Coral reef

Ocean habitats
Some of the biodiversity of a coral reef, in this case the Great Barrier Reef
	Littoral zone
	Intertidal zone
	Estuaries
	Kelp forests
	Coral reefs
	Ocean banks
	Continental shelf
	Neritic zone
	Straits
	Pelagic zone
	Oceanic zone
	Seamounts
	Hydrothermal vents
	Cold seeps
	Demersal zone
	Benthic zone

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters containing few nutrients. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups. The polyps secrete a hard carbonate exoskeleton which provides support and protection for the body of each polyp. Reefs grow best in warm, shallow, clear, sunny and agitated waters.

Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on earth. They occupy less than one tenth of one percent of the world ocean surface, about half the area of France, yet they provide a home for twenty-five percent of all marine species, including fish, molluscs, echinoderms and sponges.^[1]

Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, particularly in the Pacific Ocean, but deep water and cold water corals also exist on smaller scales.

Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection. The annual global economic value of coral reefs has been estimated at $30 billion. However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature. They are under threat from climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, 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.^[2]

Formation

Most coral reefs were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. This means that most coral reefs are less than 10,000 years old. As coral reef communities were established on the shelves, they built reefs that grew upwards, keeping pace with the rise in sea level. Reefs that didn't keep pace could become drowned reefs, covered by so much water that there was insufficient light for further survival.^[3]

Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface.

In 1842 Charles Darwin published his first monograph, The Structure and Distribution of Coral Reefs.^[4] There he set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle. His theory was that atolls were formed by the uplift and subsidence of the Earth's crust under the oceans.^[5] Darwin’s theory sets out a sequence of three stages in atoll formation. It starts with a fringing reef forming around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef, and ultimately an atoll reef.

Darwin’s theory starts with a volcanic island which becomes extinct

As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef

As the subsidence continues the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside

Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon

Darwin predicted that underneath each lagoon would be a bed rock base, the remains of the original volcano. Subsequent drilling has proved this correct. Darwin's theory followed from his understanding that coral polyps thrive in the clean seas of the tropics where the water is agitated, but can only live within a limited depth of water, starting just below low tide. Where the level of the underlying land stays the same, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef.

A fringing reef can take ten thousand years to form, and an atoll can take up to 30 million years^[6]

Where the land is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of dead coral, forming a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls don't usually form complete circles, but are broken in places by storms. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth.^[5][7]

In general, the two main variables determining the geomorphology, or shape, of coral reefs are the nature of the underlying substrate on which they rest, and the history of the change in sea level relative to that substrate.

As an example of how coral reefs have formed on continental shelves, the current living reef structure of the Great Barrier Reef began growing about 20,000 years ago. The sea level was then 120 metres (390 ft) lower than it is today.^[8][9] As the sea level rose, the water and the corals encroached on what had been the hills of the coastal plain. By 13,000 years ago the sea level was 60 metres (200 ft) lower than at present, and the hills of the coastal plains were, by then, continental islands. As the sea level rise continued most of the continental islands were submerged. The corals could then overgrow the hills, forming the present cays and reefs. The sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years,^[9] and the age of the present living reef structure is estimated to be between 6,000 and 8,000 years.^[10] Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, the same principles apply as outlined by Darwin's theory above. The Great Barrier Reef development has stopped at the barrier reef stage, since Australia is not about to submerge. It has formed the world's largest barrier reef, 300–1000 metres (330-1100 yards) from shore, and 2,000 kilometres (1,200 mi) long.^[11]

Healthy coral reefs grow horizontally from 1 to 3 centimetres (0.39 to 1.2 in) per year, and grow vertically anywhere from 1 to 25 centimetres (0.4–12 in) per year; however, they are limited to growing above a depth of 150 metres (490 ft) due to their need for sunlight, and cannot grow above sea level.^[12]

Types

The three principal reef types, as detailed in the previous section, are:

• 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.
• Atoll reef – a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island.

A small atoll in the Maldives.

