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Coral reefs

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A coral reef is a structure in the sea constructed by coral skeletons and limestone debris that remains in place after the plant or animal dies. The structure is geological, the communities include plants and animals, and they are controlled by meteorological and oceanographic conditions.

Distribution and Roles of Coral Reefs

Coral reefs include shallow-water tropical reefs, such as the Great Barrier Reef off the east coast of Australia, and deep-water reefs, such as Sula Bank off Norway. Tropical reef diversity (the number of different plants and animals) is high, with complexity similar to tropical rain forests.

Natural and Human Values.

Coral reefs function as living breakwaters. For example, on Bikini Atoll, Marshall Islands, the wave energy that strikes this reef amounts to 20 horsepower per surge channel per wave (waves arrive at the surge channel at a rate of one wave approximately every 8 seconds).

A cross section of a reef shows that it is built by multiple generations of corals and that there is significant void space. This empty space provides refuge for plants and animals. A coral cut in half would reveal small tunnels and galleries created by boring sponges, clams, and other animals that provide homes for worms, shrimp, and fish.

The tourism and fisheries industries supported by coral reefs are important to the economies of tropical areas, as well as the economies of the countries from which the tourists travel. Conservation efforts have been aided by the high visibility of reefs and their biocommunities.

Coral: Simple Animals.

Corals exist at the tissue level: they do not have organs, such as a heart. On the evolutionary ladder, corals are one step above the sponges. They are the simplest animals to have nervous and connected muscular systems and a dedicated reproductive system. They belong to the phylum Cnidaria, which also includes the jellyfish.

Each coral animal consists of an individual sac-like body called a polyp. Tentacles radiate from the mouth end of the polyp and bristle with tiny hairs; these are the triggering devices for a microscopic harpoon called a nematocyst. The harpoon includes a barb for injecting neurotoxin, and a tube to connect the barb and the toxin’s reservoir. Corals are colonial animals, and multiples of polyps form a colony.

Types of Coral

The more common corals include hydrocorals, octocorals (polyps with eight tentacles), and scleractinian corals (polyps with six, or multiples of six, tentacles).

Hydrocorals.

The hydrocorals include the fire corals (genus Millepora ). Millepora is common on shallow, tropical–subtropical reefs in the Caribbean and the Pacific. Hydrocorals have a life cycle of alternating sexual and asexual generations known as metagenesis. The hydroid asexual generation attaches itself to the reef and produces a medusa offspring by budding. The medusa generation sends eggs and sperm into the water, and the product of this union, a free-swimming larva, settles on the bottom and grows into a new hydroid coral. The upward growth for Millepora is about 1 centimeter (2.5 inches) annually.

Octocorals.

Octocorals include sea fans, plumes, mats, and rods. They are flexible and sway to and fro in the waves. The octocoral skeleton is a matrix of limestone structures called spicules and an organic connective material. Spicules provide strength like bones, and the connective material holds the spicules together.

Pillar coral, Dendrogyra cylindrus, is a Caribbean coral species noted for its cathedral-like columns or pillars. (Bottom) A coral species from the family Mussidae is shown here exhibiting the septa, a feature of the skeleton (thin plates); the red color is contributed by plant pigments in the zooxanthellae.

Octocorals reproduce sexually, with eggs and sperm released in the water column. Following fertilization, the larvae may remain in the water column for days or weeks before settling and attaching to the sea floor. An individual larva settles and grows into an adult. Octocorals in the western Atlantic grow from 1 to 4 centimeters (2.5 to 10 inches) annually. Scleractinian Corals. The scleractinian corals have many growth forms: branching, hemispheres, columns, sheets, mushrooms, and tubes. Because of the strength and persistence of their limestone skeletons, these corals are the principal reef architects.

Reproduction in the scleractinian corals is either by broadcasting eggs and sperm into the water or by internal fertilization and brooding of larva. Some corals have female and male sex organs in the same polyp and are capable of self-fertilization. Larvae either live in the water column or crawl along the bottom.

