coral reef

coral reef

Ridge or hummock formed in shallow ocean areas from the external skeletons of corals. The skeleton consists of calcium carbonate (CaCO3), or limestone. A coral reef may grow into a permanent coral island, or it may take one of four principal forms. Fringing reefs consist of a flat reef area around a nonreef island. Barrier reefs may lie a mile or more offshore, separated from the landmass by a lagoon or channel. Atolls are circular reefs without a central landmass. Patch reefs have irregular tablelike or pinnacle features. Smaller patches occur inside atoll lagoons; larger patches occur as isolated parts of any of the other three reef categories, and they sometimes occur completely separate from other kinds of reefs.

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Coral reefs are aragonite structures produced by living organisms, found in marine waters with little to no nutrients in the water. High nutrient levels such as those found in runoff from agricultural areas can harm the reef by encouraging the growth of algae. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate. The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, shallow-water reefs are formed only in a zone extending at most from 30°N to 30°S of the equator. Tropical corals do not grow at depths of over 50 m (165 ft). Temperature has less of an effect on the distribution of tropical coral, but it is generally accepted that they do not exist in waters below 18 °C.. However, deep water corals can exist at greater depths and colder temperatures. Although deep water corals also form reefs, very little is known about them.


Coral reefs can take a variety of forms, defined in following:

  • Fringing reef – a reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
  • Barrier reef – a reef separated from a mainland or island shore by a deep lagoon (see Great Barrier Reef).
  • Patch reef – an isolated, often circular reef, usually within a lagoon or embayment.
  • Apron reef – a short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
  • Bank reef – a linear or semi-circular shaped-outline, larger than a patch reef.
  • Ribbon reef – a long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
  • Atoll reef – a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island; see atoll.
  • Table reef – an isolated reef, approaching an atoll type, but without a lagoon.


Coral reefs are estimated to cover 284,300 square kilometres, with the Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) accounting for 91.9% of the total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs only account for 7.6% of the world total.

Coral reefs are either restricted or absent from the west coast of the Americas, as well as the west coast of Africa. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas. Corals are also restricted from off the coastline of South Asia from Pakistan to Bangladesh. They are also restricted along the coast around north-eastern South America and Bangladesh due to the release of vast quantities of freshwater from the Amazon and Ganges Rivers respectively.

Famous coral reefs and reef areas of the world include:

  • The Great Barrier Reef - largest coral reef system in the world, Queensland, Australia;
  • The Belize Barrier Reef - second largest in the world, stretching from southern Quintana Roo, Mexico and all along the coast of Belize down to the Bay Islands of Honduras.
  • The New Caledonia Barrier Reef - second longest double barrier reef in the world, with a length of about 1500km.
  • The Andros, Bahamas Barrier Reef - third largest in the world, following along the east coast of Andros Island, Bahamas between Andros and Nassau.
  • The Red Sea Coral Reef - located off the coast of Egypt and Saudi Arabia.
  • Pulley Ridge - deepest photosynthetic coral reef, Florida
  • Many of the numerous reefs found scattered over the Maldives


The building blocks of coral reefs are the generation of reef-building , and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner. Coralline algae are important contributors to the structure of the reef in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing also to the structural integrity of the reef.

Reef-building or hermatypic corals are only found in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form impressive reef structures. Corals can get up to 90% of their nutrients from their zooxanthellae symbionts.

Corals can reproduce both sexually and asexually. An individual polyp may use both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesentery membranes that radiate inward from the layer of tissue that lines the stomach cavity. Some mature adult corals are hermaphroditic; others are exclusively male or female. A few even change sex as they grow.

Internally fertilized eggs are brooded in the polyp for a period ranging from days to weeks. Subsequent development produces a tiny larva, known as a planula. Externally fertilized eggs develop during a synchronized spawning. Polyps release eggs and sperm into the water simultaneously. This spawning method disperses eggs over a larger area. Synchronous spawning depends on four factors: time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tides. The less water movement there is over the reef, the better the chance that an egg will be fertilized. Ideal timing occurs in the spring, release of eggs or planula larvae usually occurs at night and is sometimes in phase with the lunar cycle (3-6 days after a full moon). The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks (7, 14). They are vulnerable at this time to heavy predation and adverse environmental conditions. For the lucky few which survive to attach to substrate, the challenge comes from competition for food and space.

Ecology and biodiversity

Coral reefs support an extraordinary biodiversity; although they are located in nutrient-poor tropical waters. The process of nutrient cycling between corals, zooxanthellae, and other reef organisms provides an explanation for why coral reefs flourish in these waters: recycling ensures that fewer nutrients are needed overall to support the community.

