be developed to lessen impacts to mangroves, seagrasses and the reef ecosystem, based upon scientific data and a better understanding of the system. EPA is in the process of developing guidance for a watershed approach to coral ecosystem protection.

See also:

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For additional information contact:
Office of Wetlands, Oceans and Watersheds
Ocean and Coastal Protection Division
Mail Code 4504T
1200 Pennsylvania Avenue NW
Washington, DC 20460

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