Coral reefs


Executive Editor: George Clark; Editor: Paul Malamud
U.S. Department of State Office of International Information Programs

Great Barrier Reef in Australia. Courtesy of

Coral reefs are one of the wonders of nature, because of their enchanting beauty and unusual biology. In addition, many consider them to be second only to tropical rain forests as incubators and protectors of biodiversity.

The reefs, which grow in shallow, warm waters, consist largely of the skeletons of small, sedentary animals called polyps, which are relatives of jellyfish and sea anemones. The remains of dead polyps - in the form of calcium carbonate - constitute the main body of the reef. Living polyps form a kind of skin over the surface of the coral reef.

As they move through their life cycle, coral polyps secrete a hard skeleton or shell of calcium carbonate, into which they contract to protect themselves. When they die, their calcium carbonate remains add to the structure of the reef. For this reason, coral reefs enlarge themselves and become complex structures over the years.

Reef in Thailand. Courtesy of

Coral polyps begin their lives as larvae floating free. As mature adults, they are sessile animals, that is, they are fixed to one place. They range in size from the diameter of a teacup saucer to a pinhead. Corals feed by reaching out from their perches with tentacles to sting plankton. Their most unusual biological property, however, is a symbiotic relationship they form with a species of algae named of zooxanthellae. The algae infiltrate the bodies of the coral polyps, and use photosynthesis to produce nutrients they share with the polyp. Zooxanthellae can provide up to 90 percent of the nutrition the coral needs to survive. By living in a polyp, the algae receive some protection and are moved closer upwards through the ocean towards the light as the coral structure grows - making it easier for them to perform their photosynthesis. It is the algae that give coral reefs their hues, dramatically reflected by many of the fish and plants and other animals that dwell in and around them. The color pigment given to the polyp by the algae may even work as a kind of sunblock, protecting the polyp from solar radiation. Coral polyps form various kinds of reef structures that have been given names like "brain," "star," and "elkhorn." A non-reef-building coral, "octocoral," can look like trees and shrubs and forms "sea fans" and "sea whips."

Sea Anemone. Courtesy of

Coral reefs have been around for about 200 million years, and have survived eons of storm-induced damage and sea animal predation. Unfortunately, their survival in this century is less certain. The year 2000 report from the Global Coral Reef Monitoring Network says that approximately 25 percent of coral reefs worldwide have been effectively lost and another 40 percent "may be lost" by 2010 unless urgent action is taken.

Warming oceans, pollution from human activities, damage from careless tourists and fishermen -- even increased ultraviolet radiation from the sun due to the depletion of ozone in the upper atmosphere -- have been blamed for extensive illness and death in the coral population. Corals are uniquely vulnerable because they are near coastlines and near the surface of the ocean. There are fewer healthy coral colonies on the planet than even a few decades ago, according to marine scientists. One of the most frequently studied pathologies is known as "coral bleaching." When stressed by overly warm water, or other causes, coral polyps tend to expel their zooxanthellae, losing their pigment in the process and exposing the white calcium carbonate structure underneath. When this process continues long enough, the reefs become sickly, or die.

Fan coral in Belize. Courtesy of

In a report presented to the U.S. Coral Reef Task Force in 1999, the U.S. State Department warned: "In 1998 coral reefs around the world appear to have suffered the most extensive and severe bleaching and subsequent mortality in modern record. In the same year, tropical sea surface temperatures were the highest in modern record, topping off a 50-year trend for some tropical oceans. These events cannot be accounted for by localized stressors or natural variability alone". The geographic extent, increasing frequency, and regional severity of mass bleaching events are likely a consequence of a steadily rising baseline of marine temperatures, driven by anthropogenic global warming."

Admittedly, corals have natural predators and diseases. But while these and human damage from recreation or fishing are major threats to coral, most observers agree that an altered environment plays a crucial role. In addition to stresses due to changes in ocean temperature, reefs are increasingly exposed to sewage, agricultural runoff, and associated algae blooms near coastal shorelines, themselves the results of human pressure on coastal ecosystems. Sediment that clouds water is bad for coral, and increasing amounts are flowing into the ocean. Unsustainable and damaging fishing practices, including the use of cyanide and explosives to kill fish, trawling, and other forms of drag-netting, are also destroying reefs.

Jellyfish. Courtesy of

Coral bleaching was known over a century ago, but seems to have begun in earnest in the 1980s, and is attacking reefs in most parts of the world. In the early 1980's, 70 percent of the coral along the Pacific Central American coast bleached. It is estimated that 70 percent of coral in the Indian Ocean have died. The U.S. Coral Reef Alliance has estimated that 80 percent of coral near the Seychelles, and 90 percent near Indonesia have died. Corals offshore the Maldives, Bahrain, Sri Lanka, Singapore and Tanzania are deteriorated, and Caribbean coral is coming under pressure. Extensive coral reef structures near Jamaica, in the Caribbean, have lost their polyps and are now limestone skeletons covered with algae. Even when they survive, it is not easy for coral reefs to regenerate themselves. Estimates of reef growth range from one to 16 feet every thousand years.

Healthy coral reefs foster species diversity. Funguses, sponges, mollusks, oysters, clams, crabs, shrimps, sea urchins, turtles, and many fish seek food and shelter amid reefs. The architecture of corals provides reef fish protection from carnivorous species such as sharks and barracudas. Sea cucumbers, worms, and mollusks burrow into the reef-generated sand to hide from their enemies. According to the Worldwatch Institute publication, "State of the World 2000," reefs include only 0.3 percent of the ocean area, but "one out of every four ocean species thus far identified is a reef-dweller, including at least 65 percent of marine fish species."