Branching corals have relatively rapid growth (15 centimeters, or 6 inches, per year). The boulder corals grow 1.2 to 2.5 centimeters (0.5 to 1 inch) per year. Branching corals are more fragile, often breaking during storms and generating fragments that in turn can grow into new coral colonies. Boulder corals generally do not fragment.

Symbiosis: Living Together

Nearly all the shallow-water corals and related Cnidarians have a microscopic symbiotic algae living in the tissues. The alga, called zooxanthellae, is an important partner in the success of tropical coral reefs. Zooxanthellae provide the means for corals to sustain high growth and reproduction rates in waters that are low in nutrients . Metabolic wastes generated by animal tissues are used by the alga; the corals use the fats, oils, and sugars synthesized by the alga during photosynthesis.

For the process to function optimally, three things are necessary: sufficient light, clear water, and temperatures that range from 20 to 30°C (68 to 86°F). The process of photosynthesis is conceptualized as follows.

Carbon Dioxide + Water light energy → Sugar + Water + Oxygen + Energy

The energy provided by photosynthesis enhances coral growth. Corals grow by taking in calcium ions from the sea and combining them with bicarbonate ions. The result, calcium carbonate, bonds as a single crystal onto other crystals, creating the limestone skeleton. Energy generated from photosynthesis expedites the calcium carbonate movement from the tissues to the skeleton. The steps follow.

Calcium bicarbonate formation: Ca 2+ + 2HCO 3 − → Ca(HCO 3 ) 2

Calcium carbonate and carbonic acid formation: Ca(HCO 3 ) 2 → CaCO 3 + H 2 CO 3

Carbonic acid ionization: H 2 CO 3 → H + + HCO 3 −

Conversion to water and carbon dioxide: H 2 CO 3 → H 2 O + CO 2

Examples of Coral Reef Types

Reefs in the Pacific Island chains (Hawaii, French Polynesia, and the Marshall Islands) initially are formed on the sides of old volcanic mountains and are called fringing reefs. In the process of tectonic plate movement, the plate carries the mountain into deeper water, and the reef becomes a ring that surrounds the mountain but is separated from it by a body of water called a lagoon. The reef is now called a barrier reef. Over thousands of years, the mountain submerges, leaving a ring of reef and a few low islands called an atoll. The terms “fringing,” “barrier,” and “atoll” were first used in 1834 by Charles Darwin in his book about coral reefs.

Norway.

Deep-water coral assemblages include those off the coast of Norway. Because they live in darkness, deep-water corals do not have the symbiotic zooxanthellae that are common to the shallow reefs. The reefs are built up from the sea floor by a branching coral, Lophelia pertusa.

Coralline algae produce calcium carbonate skeletons that assist in building reefs. The coralline algae typically are found in areas with heavy wave surge.

These coral banks are often associated with petroleum gas seeps that are surrounded by bacterial mats. The biological productivity (chemosynthesis) from these mats supports a food web somewhat similar to deep-sea thermal vent communities: high biomass and productivity without the influence of light. Corals, presumably, obtain small prey animals (crustaceans, worms, and mollusks) that feed on the bacterial mats.

Lophelia pertusa banks are often miles long, up to 30 meters (100 feet) high, and equally wide. The maze of twisted and interlocking branches provides refuge for resident fish populations.

Nova Scotia.

Off the coast of Nova Scotia, on the fishing banks, most corals are large octocorals; some old and large colonies are up to 5 meters (about 16 feet) high. Paragorgia arborea, a common coral, is pink to orange in color with branches ending in blunt bundles resembling a wad of gum, living up to its common name—bubblegum coral. Although this coral is not well studied, its presumed ecological value is as a refuge for juvenile fish. This is a region that in the past had large populations of codfish; however, overfishing has decimated the fishery.

Florida.

Off the central east coast of Florida, another branching coral, Oculina varicosa, builds banks upward from the sea floor. Oculina banks are a refuge area for a variety of fish, such as the snowy grouper. These deep-water coral reefs exemplify two common themes of deep-water corals: one species builds the framework, and photosynthesis is not a major energy source.

