coral reef

Some of the biodiversity of a coral reef, in this case the Great Barrier Reef, Australia.

Coral reefs are aragonite structures produced by living organisms, found in marine waters containing few nutrients. 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 calcareous formation that supports the living corals and a great variety of other animal and plant life.

Coral reefs most commonly live in tropical waters, but deep water and cold water corals exist on a much smaller scale.

Globally, coral reefs are under threat from climate change, ocean acidification, overuse of reef resources, and harmful land-use practices. High nutrient levels such as those found in runoff from agricultural areas can harm reefs by encouraging excess algae growth.^[1]

Formations

Diagram of a fringing coral reef.

Fringing reef off the coast of Eilat, Israel.

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.
• Table reef – an isolated reef, approaching an atoll type, but without a lagoon.

Distribution

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.^{[citation needed]} 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%.^[2]

Locations of coral reefs.

20°C isotherms.

Coral reefs are rare on Americas west coast, as well as Africa west coast. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas.^[3] Corals are seldom found along the coastline of South Asia from Pakistan to Bangladesh.^[2] They are also rare along the coast around north-eastern South America and Bangladesh due to the freshwater release from the Amazon and Ganges Rivers respectively.^{[citation needed]}

Although corals exist both in temperate and tropical waters, shallow-water reefs form only in a zone extending 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.^[4], and that the optimum temperature is 26-27° Celsius for most coral reefs. The reefs in the Persian gulf however have adapted to temperatures of 13° Celsius in winter and 38° Celsius in summer. ^[5]deep water coral is more still exceptional since it can exist at greater depths and colder temperatures. Although deep water corals can form reefs, very little is known about them.^[6]

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 along the coast of Belize to the Bay Islands of Honduras.
• The New Caledonia Barrier Reef - second longest double barrier reef in the world, with a length of about 1500 km.
• The Andros, Bahamas Barrier Reef - third largest in the world, following the east coast of Andros Island, Bahamas, between Andros and Nassau.
• The Red Sea Coral Reef - located off the coast of Israel, Egypt and Saudi Arabia.
• Pulley Ridge - deepest photosynthetic coral reef, Florida
• Numerous reefs scattered over the Maldives
• Ghe Raja Ampat Islands in Indonesia's West Papua province offer the highest known marine diversity.^[7]

Biology

Anatomy of a coral polyp.

Coral consists of accumulations of individual organisms called polyps,^[8] arranged in diverse shapes. Reefs grow as polyps along with other organisms deposit calcium carbonate^[9][10], the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral's "head" or polyps upwards and outwards.^[11] Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner. Coralline algae are important contributors to reef structure in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae deposit limestone in sheets over the reef surface, thereby strengthening it.

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. Coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled organisms called zooxanthellae; these 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 get up to 90% of their nutrients from their zooxanthellae symbionts.^[12]

Table coral.

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 develop 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 synchronized spawning. Polyps release eggs and sperm into the water simultaneously. Eggs disperse over a large area. 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, the better the chance for fertilization. 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 to heavy predation and adverse environmental conditions. For the lucky few who survive to attach to substrate, the challenge comes from competition for food and space.

Ecology and biodiversity

Different colors of corals are due to symbiotic algae

Elephant ear sponge.

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

Cyanobacteria provide soluble nitrates for the reef via nitrogen fixation. Corals absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water, and they feed upon zooplankton that pass the polyps via water motion.^[13] Thus, primary productivity on a coral reef is very high, which results in high biomass per square meter, at 5-10g C m^-2 day^-1.^[14] Producers in coral reef communities include the symbiotic zooxanthellae, sponges, marine worms, seaweed, coralline algae (especially small types called turf algae), ... although scientists disagree about the importance of these particular organisms.^[13]

Coral reefs often depend on other habitats, such as seagrass meadows and mangrove forests in the surrounding area for the supply of nutrients. Seagrass and mangroves supply dead plants and animals which are rich in nitrogen and also serve to feed fish and animals from the reef by supplying wood and vegetation. Reefs in turn protect mangroves and seagrass from waves and produce sediment for the mangroves and seagrass to root in.^[15]

