Each year divers, fishermen and -women, scientists, and sightseers visit coral reefs. These brightly colored marine communities are found off the coasts of more than 100 countries, including the United States, Australia, India, China, Japan, Mexico, and Belize. At first glance, the reefs appear to be magnificent underwater structures built from stone. Closer inspection reveals that these aquatic complexes are actually composed of millions of living organisms resting atop the skeletons of their ancestors.
The living and growing parts of the reef form only a thin veneer on top of the remains of dead corals, algae, mollusks, and sponges. As organisms die, they leave behind their skeletons, expanding the base on which the next generation builds. Over thousands of years, coral reefs grow to gigantic sizes, reaching lengths of several miles.
Although visits to coral reefs reveal colossal structures and abundant life, these systems are rare, occurring in less than 0.4 percent of the ocean’s waters. Their scarcity is due to their requirements for precise physical conditions. Reefs develop and thrive in seawater within a narrow range of temperatures. Coral animals require some nutrients but are intolerant of extremely high levels. The water of reefs must be energetic enough to dissolve and incorporate oxygen, and it must be shallow enough to be penetrated by light. This unique set of conditions is most likely to occur in locations near the equator.
People are interested in coral reefs for a variety of reasons. Many gain their living from these aquatic gardens, harvesting their bounty or marketing their beauty. Some coastal communities are protected from the brunt of the ocean’s forces by the barrier provided by the reef’s physical structure. Leaders in the fight against disease are exploring the reef’s collection of unique chemicals, looking for those with potential as medications. As reefs gain attention, citizens of the world are becoming increasingly aware of the uniqueness and fragility of these ecosystems. More and more, coral reefs are being recognized as wild places whose existence may be endangered by human activities. The key to their survival may hinge on humankind’s ability to understand them better.
Covering only about 108,000 square miles (about 280,000 sq km) in total, reefs make up a relatively small part of the ocean; however, they are remarkably important ecosystems, supporting more than 25 percent of all known marine species. Coral reefs serve as homes, nurseries, feeding grounds, and gathering places for thousands of kinds of living things, such as the pyramid bluefish in Figure above. The great variety of organisms found among the coral reefs makes them the most biodiverse marine ecosystems on the planet. For that reason, some scientists refer to them as “the tropical rain forests of the ocean” because, like rain forests, reefs support great biodiversity.
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| The pyramid bluefish is one of hundreds of brightly colored species that live on coral reefs. (Courtesy Google Images) |