Inhabited cay in the Maldives

Other reef types or variants are:

• Patch reef – an isolated, comparatively small reef outcrop, usually within a lagoon or embayment, often circular and surrounded by sand or seagrass. Patch reefs are common.
• 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.
• Table reef – an isolated reef, approaching an atoll type, but without a lagoon.
• Habili - (from the Arabic for "unborn"), a type of coral reef in the Red Sea that does not reach the surface near enough to cause visible surf, although it still may be near enough to be a hazard to ships.
• Microatolls – certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analysing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating. Such methods have been used to reconstruct Holocene sea levels.^[13]

• Cays – small, low-elevation, sandy islands formed on the surface of a coral reef. Material eroded from the reef piles up on parts of the reef or lagoon, forming an area above sea level. Plants can stabilize cays enough for them to be habitable by humans. Cays occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including in the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land for hundreds of thousands of people. Their surrounding reef ecosystems also provide food and building materials for island inhabitants.

• When a coral reef cannot keep up with the sinking of a volcanic island, a seamount or guyot is formed. Seamounts and guyots are below the surface of the ocean and may host many species, depending on their location and depth. Seamounts are rounded at the top and guyots are flat. The flat top of the guyot, also called a tablemount, is due to erosion by waves, winds, and atmospheric processes.

Zones

The three major zones of a coral reef: the fore reef, reef crest, and the back reef

Coral reef ecosystems contain a number of distinctive zones that represent different kinds of habitats for fishes and invertebrate animals. Usually, three major zones are recognized: the fore reef (outermost and deepest part of the reef), reef crest (shallowest and narrowest zone over which ocean waves break), and the back reef (behind the reef crest and closest to shore, with calm protected waters) which is also frequently referred to as the reef lagoon.

All three zones are physically and ecologically interconnected to some extent, with reef life and oceanic processes creating ample opportunities for exchange of ocean water, sediments, nutrients, and marine life between one another.

Thus, they should properly be viewed as integrated components of the coral reef ecosystem, each playing a role in the support of the abundant and diverse fish assemblages that characterize coral reefs.

Most coral reefs exist in shallow waters less than fifty metres deep. Some are found on tropical continental shelves where cool, nutrient rich upwelling does not occur, such as Great Barrier Reef. Others are found in the deep ocean surrounding islands or as atolls, such as in the Maldives. The reefs surrounding islands form when islands subside into the ocean, and atolls form when an island subsides below the surface of the sea.

Moyle and Cech^[14] distinguish six zones, though most reefs possess only some of the zones.

Water in the reef surface zone is often agitated. This diagram represents a reef on a continental shelf. The water waves at the left travel over the off-reef floor until they encounter the reef slope or fore reef. Then the waves pass over the shallow reef crest. When a wave enters shallow water it shoals, that is, it slows down and the wave height increases.

• The reef surface is the shallowest part of the reef. It is subject to the constant surge of waves and the rise and fall of tides. When water waves pass over shallow areas, they shoal, as shown in the diagram at the right. This means that the water in the reef surface zone is often agitated. These are the precise condition under which coral flourish. Shallowness means there is plenty of light for photosynthesis, and agitated water promotes the ability of coral to feed on plankton. However other organisms, such as fish and invertebrates, must be able to withstand the robust conditions to flourish in this zone.

• The off-reef floor is the shallow sea floor surrounding a reef. This zone applies to reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths, and don't have an off-reef floor. Usually sandy, the off-reef floor often supports seagrass meadows which are important foraging areas for reef fish.

• The reef drop-off is, for its first 50 metres, habitat for many reef fish who find shelter on the cliff face and plankton in the water nearby. The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls.

