Reefs are any rigid, water-resistant structure, usually in shallow water, that is inhabited by aquatic organisms. Most reefs are found in shallow, tropical ocean waters and are built primarily by invertebrates called corals. Corals exude a calcium carbonate skeleton, which over time forms a three-dimensional structure containing many nooks and caves. This reef structure provides abundant habitat for sessile, encrusting, and burrowing animals. Fish, swimming invertebrates, and some reptiles also flourish on coral reefs. As a result, coral reefs are among the most diverse and productive ecosystems in the world.
Historical Background and Scientific Foundations
Reefs are shallow water areas that create abundant habitat for marine organisms. Reefs can be formed by some species of worm, oysters, cyanobacteria, and algae. Engineers also create artificial reefs by sinking large machinery in shallow waters. However, the majority of reefs in the ocean result from the accumulation of calcium carbonate skeletons of tiny invertebrates called corals.
Corals are invertebrates, classified in the phylum Cnidaria. They are related to jellyfish and sea anemones. Reef-building coral are generally colonial, growing in clusters. Each individual has a shape similar to a flower: a cuplike body with tentacles in the place of petals. Within the center of the tentacles is a structure that serves both as a mouth for ingesting small plankton food particles as well as an excretory structure. Coral secrete a hard, cementlike substance called aragonite that makes up a skeleton in which the coral live. As subsequent generations of coral lay layers of aragonite on top of older skeletons, a reef is formed.
Symbiotic algae called zooxanthellae live within the tissue of coral. As these symbionts perform photosynthesis, they facilitate the conversion of dissolved calcium into the calcium carbonate that forms aragonite. In particular, zooxanthellae create an alkaline environment in which calcium carbonate deposition proceeds easily. In addition, zooxanthellae can provide a very significant portion of a coral’s energy requirements. In turn the coral provide a stable environment in which the algae can live and grow as well as a constant source of carbon dioxide that is required for photosynthesis.
Because coral rely heavily on their relationship with zooxanthellae, they must grow in environments in which light is sufficient for photosynthesis. This limits the range of coral reefs to shallow waters where light penetrates. Zooxanthellae also restrict in the temperatures at which coral can live. These algae require water temperatures between about 70 to 79°F (21 to 26°C), which limit corals to tropical waters roughly between latitudes of 25°N and 25°S. Corals themselves are susceptible to changes in salinity. As a result reefs stay below the surface of the water because rain mixing with surface waters can decrease the salinity of ocean water and cause osmotic shock to corals. Sediment can be lethal to corals, as it can clog their feeding appendages, so reefs are rarely found near river mouths or other areas of significant erosion.
Coral reefs create abundant ecological niches for many different types of marine organisms. The three-dimensional structure of the reef provides holes to hide in, surface to grow on, and substrate to burrow through. As animals come to reefs to perform these functions, predators and mates are attracted to the reefs as well. Reefs become places teeming with life from the microscopic diatoms that swarm across the surfaces of encrusting algaes to enormous manta rays that cruise the reef for food. Reefs are home to nearly every phylum of invertebrate as well as many classes of fish and shark, and are often inhabited by sea turtles and ocean snakes.
A typical reef ecosystems might include staghorn corals shaped like deer antlers, green corals shaped like brains, sea stars of blue, red, and orange, and pink and orange sea anemones with orange and white striped clownfish swimming among their tentacles. Large, purple fanlike gorgonians wave in the currents as speckled sea cucumbers crawl along the reef bottom. Spider crabs, mantis shrimp, spiny lobsters, and sea slugs all hunt along the reefs for meals of smaller invertebrates and plant material. Bryozoans, both circus-like and drab, filter water through their elaborate feeding apparatus. Jellyfish and ctenophores float through the tropical waters hunting for small crustaceans that they sting with specialized dart cells. Fish of every color and size from tiny purple gobys to enormous white-tipped reef sharks are found in reef ecosystems.
The enormous diversity in the ecosystem drives intense competition among the members of the reef community. Organisms compete for space, nutrients, light, mates, and hiding spaces from predators. As a result, many reef animals have developed bright colors to warn away predators and competitors and to attract mates. Some reef animals have also developed sophisticated defenses, like poisons, stinging cells, barbs, and spines.
There are three major types of coral reefs, as first classified by Charles Darwin (1809–1882), and still recognized today. Fringing reefs border a coastline. They are common near Hawaii and in the Caribbean. Barrier reefs are found farther offshore than fringing reefs. A
WORDS TO KNOW
CORAL: Invertebrate organisms in the phylum Cnidaria that form reefs in tropical ocean waters.
CYANOBACTERIA: Photosynthetic bacteria, commonly known as blue-green algae.
ECOSYSTEM: The community of individuals and the physical components of the environment in a certain area.