Cyanobacteria also provide soluble nitrates for the coral reef through the process of nitrogen fixation. Corals absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water, and they feed upon zooplankton that are carried past the polyps by water motion. Thus, primary productivity on a coral reef is very high, which results in the highest values per square meter, at 5-10g C m-2 day-1. Producers in coral reef communities include the symbiotic zooxanthellae, coralline algae, and various seaweeds, especially small types called turf algae, although scientists disagree about the importance of these particular organisms.

Coral reefs are home to a variety of tropical or reef fish which can be grouped into several types:

Generally, fish that swim in coral reefs are just as colourful as the reef itself. Examples are the beautiful parrotfish, angelfish, damselfish, Pomacanthus paru, Clinidae and butterflyfish. Other fish groups found on coral reefs include groupers, grunts and wrasses. Over 4,000 species of fish inhabit coral reefs. It has been suggested that the high number of fish species that inhabit coral reefs are able to coexist in such high numbers because any free living space is rapidly inhabited by the first planktonic fish larvae that occupy it. These fish then inhabit the space for the rest of their life. The species that inhabit the free space are random and have therefore been termed 'a lottery for living space'.

Reefs are also home to a large variety of other organisms, including sponges, Cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main group. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs.

A number of invertebrates, collectively called cryptofauna, inhabit the coral skeletal substrate itself, either boring into the skeletons (through the process of bioerosion) or living in pre-existing voids and crevices. Those animals boring into the rock include sponges, bivalve molluscs, and sipunculans. Those settling on the reef include many other species, particularly crustaceans and polychaete worms.

Due to their vast biodiversity, many governments worldwide take measures to protect their coral reefs. In Australia, the Great Barrier Reef is protected by the Great Barrier Reef Marine Park Authority, and is the subject of much legislation, including a Biodiversity Action Plan.

Algae and coral reef

Researchers found evidence of algae dominance in locations of healthy coral reefs. In surveys done around largely uninhabited US Pacific islands, algae consists of a large percentage of the surveyed coral locations. The algae population consists of turf algae, coralline algae, and macroalgae.


Human activity may represent the greatest threat to coral reefs living in Earth's oceans. In particular, pollution and over-fishing are the most serious threats to these ecosystems. Physical destruction of reefs due to boat and shipping traffic is also a problem. The live food fish trade has been implicated as a driver of decline due to the use of cyanide and disaster for people living in the tropics. Hughes, et al, (2003), writes that "with increased human population and improved storage and transport systems, the scale of human impacts on reefs has grown exponentially. For example, markets for fishes and other natural resources have become global, supplying demand for reef resources far removed from their tropical sources.

Currently researchers are working to determine the degree various factors impact the reef systems. The list of factors is long but includes the oceans acting as a carbon dioxide sink, changes in Earth's atmosphere, ultraviolet light, ocean acidification, biological virus, impacts of dust storms carrying agents to far flung reef systems, various pollutants, impacts of algal blooms and others. Reefs are threatened well beyond coastal areas and so the problem is broader than factors from land development and pollution though those are too causing considerable damage.

Land development and pollution

Extensive and poorly managed land development can threaten the survival of coral reefs. Within the last 20 years, once prolific mangrove forests, which absorb massive amounts of nutrients and sediment from runoff caused by farming and construction of roads, buildings, ports, channels, and harbors, are being destroyed. Nutrient-rich water causes fleshy algae and phytoplankton to thrive in coastal areas in suffocating amounts known as algal blooms. Coral reefs are biological assemblages adapted to waters with low nutrient content, and the addition of nutrients favors species that disrupt the balance of the reef communities. Both the loss of wetlands and mangrove habitats are considered to be significant factors affecting water quality on inshore reefs.

Poor water quality has also been shown to encourage the spread of infectious diseases among corals.

Copper, a common industrial pollutant, has been shown to interfere with the life history and development of coral polyps. Fish Trade The hobby of keeping saltwater aquaria has experienced an increase in world popularity since the 1990s. Beyond sales of aquaria, air pumps, food, medications and other supplies, the primary product of the aquarium industry is fish. However, the world market is limited in the diversity of collected species. For example, among 4000 coral reef fish species, only 200–300 are exploited. Selection of species results from a demand for fish being highly colorful and being able to be maintained and fed in aquaria. The last point is very important in the choice of imported species.