Physical and Biological Controls

Tropical cyclones are the most important natural force controlling reef development. Storms generate massive waves that break up coral formations, heavy rains that reduce salinity, and silt deposits in reefs close to high islands. Damage and recovery depends on storm strength, wind direction, duration, and the frequency of events.

The crown-of-thorns starfish (or acanthaster) eats coral polyps. If present in sufficient numbers, this voracious starfish can decimate coral populations.

Crown-of-Thorns Starfish.

The biological controls on coral reefs generally are not as catastrophic as a tropical cyclone; however, a number of reefs have been virtually picked clean of coral thanks to a voracious predator. In the Pacific and Indian Oceans, a starfish called the crown-of-thorns (Acanthaster planci), has a history of population explosions. When the crown-of-thorns (COT) reaches abundances of several per square meter, they will eat virtually every coral in sight.

In the 1970s, conventional wisdom held that COT outbreaks were related to insufficient predators to control them. In Australia and Guam, a bounty of $1 per COT collected was instituted. Others placed the blame on pollution.

However, geologists sampled deep sediments on the reefs and found pockets of COT spines. Determining the age of these spines indicated that COT population explosions occurred thousands of years before the arrival of Europeans in Australia. The COT outbreaks seem to be related to the typhoons that occasionally strike the east coast of Australia. The torrential rainfall from the typhoon causes heavy flooding, bringing nutrients into the near-shore waters around the Great Barrier Reef.

The COT starfish usually lays millions of eggs, which develop into floating (planktonic) larvae. Normally, the larvae would have a poor chance for survival because of a lack of food. After a flood event with the enriched water, phytoplankton blooms occur. Such blooms are key to COT larval survival. The COT larvae thrive on the rich soup of plankton, and when they metamorphose into the adult COT, huge swarms of COT begin feeding on coral.

If a COT event occurs approximately once per decade, there is sufficient time for the coral to recover before the next COT disturbance. However, if a COT event occurs every three years, the reef does not have sufficient time for the coral larvae to settle and grow and replace the lost corals.

Long-Black-Spine Sea Urchin.

The concept of keystone species for coral reefs is not subscribed to universally; however, there is some validity to the idea. In western Atlantic coral reefs prior to 1983, a prodigious algae grazer was held up as the keystone for controlling the algal growth on the reefs and for creating space for the coral larvae to settle. Studies have documented the value of the long-black-spine sea urchin in removing algae and enhancing coral recruitment on Caribbean reefs.

In 1983 and 1984, a pandemic disease killed 99 percent of the urchins on reefs from Barbados to Panama. Since that disease event, the urchin has not recovered to reclaim its role as a major herbivore . Today, most Caribbean reefs have moderate to high algal cover. There is continuing debate as to whether the algal cover seen today is the result of higher levels of nutrients in the waters, or whether it is due to the lack of grazing.

Hazards and Global Stresses

Various environmental hazards can threaten coral reefs. Some stresses are natural, whereas others are magnified by human activities.

Thermal Stress.

Elevated surface sea-water temperatures cause coral bleaching at the time of seasonal maximum heating (late summer to early fall). This thermal stress results in the expulsion of the zooxanthellae (symbiotic alga), causing the corals and similar reef inhabitants to turn white and, in severe cases, to die. Loss of the zooxanthellae results in reduced or no growth, no reproduction, and susceptibility to disease.

Corals in shallow tropical reefs are at the near-margin of upper thermal tolerance. Most corals will tolerate water of 30°C (86°F); however, if the temperature reaches 32°C (90°F), corals will bleach.

Factors that affect temperature include tide, wind, time of year, cloud cover, and currents. In the 1960s and 1970s, bleaching events were rare, occurring about once a decade in a region. By the 1990s, bleaching events were occurring every other year or every third year. The magnitude of the events (oceanwide and the extended duration of the episode) resulted in massive coral mortalities in reefs in the Indian Ocean and western Pacific. An El Niño event could have played a major role in a vast coral bleaching event in French Polynesia’s Rangiroa Atoll in 1998.