• The reef face is the zone above the reef floor or the reef drop-off. "It is usually the richest habitat for fish and invertebrates. Its complex growths of coral and calcareous algae provide innumerable cracks and crevices for protection, and the abundant invertebrates and epiphytic algae provide an ample source of food."^[14]

• The reef flat – sandy bottomed flat can be behind the main reef, containing chunks of coral. "The reef flat may be a protective area bordering a lagoon, or it may be a flat, rocky area between the reef and the shore. In the former case, the number of fish species living in the area often is the highest of any reef zone."^[14]

• The reef lagoon – "many coral reefs completely enclose an area, thereby creating a quiet-water lagoon that usually contains small patched of reef."^[14]

However, the "topography of coral reefs is constantly changing. Each reef is made up of irregular patches of algae, sessile invertebrates, and bare rock an sand. The size, shape and relative abundance of these patches changes from year to year in response to the various factors that favour one type of patch over another. Growing coral, for example, produces constant change in the fine structure of reefs. On a larger scale, tropical storms may knock out large sections of reef and cause boulders on sandy areas to move." (Connell 1978)

Distribution

Locations of coral reefs.

Boundary for 20 °C isotherms. Most corals live within this boundary. Note the cooler waters caused by upwelling on the south west coast of Africa and off the coast of Peru.

This map shows areas of upwelling in red. Coral reefs are not found in coastal areas where colder and nutrient-rich upwellings occur

Coral reefs are estimated to cover 284,300 square kilometers (109,800 sq mi), which is just under one tenth of one percent of the surface area occupied by the world oceans. The Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) account for 91.9% of this 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%.^[15]

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 meters (160 ft). The optimum temperature for most coral reefs is 26–27 °C, and few reefs exist in waters below 18 °C.^[16] However reefs in the Persian Gulf have adapted to temperatures of 13 °C in winter and 38 °C in summer.^[17]

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.

Coral reefs are rare along the American west coast, as well as along the African west coast. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas (respectively the Peru, Benguela and Canary streams).^[18] Corals are seldom found along the coastline of South Asia from the eastern tip of India (Madras) to the border of Bangladesh and Myanmar.^[15] 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.

Principal coral reefs and reef areas of the world

• 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 1,500 kilometers (930 mi).
• 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, Sudan, Eritrea, Djibouti, Somalia, Jordan, Saudi Arabia, and Yemen.
• 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.^[19]

Biology

Anatomy of a coral polyp.

Live coral can be thought of as small live animals embedded in calcium carbonate. It is a mistake to think of coral as plants or rocks. Coral consists of accumulations of individual animals called polyps, arranged in diverse shapes.^[20] Polyps are usually tiny, but they can range in size from a pinhead to a foot across. Reefs grow as polyps along with other organisms deposit calcium carbonate,^[21][22] the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral's "head" or polyps upwards and outwards.^[23] Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms act as bioeroders, breaking down coral skeletons into fragments that settle into spaces in the reef structure or form sandy bottoms in associated reef lagoons. 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 found only 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.^[24]

Table coral

Close up of polyps arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.

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.

There are about one thousand species of coral, which build different shapes such as wrinkled brains, cabbages, table tops, stag antlers, wire strands and pillars.

Brain coral

Staghorn coral

Spiral wire coral

Pillar coral

Darwin's paradox

During his voyage on the Beagle, Darwin described tropical coral reefs as oases in the desert of the ocean. He reflected on the paradox that tropical coral reefs, which are among the richest and most diverse ecosystems on earth, flourish when they are surrounded and supported by tropical ocean waters that provide hardly any nutrients. It has been a challenge for scientists to explain this paradox.

Coral reefs cover just under one percent of the surface of the world’s ocean, yet they support over one-quarter of all marine species. This huge number of species results in complex food webs, with large predator fish eating smaller forage fish that eat yet smaller zooplankton and so on. However, all food webs eventually depend on plants, which are the primary producers. The primary productivity on a coral reef is very high, resulting in a typical biomass production of 5-10g C m⁻² day⁻¹.^[26]

Tropical waters are often described as crystal clear. This is because they are deficient in nutrients and drifting plankton. The sun shines year round in the tropics, warming the surface ocean layer so it is less dense than subsurface layers. The warmer water is separated from the cooler water by a stable thermocline, where the temperature makes a rapid change. This keeps the warm surface waters floating above the cooler deeper waters. There is little exchange between these layers. Organisms that die in aquatic environments generally sink to the bottom where they decompose. This decomposition releases nutrients in the form of nitrogen (N), phosphorus (P) and potassium (K). These nutrients are necessary for plant growth, but in the tropics they are not directly recycled back to the surface.^[7]