Although a few fish species (e.g. Pomacentridae) can be reproduced in aquaria, 95% of exploited fish are directly collected in the coral environment. Intense sampling of coral reef fish, especially in South-East Asia (including Indonesia and the Philippines), has caused great damage to the environment. A major catalyst of cyanide fishing is poverty within fishing communities. In areas like the Philippines where cyanide is regularly used to catch live aquarium fish, the percentage of the population below the poverty line is 40%. In such developing countries, a fisherman might resort to such unethical practices in order to prevent his or her family from starving.

Most, 80–90%, of aquarium fish exported from the Philippines are captured with sodium cyanide.. This toxic chemical is dissolved in sea water and released into fish shelters. It has a rapid narcotic effect on fish, which are then easily captured. However, most fish collected with cyanide die a few months after capture from extensive liver damage. Moreover, other fish species that are not interesting for the aquarium market also die in the field.

Dynamite fishing

Dynamite fishing is another extremely destructive method that fishermen use to harvest small fish. Sticks of dynamite, grenades, or home-made explosives are lit or activated and thrown in the water. Once the dynamite goes off the explosion brings about an underwater shockwave, causing the internal organs of fish to liquefy, killing them almost instantly. A second blast is often set off after the first to kill any larger predators that are attracted to the initial kill of the smaller fish. This method of fishing not only kills the fish within the main blast area, but also takes the lives of many reef animals that are not edible or wanted. Also, many of the fish do not float to the surface to be collected, but sink to the bottom. The blast also kills the corals in the area, eliminating the very structure of the reef, destroying the habitat for fish and other animals important for the maintenance of a healthy reef. Areas that used to be full of coral become deserts, full of coral rubble, dead fish and little else after dynamite fishing. With dynamite fishing especially around the Maldives in the Indian Ocean, have caused a vast majority of problems. With the rising sea level already the coral reefs act as a natural defence against flooding. With the dynamite fishing, the coral reefs are destroyed making the islands more vulnerable to flooding.


During the 1998 and 2004 El Niño weather phenomena, in which sea surface temperatures rose well above normal, many tropical coral reefs were bleached or killed. Some recovery has been noted in more remote locations, but global warming could negate some of this recovery in the future. High seas surface temperature (SSTs) coupled with high irradiance (light intensity), triggers the loss of zooxanthellae, a symbiotic algae, and its dinoflagellate pigmentation in corals causing coral bleaching. Zooxanthellae provide 95% of the energy to the coral host. Refer to Hoegh-Guldberg 1999 for more information.

Ocean acidification

The decreasing ocean surface pH is of increasing long-term concern for coral reefs. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. Carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.3 - 0.4 units by 2100 as the ocean absorbs more anthropogenic CO2. Under normal conditions, the conditions for calcium carbonate production are stable in surface waters since the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes under-saturated, structures made of calcium carbonate are vulnerable to dissolution. Research has already found that corals experience reduced calcification or enhanced dissolution when exposed to elevated CO2.

African and Asian dust outbreaks

Dust from the Sahara moving around the southern periphery of the subtropical ridge moves into the Caribbean and Florida during the warm season as the ridge builds and moves northward through the subtropical Atlantic. Dust can also be attributed to a global transport from the Gobi and Taklamakan deserts across Korea, Japan, and the Northern Pacific to the Hawaiian Islands. Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year; however, the flux of dust is greater during positive phases of the North Atlantic Oscillation. Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s. Studies have shown that corals can incorporate dust into their skeletons as identified from dust from the 1883 eruption of Krakatoa in Indonesia in the annular bands of the reef-building coral Montastraea annularis from the Florida reef tract. The relative abundance of chemical elements, particularly metals, has been used to distinguish soil derived from volcanic dust from mineral dust.

Destruction worldwide

Southeast Asian coral reefs are at risk from damaging fishing practices (such as cyanide and blast fishing), overfishing, sedimentation, pollution and bleaching. A variety of activities, including education, regulation, and the establishment of marine protected areas are under way to protect these reefs. Indonesia, for example has nearly of coral reefs. Its waters are home to a third of the world’s total corals and a quarter of its fish species. Indonesia's coral reefs are located in the heart of the Coral Triangle and have been victim to destructive fishing, unregulated tourism, and bleaching due to climatic changes. Data from 414 reef monitoring stations throughout Indonesia in 2000 found that only 6% of Indonesia’s coral reefs are in excellent condition, while 24% are in good condition, and approximately 70% are in poor to fair condition (2003 The Johns Hopkins University).

On September 24, 2007, Reef Check (the world’s largest reef conservation organization) stated that only 5% of Philippines 27,000 square-kilometers of coral reef are in “excellent condition” : Tubbataha Reef, Marine Park in Palawan, Apo Island in Negros Oriental, Apo Reef in Puerto Galera, Mindoro, and Verde Island Passage off Batangas. Philippine coral reefs is 2nd largest in Asia.