Global temperature is rising, and this in turn elevates surface sea-water temperatures. Global warming is believed to be the result of increased greenhouse gases (such as carbon dioxide) and an increase in ultraviolet radiation due to the loss of ozone in the atmosphere caused by the release of chlorofluorocarbon compounds (CFCs).

Dust.

Satellite images show the movement of dust across the oceans from other continents. * African dusts from the sub-Saharan region are entrained in the upper winds and carried across the Atlantic. The dust includes spores from bacteria and fungi, and mineral elements such as iron, copper, mercury, and arsenic. A well-documented disease in a Caribbean sea fan (Gorgonia ventalina) is caused by a fungus whose spores are carried in African dust. The ocean, land, and atmosphere are interconnected; no part of the global community is insulated from influences that come from other continents or oceans.

Fish and Agriculture.

Reefs are under siege in some places by destructive fishing practices: dynamite, chemicals, dredging, poor land-use practices, slash-and-burn agriculture on hillslopes, and fishing efforts beyond maximum sustainable yield. The recent trend in protecting reefs is to designate large areas as parks or reserves and not to allow the harvest of any plant or animal. This maximizes conservation of the entire reef community.

Tropical settings and a diversity of easily viewable biota make snorkeling a popular tourist activity on shallow-water coral reefs. Deep-water reefs are less accessible and hence have not captured as much popular appeal.

Preserving Coral Reefs.

Marine protected areas (MPAs) permit visitors to look at, but not take, anything; these areas therefore are a reasonable way to protect reef resources. An MPA protects the habitat and the target species that the fishers are harvesting.

The Florida Keys National Marine Sanctuary and the Great Barrier Reef Marine Park in Australia have created “no-take” zones to protect all the plants and animals. The spillover effect provides a source of animals to replenish other areas; that is, as the MPA becomes saturated, some animals move away. Within 2 years of creating the Florida Keys no-take zones, populations of grouper, snapper (fish), and lobster increased dramatically.

SEE ALSO B IODIVERSITY ; E L N IÑO AND L A N IÑA ; G LOBAL W ARMING AND THE O CEAN ; L AND -U SE P LANNING ; L IFE IN W ATER ; O CEANS , T ROPICAL .

Walter C. Jaap

Bibliography

Connell, Joseph H. “Diversity in Tropical Rain Forests and Coral Reefs.” Science 199 (1978):1302–1310.

Jaap, Walter C., and Pamela Hallock. “Coral Reefs.” In Ecosystems of Florida, eds. Ronald L. Myers and John J. Ewel. Orlando: University of Central Florida Press, 1990.

Littler, Dianne S., and Mark M. Littler. Caribbean Reef Plants. Washington, D.C.: OffShore Graphics, 2000.

Thorne-Miller, Boyce, and John G. Catena. The Living Ocean: Understanding and Protecting Marine Biodiversity. Washington, D.C.: Island Press, 1991.

Veron, John E. Corals of the World, Vols. 1–3. Townsville: Australian Institute of Marine Science, 2000.

Wells, Sue, and Nick Hanna. The Greenpeace Book of Coral Reefs. London, U.K.: Blanford, 1992.

Internet Resources

Corals. Australian Institute of Marine Science. <http://www.aims.gov.au/pages/search/search-corals.html> .

NOAA’s Coral Health and Monitoring Program. National Oceanic and Atmospheric Administration. <http://www.coral.noaa.gov> .

CORALS AS HISTORIC RECORDKEEPERS

Just as tree rings or layers of sediment provide environmental records of past events, skeletons of massive corals provide records that are useful in climate research. Each year, the coral precipitates calcium carbonate in two density layers that, if X-rayed, provide relative growth rates and coral age. Isotopes from the skeleton yield information on temperature and salinity. Corals exposed to floods of fresh water have fluorescent bands (seen under black light) because they retain humic and fulvic acids in their skeletons.

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