Coral reefs are home to a variety of tropical or reef fish which can be distinguished. These include:

• fish that adjust the coral (such as Labridae and parrotfish) These types of fish feed either on small animals living near the coral, seaweed, or on the coral itself. Fish that feed on small animals include cleaner fish (these fish feed between the jaws of larger predatory fish), bullet fish and Balistidae (these eat sea urchins) while seaweed eating fish include the Serranidae. Serranidae cultivate the seaweed by removing creatures feeding on it (as sea urchins), and they remove inedible seaweeds. Fish that eat coral include parrotfish and butterflyfish.
• fish that swim nearby the reef. These include predatory fish such as pompanos, groupers, Horse mackerels, certain types of shark, Epinephelus marginatus, barracudas, snappers, ...) They also include herbivorous and plankton-eating fish. Fish eating seagrass include Horse mackerel, snapper, Pagellus, Conodon, ... Fish eating plankton include Caesio, manta ray, chromis, Holocentridae, pterapogon kauderni, ...

Generally, fish that swim in coral reefs are as colorful as the reef itself. Examples are the beautiful parrotfish, angelfish, damselfish, Pomacanthus paru, Clinidae and butterflyfish. At night, some change to a less vivid color. Besides colorful fish matching their environment, other fish (e.g., predatory and herbivorous fish such as Lampanyctodes hectoris, Holocentridae, Pterapogon kauderni, ...) as well as aquatic animals (Comatulida, Crinoidea, Ophiuroidea, ...) emerge and become active while others rest.

Other fish groups found on coral reefs include groupers, grunts and wrasses. Over 4,000 species of fish inhabit coral reefs.^[2] It has been suggested that the fish species that inhabit coral reefs are able to coexist in such high numbers because any free living space is inhabited by the first planktonic fish larvae that find it in what has been termed "a lottery for living space". ^[16]

Soft coral, cup coral, sponges and ascidians on a reef in Indonesia

. 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, cleaner shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, turtles such as the sea turtle, green turtle and hawksbill turtle and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main exception. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs.^[2][13]

These other organisms have their part in the food-chain of the reef. For example, sea urchins eat seaweed, while the Hawksbill turtle eats sponges. Nudibranchia eat sponges too, as well as sea anemones. Dotidae and sea slugs eat seaweed.

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.^[3]

Researchers have found evidence of algae dominance in locations of healthy coral reefs. In surveys done around largely uninhabited US Pacific islands, algae inhabit a large percentage of surveyed coral locations. ^[17] The algae population consists of turf algae, coralline algae, and macroalgae.

Darwin's Paradox names the phenomenon in which coral reefs have high biodiversity in areas that contain few nutrients. How can such a diverse ecosystem exist with so few nutrients? The main reasons for this are:

• Symbiosis between coral and zooxanthellae
• A high nutrient flux
• Very efficient nutrient cycling
• Photosynthetic efficiency within the ecosystem.

Threats

Bioerosion (coral damage) such as this may be caused by coral bleaching.^[18]

Human activity may represent the greatest threat to coral reefs. In particular, global warming, coral mining, pollution (organic and non-organic/chemical), over-fishing, blast fishing and the digging of canals and access into islands and bays are serious threats to these ecosystems. Agricultural runoff may also threaten reefs by encouraging the growth of harmful algae.^[1]

In order to find answers for these problems, researchers study the various factors that impact reefs. The list of factors is long, including the oceans role as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, biological virus, impacts of dust storms carrying agents to far flung reefs, pollutants, algal blooms and others. Reefs are threatened well beyond coastal areas.

Overfishing

The use of cyanide has been implicated as a driver of decline. Hughes, et al., (2003), write 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 fish and other natural resources have become global, supplying demand for reef resources."^[19]

Overfishing (and particularly selective overfishing) results in excessive growth of organisms that can damage the reef if they appear in great numbers. For example overfishing of bullet fish, Balistidae and other natural predators such as lobsters ^[20] promote the growth of sea urchins. Also, overpopulation of acanthaster planci, Drupella, Tapiro, Terpios, and Rhodactis can destroy reefs.