Mahi mahi. Courtesy of

Historically, coral reefs have been important to fishermen; increasingly, they stimulate local economies by drawing tourism. They protect coastlines from erosion, and over the eons have helped create brilliant beaches as their calcium carbonate leached on to the shore. According to the U.S. Fish and Wildlife Service, "Reef systems are storehouses of immense biological wealth" that provide "sources of food, pharmaceuticals, jobs, and revenues". Reef habitats provide humans with services worth about $375 billion [thousand million] each year, despite the fact that they cover less than one percent of the earth's surface." The U.S. State Department estimates that "reefs provide one-quarter of the fish catch in developing countries and employment for millions of fishers." Corals are also sensitive indicators of the health of the aquatic environment. They flourish in a fairly narrow range of temperatures, salinity, and water purity. The die-off of corals going on in many oceans does not bode well for the health of the oceans themselves; and healthy oceans are essential if life on the planet is to be sustained in its current form.

Attempts to restore coral reefs and manage their biological richness better include efforts being carried out to inventory and protect the structures themselves. Watershed management, including protection and conservation of wetlands with their mud flats, mangrove forests, and sea grasses, can help the estuarine system, including corals, to remain clean and healthy.

Salmon. Courtesy of

Coral reefs have come to the attention of the public only recently, perhaps because fewer people visit a coral reef than a forest or prairie. Governments and private-sector organizations have taken note of the deterioration of the world's reefs, and are trying to find solutions. In 1994, the U. S. government helped found the International Coral Reef Initiative, a partnership designed to address threats to coral reefs. In 1996, the U.S. Coral Reef Initiative was launched to support these efforts and aid them domestically. And in 1998, the president issued an executive order directing U.S. government agencies to protect coral reefs. This executive order also established the United States Coral Reef Task Force, co-chaired by the Secretary of the Interior and the Secretary of Commerce, including other federal departments. Its duties include the promotion of reef mapping, scientific research, restoration, and collaboration with other nations. As it begins to operate, the task force has focused in on illegal trade in corals and associated sea life as one important cause of reef destruction. Even aquaculture of species such as shrimp can harm reef environments, in part by undermining biodiversity to produce large amounts of a single species. Efforts are under way in the U.S. Congress to support the task force and improve mapping and conservation of reef systems under U.S. jurisdiction.

GBR, Australia. Courtesy of

One way to help restore reef environments is simply to protect them from undue exploitation. Writing in Issues in Science and Technology, marine biologist Tundi Agardy observed, "Scientific studies on the effect of no-take reserves in East Africa, Australia, Jamaica, the Lesser Antilles, New Zealand, the Philippines, and elsewhere all suggest that small, strictly protected no-take areas result in increased fish production." In addition: "Preliminary evidence from a 1997 fishing ban in 23 small coral reef reserves by the Florida Keys Marine Sanctuary, indicates that several important species, including spiny lobsters and groupers, are already beginning to rebound."

Coral reef off Cancun, Mexico. Courtesy of

Australia's Great Barrier Reef Marine Park is often cited as one example of enlightened coral reef management. Nations as diverse as Guinea Bissau, Spain, and Croatia have established marine and watershed reserves. In many instances, national governments initiate the conservation measures; in others, local communities initiate conservation efforts, with the assistance of the government. Like all ecosystems, reefs have sections and areas more crucial to biodiversity than others. Determining which these are can be an important part of conservation. "Zoned" networks of vital areas of reef can be easier to protect than an entire system. When the most biologically vital parts of the reef are put off-limits, other areas can be made available for commercial use and tourism. The Florida Keys National Marine Sanctuary management plan, for example, establishes protective zones, as well as recreational and commercial zones -- and sets aside areas for scientific research.

University of Maryland zoologist Marjorie L. Reaka-Kudla has estimated the number of species living on coral reefs at 950,000, of which about 10 percent have been studied and described. Mankind is just beginning to perceive the value of coral reefs, with their known supplies of food and as-yet-unexplored biota that could lead to the development of new medicines. The U.S. State Department estimates "half the potential pharmaceuticals being explored are from the oceans, many from coral reef ecosystems."

Sunfish. Courtesy of

In Reef Research, Dr. Patrick Colin, a marine "bioprospector," clearly described the hopes that had led him to spend the 1990s collecting marine samples in the Pacific for the U.S. National Cancer Institute (NCI). "Over the past 20 some years the NCI has been screening terrestrial plants and marine organisms worldwide for bioactivity against cancer and AIDS, and has come up with a number of hot prospects, a number of which are in clinical trials". We try to collect from all environments possible, from shoreline areas with mangroves, beaches or rocks to deep offshore reef environments". We do not collect any hard (stony) corals, threatened, endangered or locally protected species. We are mostly interested in soft-bodied sessile invertebrates which rely on their chemistry, rather than stinging cells, spines, jaws or teeth for their survival."

Clearly, conservation of coral, and oceans in general, is linked to human survival and will continue to be an urgent issue in the 21st century.



Benefits of Biodiversity
Traditional crop varieties and food security
Plant-based medicines
The death of global biodiversity
How to save biodiversity

Coral reefs
Endangered Forests [page 2]
Biodiversity of Wetlands
Arctic Biodiversity