Plants are the base of the food chain, and need sunlight and nutrients if they are to grow. In the ocean these plants are mainly a type of plankton, microscopic phytoplankton which drift in the water column. They need sunlight for photosynthesis, which powers carbon fixation, so they are found only in the surface waters. But they also need nutrients. Phytoplankton rapidly use any nutrients in the surface waters, and in the tropics these nutrients are not usually replaced because of the thermocline.^[7]

Coral polyps

The situation with coral reefs is different. The lagoons that are formed by the upward growth of coral reefs fill in with material eroded from the reef and the island. They become havens for marine life, providing protection from waves and storms.

Most importantly, nutrients are recycled, and not lost like they are in the open ocean. In coral reefs and lagoons, the producers include phytoplankton as well as marine worms, seaweed, and coralline algae, especially small types called turf algae, which pass their nutrients to the corals.^[27] The phytoplankton are eaten by fish and crustaceans, who also pass nutrients along the food web. Recycling ensures that fewer nutrients are needed overall to support the community.

Corals harbour numerous symbiotic organisms. In particular, there is a remarkable symbiosis between coral and a microscopic algae, the single cell dinoflagellate known as a zooxanthella. The zooxanthella forms an endosymbiosis with a coral polyp, that is, it lives within the tissues of the polyp. There it absorbs solar energy with special pigments, using photosynthesis to supply the polyp with organic nutrients in the form of glucose, glycerol, and amino acids.^[28] Zooxanthellae can provide up to 90% of a coral’s energy requirements.^[29] In return, as an example of mutualism, the coral provides the zooxanthellae, averaging one million for every cubic centimetre of coral, with a relatively safe place to live and a constant supply of the carbon dioxide it needs for photosynthesis.

The colour of corals depends on the type of zooxanthella they host

Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water. Many corals extend their tentacles at night to catch zooplankton that brush them when the water is agitated. Zooplankton provides the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element.^[27] The pigments in different species of zooxanthellae give corals their different colours. Coral which loses its zooxanthellae becomes white and is said to be bleached, a condition which unless corrected can lead to the death of the coral.

A 2001 paper reported that sponges are another key to explaining Darwin’s paradox. These sponges live in crevices in the coral reefs. They are efficient filter feeders, and in the Red Sea they consume about sixty percent of the phytoplankton that drifts by. The nutrients the sponges absorb from the phytoplankton are then excreted in a form the corals can use.^[30]

Most coral polyps are nocturnal feeders. Here, in the dark, polyps have extended their tentacles to feed on zooplankton

Researchers in 2002 explained why coral thrives better in agitated waters. They found the roughness of coral surfaces is the key. Normally there is a boundary layer of still water around a submerged object which acts as a barrier. But when waves break on the extremely rough edges of corals the boundary layer is disrupted, allowing the corals access to the few nutrients that are there. The researchers claim that turbulent water promotes rapid reef growth and lots of branching. Although coral ecosystems are great at recycling, with the wastes of one species becoming the food of another, the researchers also claim that, without the nutritional gains achieved by rough coral surfaces, even the most effective recycling would leave corals wanting in nutrients.^[31]

In 2004, another symbiotic organism, a bacterium called Cyanobacteria, was discovered to provide soluble nitrates for the reef via nitrogen fixation.^[32]

Coral reefs also 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.^[33]

Biodiversity

Tube sponges attracting cardinal fishes, glassfishes and wrasses

Reefs are also home to a large variety of other organisms, including fish, seabirds, 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, sea turtles 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.^[15][27]

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.^[34] The algae population consists of turf algae, coralline algae, and macroalgae.

Fish

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 Pomacentridae (damselfishes). 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 cruise the boundaries of the reef or nearby seagrass meadows. 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, ...

Organisms can cover every square inch of a coral reef.