General estimates show approximately 10% of the coral reefs around the world are already dead.Problems range from environmental effects of fishing techniques, described above, to ocean acidification. Coral bleaching is another manifestation of the problem and is showing up in reefs across the planet.

Threatened species

The global standard for recording threatened marine species is the IUCN Red List of Threatened Species. This list is the foundation for marine conservation priorities worldwide. A species is listed in the threatened category if it is considered to be critically endangered, endangered, or vulnerable. Other categories are near threatened and data deficient. By 2008, the IUCN had assessed all known reef-building corals species as follows
Group Species Threatened Near threatened Data deficient
Reef-building corals 845 27% 20% 17%

According to the IUCN, the primary threats to coral reefs are bleaching and disease which has been linked to an increase in sea temperatures. Other threats include coastal development, coral extraction, sedimentation and pollution. The coral triangle (Indo-Malay-Philippine archipelago) region has the highest number of reef-building coral species in threatened category as well as the highest coral species diversity. The loss of coral reef ecosystems will have devastating effects on many marine species, as well as on people that depend on reef resources for their livelihoods.

Protection and restoration

Inhabitants of Ahus Island, Manus Province, Papua New Guinea, have followed a generations-old practice of restricting fishing in six areas of their reef lagoon. While line fishing is permitted, net and spear fishing are restricted based on cultural traditions. The result is that both the biomass and individual fish sizes are significantly larger in these areas than in places where fishing is completely unrestricted. It is estimated that about 60% of the world’s reefs are at risk due to destructive, human-related activities. The threat to the health of reefs is particularly strong in Southeast Asia, where an enormous 80% of reefs are considered endangered.

Organisations as Coral Cay, Counterpart , U.S. Coral Reef Task Force (CRTF), National Coral Reef Institute (NCRI), U.S. Department of Commerce’s National Oceanic and Atmospheric Administration (NOAA): Coral Reef Conservation Program, National Center for Coral Reef Research (NCORE), Reef Ball, Southeast Florida Coral Reef Initiative (SEFCRI) and the Foundation of the peoples of the South Pacific are currently undertaking coral reef/atoll restoration projects. They are doing so using simple methods of plant propagation. Other organisations as Practical Action have released informational documents on how to set-up coral reef restoration to the public.

Marine Protected Areas

One method of coastal reef management that has become increasingly prominent is the implementation of Marine Protected Areas (MPAs). MPAs have been introduced in Southeast Asia and elsewhere around the world to attempt to promote responsible fishery management and habitat protection. Much like the designation of national parks and wild life refuges, potentially damaging extraction activities are prohibited. The objectives of MPAs are both social and biological, including restoration of coral reefs, aesthetic maintenance, increased and protected biodiversity, and economic benefits. Conflicts surrounding MPAs involve lack of participation, clashing views and perceptions of effectiveness, and funding.

Reef Restoration Technology

Low voltage electrical currents applied through seawater crystallizes dissolved minerals onto steel structures. The resultant white carbonate (aragonite) is the same mineral that makes up natural coral reefs. Corals rapidly colonize and grow at faster than normal rates onto these coated structures. The change in the environment produced by electrical currents also accelerates formation and growth of both chemical limestone rock and the skeletons of corals and other shell-bearing organisms. Within the vicinity of the anode and cathode is a high pH environment which inhibits the growth of filamentous and fleshy algae, which compete with coral for space. This, and the increased growth rates cease when the mineral accretion process stops.

The effects of mineral accretion is, however, only temporary. During the process the settled corals have an increased growth rate, and size, and density, but after the process is complete the corallites are comparable to naturally growing corallites in growth rate and density, and are about the same size or slightly smaller.

Reefs in the past

Throughout the Earth history, from a few million years after hard skeletons were developed by marine organisms, there were almost always reefs formed by reef-building organisms in the ancient seas. The times of maximum development were in the Middle Cambrian (513-501 Ma), Devonian (416-359 Ma) and Carboniferous (359-299 Ma), due to Order Rugosa extinct corals, and Late Cretaceous (100-65 Ma) and all Neogene (23 Ma - present), due to Order Scleractinia corals.

Not all reefs in the past were formed by corals: in the Early Cambrian (542-513 Ma) resulted from calcareous algae and archaeocyathids (small animals with conical shape, probably related to sponges) and in the Late Cretaceous (100 -65 Ma), when there also existed reefs formed by a group of bivalves called rudists; one of the valves formed the main conical structure and the other, much smaller valve acted as a cap.


General references

External links

By Jack Webb

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