SESSILE: Any animal that is rooted to one place. Barnacles, for example, have a mobile larval stage of life and a sessile adult stage of life.
SYMBIOSIS: A pattern in which two or more organisms of different species live in close connection with one another, often to the benefit of both or all organisms.
ZOOXANTHELLAE: Algae that live in the tissues of coral polyps and, through photosynthesis, supply them with most of their food.
large channel usually forms between the reef and the shoreline due to tectonic activity and erosion. Barrier reefs can be found in the Caribbean and along the eastern coast of Australia. Atolls form when a fringing reef forms around a small island and the island subsides below the surface of the ocean, leaving a circular reef formation. Atolls are found most often in the Indo-Pacific. Some of the coral reefs that are in existence today probably formed as early as 10,000 years ago when the most
recent glacial epoch ended and sea levels rose to their current height.
Impacts and Issues
Corals play an important role in the global ocean and atmosphere system. Because of their intricate symbiosis with zooxanthellae, they act as a sink for carbon dioxide. The zooxanthellae remove carbon dioxide from the atmosphere through photosynthesis and synthesize carbohydrates that provide energy to coral for metabolic processes. Among these processes is the production of aragonite, which contains carbon, thereby removing this important greenhouse gas from the atmosphere for a significant period of time.
Humans directly benefit from reef ecosystems. Reefs themselves act as a physical barrier to the shoreline, creating a buffer from floods, storms, and strong waves. When reefs are destroyed, erosion of shoreline often accelerates. This can result in significant destruction of property during serious storms and extreme tidal events. Reefs provide an important fishery resource. Crabs, lobster, mollusks, shrimp, and fish are often harvested from reef areas. In addition, reefs are popular tourist spots, often creating important economic value in tropical regions.
Because of the great diversity of organisms found in coral reef ecosystems, scientists suspect that inhabitants of the reef could be a potential source of medicine and other chemicals useful to humans. Research has suggested that effective sunscreens could be derived from chemicals found in coral reef organisms. Corals are used in bone replacement therapy because aragonite has been shown to be an effective matrix into which new human bones can grow.
Perhaps the most important problem facing coral reefs is known as bleaching. With ever-increasing frequency, corals go through periods where they eject the zooxanthellae from their tissues. Without their important contribution to the energy requirements of the coral, the coral start to die. Although the exact mechanism that causes bleaching is not completely understood, increases in water temperature likely act as a trigger. Since 1979 there have been six significant bleaching events in the world’s reefs. In 1998 during a severe El Niño event, 48% of the reefs in the Western Indian Ocean underwent bleaching. Nearly 16% of the world’s reefs died by the end of the year. It is estimated that between 60 and 95% of the coral in the Great Barrier Reef also suffered from bleaching. As water temperatures continue to rise in the future due to changes in the global climate, bleaching events are predicted to become even more severe and frequent.
Another concern regarding the health of coral reef ecosystems is acidification of the ocean due to increases in dissolved carbon dioxide into ocean water. As carbon dioxide accumulates in the atmosphere, some of it dissolves into the ocean. This strips carbonate ions out of the water, leaving it more acidic. Coral cannot produce aragonite in acidic environments. Currently atmospheric carbon dioxide is around 380 ppm (parts per million). At concentrations of 500 ppm, scientists estimate that corals will not be able to lay down calcium carbonate skeletons. At current rates of atmospheric carbon dioxide accumulation, the Intergovernmental Panel on Climate Change (IPCC) estimates that this threshold will be reached in about 2050.
Reef ecosystems are impacted by human activities in other ways as well. Oil, pesticides, heavy metals, and garbage all damage or poison both corals and the organisms that live on them. Because they occur in coastal areas, they are impacted by runoff. In particular, when sediments, soils, and other urban wastes are washed into ocean waters, they can clog the feeding structures of corals and cause them to die. Dredging for harbors, overfishing, fishing with dynamite and cyanide, careless underwater divers, the seashell industry, and the aquarium hobbyists have all contributed to coral reef destruction throughout the world.
In response to these human-induced threats, many preserves have been established to protect reef ecosystems. In 1975, the Great Barrier Reef Marine Park was established by the Australian government, protecting the largest coral reef ecosystem on Earth. Many Caribbean countries, such as the Virgin Islands, Belize, and Panama have also established marine preserves and parks. The Conference on International Trade in Endangered Species of Wild Fauna and Flora (CITES) has developed regulations that oversee the use and trade of reef organisms. The United States, Guam, and Puerto Rico have banned the collection of corals, both living and dead.
See Also Aquatic Ecosystems; Benthic Ecosystems; Climate Change; Coastal Ecosystems; Conservation; Coral Reefs and Corals; Ecosystem Diversity; Marine Ecosystems; Marine Water Quality; Oceans and Coastlines; Oil Pollution Acts; Runoff; Sea Level Rise; Temperature Records; Water Pollution
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