Generally, fish that swim in coral reefs are as colourful 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.^[15] 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".^[35]

Seabirds

Coral reef systems provide important habitats for seabird species, some endangered. For example, Midway Atoll supports nearly three million seabirds, including two-thirds (1.5 million) of the global population of Laysan Albatross, and one-third of the global population of black-footed albatross.^[36] Each seabird species have specific sites on the atoll where they nest. Altogether, 17 species of seabirds live on Midway. The short-tailed albatross is the rarest, with fewer than 2,200 surviving after excessive feather hunting in the late nineteenth century.^[37]

Invertebrates

Invertebrates have their part in the food-chain of the reef. For example, sea urchins, Dotidae and sea slugs eat seaweed. Some species of sea urchins, such as Diadema antillarum, can play a pivotal part in preventing algae overrunning reefs.^[38] Hawksbill turtles, Nudibranchia and sea anemones eat sponges.

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.^[18]

Other

Sea snakes feed exclusively on fish and their eggs. Many tropical birds forage on reef fish, such as herons, gannets, pelicans and boobies. Some land based reptiles can be intermittently associated with reefs, such as monitor lizards, the marine crocodile and semi-aquatic snakes like Laticauda colubrina.

Schooling reef fish	Caribbean reef squid	Banded coral shrimp	The whitetip reef shark almost exclusively inhabits coral reefs
Green turtle	Giant clam	Soft coral, cup coral, sponges and ascidians	Banded sea krait

Economic value

Coral reefs deliver ecosystem services to tourism, fisheries and coastline protection. The global economic value of coral reefs has been estimated at $30 billion.^[39] Coral reefs protect shorelines by absorbing wave energy, and many small islands would not exist without their reef to protect them. According to the WWF, the economic cost over a 25 year period of destroying one kilometre of coral reef is somewhere between $137,000 and $1,200,000.^[40] About 6 million tons of fish are taken each year from coral reefs. Well managed coral reefs have an annual yield of 15 tons seafood on average per square kilometre. Southeast Asia's coral reef fisheries alone yield about $ 2.4 billion annually from seafood.^[40]

Issues

Island with fringing reef off Yap, Micronesia. Coral reefs are dying around the world.^[41]

Coral reefs are dying around the world.^[41] In particular, coral mining, pollution (organic and non-organic), overfishing, blast fishing and the digging of canals and access into islands and bays are serious threats to these ecosystems. Coral reefs also face high dangers from pollution, diseases, destructive fishing practices and warming oceans."^[42] In order to find answers for these problems, researchers study the various factors that impact reefs. The list of factors is long, including the ocean's 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.

General estimates show approximately 10% world's coral reefs are already dead.^[43][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 80% of reefs are endangered.

Protection

Coral reefs and fish in Papua New Guinea

Marine Protected Areas (MPAs) have become increasingly prominent for reef management. MPAs in Southeast Asia and elsewhere around the world 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.

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 unrestricted.^[46][47]

Restoration

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.^[48]

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.^[48]

In large restoration projects, depending on the type of coral, placement of propagated hermatypic coral unto substrate is often done with metal pins, superglue or milliput.^[49] Needle and thread can also attach A-hermatype coral to substrate.^[50] 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:

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

Reefs in the past

Ancient coral reefs

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

• Census of Coral Reefs
• Marine biology

Notes

References

External links

External images
	Coral Reefs: Rainforests of the Sea ORG Educational films.

• Coral Reefs- At the Smithsonian Ocean Portal
• How Coral Reefs Work
• 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
• ReefBase: A Global Information System on Coral Reefs
• National Coral Reef Institute Nova Southeastern University
• Marine Aquarium Council
• NCORE National Center for Coral Reef Research University of Miami
• Science and Management of Coral Reefs in the South China Sea and Gulf of Thailand
• NBII portal on coral reefs
• Microdocs: 4 kinds of Reef & Reef structure
• [7] Reef Relief Founders site: Coral reef resources, images, education, threats, solutions

Organisations

• Reef Fest Concert Series Non-profit effort to benefit coral reef conservation globally, starting in 2008, the International Year of the Reef
• The Coral Reef Alliance (CORAL)
• Global Coral Reef Alliance
• Global Coral Reef Monitoring Network (GCRMN)

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