Aquarium fish

Although a few fish species (e.g. Pomacentridae) can reproduce in aquaria, 95% are collected on the reef. Intense harvesting, especially in South-East Asia (including Indonesia and the Philippines), severely damages the reefs. A major catalyst of cyanide fishing is poverty within fishing communities. In areas like the Philippines where cyanide is regularly used, the percentage of the population below the poverty line is 40%.^[21] In such developing countries, a fisherman might resort to such practices in order to protect his family from starvation.

Most, 80–90%, of aquarium fish from the Philippines are captured with sodium cyanide.. This toxic chemical is dissolved in sea water and released into fish shelters. It narcotizes fish, which are then easily captured. However, most fish collected with cyanide die a few months later from liver damage. Moreover, non-marketable species die in the field.^[22]

Pollution

Some nutrients favor species (such as algae, seaweed, ...) that disrupt the balance of reef communities. ^[23] Some algae are toxic, and both plants reduce the levels of sunlight and oxygen, killing marine organisms such as fish and coral. The addition of nutrients such as phosphates and nitrates are very damaging to reefs. High nitrate levels are toxic to corals, while phosphates slow down the growth of coral skeleton.

Poor water quality has also been shown to encourage the spread of infectious diseases among corals.^[24]

Organic pollutants

Soil runoff

Extensive and poorly managed land development can threaten the survival of coral reefs. Runoff caused by farming and construction of roads, buildings, ports, channels, and harbors, can carry soil laden with carbon, nitrogen, phosphorus, and minerals. This nutrient-rich water can cause fleshy algae and phytoplankton to thrive in coastal areas, known as algal blooms, which have the potential to create hypoxic conditions by using all available oxygen.

Windborne

Barbados dust graph

In addition to local soil runoff, additional soil (sand) is blown in from other regions. 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.^[25] Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in dust transport to the Caribbean and Florida from year to year;^[26] however, the flux is greater during positive phases of the North Atlantic Oscillation.^[27] The USGS links dust events to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s.^[28] 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.^[29] The relative abundance of chemical elements, particularly metals, has been used to distinguish soil derived from volcanic dust from mineral dust.^[30]

Sewage

Another major pollutant is generated by people. Most islanders in developing countries send sewage unfiltered into the sea. While most experts now agree that composting toilet alongside an ecological sanitation approach is appropriate in small island nations, these countries for the moment prefer to keep using traditional approaches.

Mines

Inland mines of copper, gold and others form a major center of pollution. Most of the pollution is simply soil, which ends up in rivers flowing to the sea and ultimately covers the coral, but small mineral fractions may also introduce trouble. Copper, a common industrial pollutant, has been shown to interfere with the life history and development of coral polyps.^[31]

Non-organic

Leaked oil and chemicals (e.g. from detergents, paints, ...) flowing into the sea from factory outlets are a key threat.

Chemical fertilizers (based on ammonium nitrate) are another pollutant.

Radioactive waste is often dumped by the USA near its military installations (Mororua, Fangataufa, Johnston Atoll, ... Nuclear tests (eg at Kwajalein, Bikini, Enewetak) may produce harmful fallout, yet compared to the other forms of pollution noted, their impact is small.

Global warming

Corals and their enemy, the crown-of-thorns starfish, at Madagascar

Global warming introduces sea level rise, effectively asking the coral to grow faster to keep up. Also, the sea temperature increases, which is very disturbing to the coral. This was seen during the 1998 and 2004 El Niño weather phenomena, in which sea surface temperatures rose well above normal, bleaching or killing many coral reefs. 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 provides up to 90% of the energy to the coral host. Reefs can often recover from bleaching if they are healthy to begin with and water temperatures cool. However, recovery will not be possible if CO₂ levels rise to 500 ppm because there will not be enough carbonate ions present.^[32] Refer to Hoegh-Guldberg 1999 for more information.

Global warming is also the basis of a new emerging problem: increasing coral diseases. Global warming (which is the main cause of coral bleaching), weakens corals. In their weakened state, coral is much more prone to disease. As such, coral diseases have begun to spread more rapidly. These include Black band disease and White band disease. With the projected 2°C temperature increase, it is likely that coral will not be able to adapt enough physiologically or genetically to keep up with climate change^[33]

Ocean acidification

Bamboo coral is an early harbinger of ocean acification

A related problem to global warming is ocean acidification, which is caused by one and the same problem; namely increasing CO₂ emissions.

The decreasing ocean surface pH is of increasing long-term concern for coral reefs.^[34] Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans.^[35] 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,^[36] 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.^[37] Normally, 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.^[38]

Deep sea bamboo coral supports deep sea life and also may be among the first organisms to display the effects of changes in ocean acidification caused by excess carbon dioxide, since they produce growth rings similar to those of tree and can provide a view of changes in the condition in the deep sea over time. This coral is especially long-lived; coral specimens as old as 4,000-year-old were found at the Monument, giving scientists "4,000 years worth of information about what has been going on in the deep ocean interior".^[39]

Mangroves and seagrassbeds

Within the last 20 years, once prolific seagrassbeds and mangrove forests, which absorb massive amounts of nutrients and sediment have been destroyed. Both the loss of wetlands, mangrove habitats and seagrassbeds are considered to be significant factors affecting water quality on inshore reefs.^[40]

Coral mining

Coral mining is another threat. Both small-scale harvesting by villagers, industrial-scale mining by companies are serious threats. Mining is often done to produce construction material, and is of particular value as these rocks are up to 50% cheaper than other rocks (eg from quarries).^[5] The rocks are ground and mixed with other materials such as cement to make concrete. Ancient coral used for construction is known as "coral rag".

Other physical destruction

Dynamite fishing is an extremely destructive method for gethering fish. Sticks of dynamite, grenades, or home-made explosives are simply thrown in the water. This method of fishing kills the fish within the main blast area, along with many inedible and/or unwanted reef animals. 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.^[41]

Other types of fishing like muro-ami and kayakas kills all fish in certain areas, causing havoc on the ecosystem of the reef.^[41]

Boats and ships require an access point into bays and islands to load/unload cargo/people. For this, often parts of the reef are chopped away to clear a path. Although this seems but minor destruction of the reef, potential negative consequences include altered water circulation and altered tidal patterns, which then cause a turnaround in the reef's supply of nutrients; sometimes destroying a great part of the reef.

Fishing boats and other large vessels occasionally run aground on a reef. Two types of damage can result. Collision damage occurs when a coral reef is crushed and split by a vessel's hull into multiple fragments. Scarring occurs when boat propellers tear off the live coral and expose the skeleton. The physical damage can be noticed as striations in the reefs.

Mooring also causes considerable damage. To reduce the (considerable) amount of devastation, boats can use mooring buoys. They are available in most major wetparks and marine sanctuaries. Most buoys are of the Halas Mooring Buoy System-type. ^[42]

Construction also takes its toll. Building directly on the reef can alter water circulation and tides (which bring the nutrients for the reef). The main reason for building on reefs (despite possible moisture problems) is simply the lack of space.

Destruction worldwide

Coral reefs and fishes in Papua New Guinea

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, is home to a third of the world's total corals and a quarter of its fish species, nearly 33,000 square miles (85,000 km²). Indonesia's coral reefs are located in the heart of the Coral Triangle and have fallen victim to destructive fishing, unregulated tourism, and bleaching due to climatic changes. Data from 414 reef monitoring stations in 2000 found that only 6% 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 second largest in Asia.^[43]

General estimates show approximately 10% world's coral reefs are already dead.^[34][44][45]

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.

Threatened species

The global standard for recording threatened marine species is the IUCN Red List of Threatened Species.^[46] 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^[47]

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.^[47]

Protection and restoration

Aerial photo of Ahus Island, Papua New Guinea

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. Their cultural traditions allow line fishing but not net and spear fishing. 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.^[48][49]

Protected areas

Many governments worldwide take measures to protect their coral reefs.

Marine Protected Areas (MPAs) have become increasingly prominent for reef management. MPAs in Southeast Asia and elsewhere around the world to attempt to promote responsible fishery management and habitat protection. Much like national parks and wild life refuges, MPAs prohibit potentially damaging extraction activities. The objectives of MPAs are both social and biological, including reef restoration, aesthetics, increased and protected biodiversity, and economic benefits. Conflicts surrounding MPAs involve lack of participation, clashing views and perceptions of effectiveness, and funding.

Biosphere reserves are other protected areas that may protect reefs.

Also, Marine parks, as well as world heritage sites can protect reefs. World heritage designation can also play a vital role. For example the Chagos archipelago, Sian Ka'an, the Great Barrier Reef, Henderson Island, the Galapagos islands, Belize's Barrier reef and Palau have been designated as protected by nomination as a world heritage site.

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.

Restoration technologies

Low voltage electrical currents applied through seawater crystallize 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 accelerated rates on these coated structures. The electrical currents also accelerate formation and growth of both chemical limestone rock and the skeletons of corals and other shell-bearing organisms. The vicinity of the anode and cathode provides a high pH environment which inhibits the growth of filamentous and fleshy algae, which compete with coral for space. The increased growth rates cease when the mineral accretion process stops.^[50]

During mineral accretion, the settled corals display an increased growth rate, and size, and density, but after the process is complete, growth rate and density return to levels that are comparable to naturally growing corallites, and are about the same size or slightly smaller.^[50]

In large restoration projects, depending on the type of coral, placement of propagated hermatype coral unto substrate is often done with metal pins, superglue or milliput ^[51]. Needle and thread can also attach A-hermatype coral to substrate^[52]. Concrete has also been used to restore large sections of broken coral reef. Finally, special structures as reef balls can be placed to provide corals a base to grow on.

Organizations

Organizations which currently undertake coral reef/atoll restoration projects using simple methods of plant propagation:

• Coral Cay
• Counterpart ^[53]
• U.S. Coral Reef Task Force (CRTF)
• National Coral Reef Institute (NCRI)
• US 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)
• Foundation of the peoples of the South Pacific
• WorldFishCenter: promotes sustainable mariculture techniques to grow reef organisms as tridacnidae

Organizations which carry out research to better understand the workings of coral reefs:

• Tropical Marine Research Unit

Organizations which promote interest, provide knowledge bases about coral reef survival, and promote activities to protect and restore coral reefs:

Reefs in the past

Throughout Earth history, from a few thousand years after hard skeletons were developed by marine organisms, there were almost always reefs. 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.

See also

	Environment portal
	Ecology portal
	Earth sciences portal
	Sustainable development portal

References

General references

External links

Wikimedia Commons has media related to: Coral reefs

• How Coral Reefs Work
• Reef Fest Concert Series Non-profit effort to benefit coral reef conservation globally, starting in 2008, the International Year of the Reef
• International Year of the Reef in 2008
• Moorea Coral Reef Long Term Ecological Research Site (US NSF)
• ARC Centre of Excellence for Coral Reef Studies
• NOAA's Coral-List Listserver for Coral Reef Information and News
• NOAA's Coral Reef Conservation Program
• Exhibition of the Mexican Caribbean coral reef biodiversity aquarium in Xcaret Mexico
• NOAA's Coral Reef Information System
• NOAA Report: The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States: 2005
• ReefBase: A Global Information System on Coral Reefs
• National Coral Reef Institute Nova Southeastern University
• Global Coral Reef Alliance
• Global Coral Reef Monitoring Network (GCRMN)
• Marine Aquarium Council
• NCORE National Center for Coral Reef Research University of Miami
• The Coral Reef Alliance (CORAL)
• Science and Management of Coral Reefs in the South China Sea and Gulf of Thailand
• A special report on the plight of the planet's coral reefs—and how you can help—from Mother Jones magazine
• A guide to finding sources and literature about coral reefs
• NBII portal on coral reefs
• Microdocs: 4 kinds of Reef & Reef structure

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