Coral reef

Most coral reefs were formed after the Last Glacial Period when melting ice caused sea level to rise and flood continental shelves. Most coral reefs are less than 10,000 years old. As communities established themselves, the reefs grew upward, keeping pace with rising sea levels. Reefs that rose too slowly could become drowned, without sufficient light. Coral reefs are also found in the deep sea away from continental shelves, around oceanic islands and atolls. The majority of these islands are volcanic in origin. Others have tectonic origins where plate movements lifted the deep ocean floor. In The Structure and Distribution of Coral Reefs, Charles Darwin set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle. He theorized that uplift and subsidence of Earth's oceanic crust beneath the oceans formed the atolls. Darwin set out a sequence of three stages in atoll formation. A fringing reef forms around an extinct volcanic island as the island and ocean floor subside. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. File:Atoll forming-volcano.png|Darwin's theory starts with a volcanic island which becomes extinct File:Atoll forming-Fringing reef.png|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. File:Atoll forming-Barrier reef.png|As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside. File:Atoll forming-Atoll.png|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 bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the underlying earth allows, corals grow along the coast to form fringing reefs, which can eventually become barrier reefs. Where the bottom is rising, fringing reefs can grow around the coast, but coral raised above sea level dies. If the land subsides slowly, the fringing reefs keep pace by growing upward on a base of older, dead coral, forming a barrier reef that encloses 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 do not usually form complete circles but are broken in places by storms. Like sea level rise, a rapidly subsiding bottom can overwhelm coral growth, killing the coral and the reef, due to what is called coral drowning. Corals that rely on zooxanthellae can die when the water becomes too deep for their symbionts to adequately photosynthesize, due to decreased light exposure. The two main variables determining the geomorphology, or shape, of coral reefs are the nature of the substrate on which they rest, and the history of the change in sea level relative to that substrate. The approximately 20,000-year-old Great Barrier Reef offers an example of how coral reefs formed on continental shelves. Sea level was then lower than in the 21st century. As sea level rose, the water and the corals encroached on what had been hills of the Australian coastal plain. By 13,000 years ago, sea level had risen to lower than at present, and many hills of the coastal plains had become continental islands. As sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming cays and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years. Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, Darwin's principles apply. Development stopped at the barrier reef stage, since Australia is not about to submerge. It formed the world's largest barrier reef, from shore, stretching for . Healthy tropical coral reefs grow horizontally from per year, and grow vertically anywhere from per year; however, they grow only at depths shallower than because of their need for sunlight, and cannot grow above sea level. Material As the name implies, coral reefs are made up of coral skeletons from mostly intact coral colonies. As other chemical elements present in corals become incorporated into the calcium carbonate deposits, aragonite is formed. However, shell fragments and the remains of coralline algae such as the green-segmented genus Halimeda can add to the reef's ability to withstand damage from storms and other threats. Such mixtures are visible in structures such as Eniwetok Atoll. In the geologic past The times of maximum reef development were in the Middle Cambrian (513–501 Ma), Devonian (416–359 Ma) and Carboniferous (359–299 Ma), owing to extinct order Rugosa corals, and Late Cretaceous (100–66 Ma) and Neogene (23 Ma–present), owing to order Scleractinia corals. Not all reefs in the past were formed by corals: those 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 (100–66 Ma), when reefs formed by a group of bivalves called rudists existed; one of the valves formed the main conical structure and the other, much smaller valve acted as a cap. ==Types==

Since Darwin's identification of the three classical reef formations – the fringing reef around a volcanic island becoming a barrier reef and then an atoll – scientists have identified further reef types. While some sources find only three, Thomas lists "Four major forms of large-scale coral reefs" – the fringing reef, barrier reef, atoll and table reef based on Stoddart, D.R. (1969). Spalding et al. list four main reef types that can be clearly illustrated – the fringing reef, barrier reef, atoll, and "bank or platform reef"—and notes that many other structures exist which do not conform easily to strict definitions, including the "patch reef". Fringing reef at the southern tip of Israel A fringing reef, also called a shore reef, or borders it with an intervening narrow, shallow channel or lagoon. Fringing reefs follow coastlines and can extend for many kilometres. They are usually less than 100 metres wide, but some are hundreds of metres wide. Fringing reefs are initially formed on the shore at the low water level and expand seawards as they grow in size. The final width depends on where the seabed begins to drop steeply. The surface of the fringe reef generally remains at the same height: just below the waterline. In older fringing reefs, with outer regions pushed far out into the sea, the inner part is deepened by erosion and eventually forms a lagoon. Fringing reef lagoons can become over 100 metres wide and several metres deep. Like the fringing reef itself, they run parallel to the coast. The fringing reefs of the Red Sea are "some of the best developed in the world" and occur along all its shores except off sandy bays. Barrier reef Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon. Other major examples are the Mesoamerican Barrier Reef System and the New Caledonian Barrier Reef. Platform reefs are found in the southern Great Barrier Reef, the Swain and Capricorn Group on the continental shelf, about 100–200 km from the coast. Some platform reefs of the northern Mascarenes are several thousand kilometres from the mainland. Unlike fringing and barrier reefs, which extend only seaward, platform reefs grow in all directions. They are usually formed from fringing reefs around volcanic islands. Other reef types or variants in the Maldives in the Maldives • Apron reef – short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore. The initial stage of a fringing reef. • Bank reef – isolated, flat-topped reef larger than a patch reef and usually on mid-shelf regions and linear or semi-circular in shape; a type of platform reef. • Ribbon reef – long, narrow, possibly winding reef, usually associated with an atoll lagoon and also called a shelf-edge reef or sill reef. • Habili – reef specific to the Red Sea; does not reach near enough to the surface to cause visible surf; may be a hazard to ships (from the Arabic for "unborn") • Microatoll – community of species of corals; vertical growth limited by average tidal height; growth morphologies offer a low-resolution record of patterns of sea level change; fossilized remains can be dated using radioactive carbon dating and have been used to reconstruct Holocene sea levels • Cays – small, low-elevation, sandy islands formed on the surface of coral reefs from eroded material that piles up, creating an area above sea level; can be stabilized by plants to become habitable; occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land • Seamount or guyot – formed when a coral reef on a volcanic island subsides; tops of seamounts are rounded, and guyots are flat; flat tops of guyots, or tablemounts, are due to erosion by waves, winds, and atmospheric processes == Zones ==

Coral reef ecosystems contain distinct zones that host different kinds of habitats. Usually, three major zones are recognized: the fore reef, the reef crest, and the back reef (also called the reef lagoon). The three zones are physically and ecologically interconnected. Reef life and oceanic processes create opportunities for the exchange of seawater, sediment, nutrients, and marine life. Most coral reefs exist in waters less than 50 m deep. Some inhabit tropical continental shelves where cool, nutrient-rich upwelling does not occur, such as the 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. Alternatively, Moyle and Cech distinguish six zones, though most reefs possess only some of the zones. . 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 surge and tides. When waves pass over shallow areas, they shoal, as shown in the adjacent diagram. This means the water is often agitated. These are the precise conditions under which corals flourish. The light is sufficient for photosynthesis by the symbiotic zooxanthellae, and agitated water brings plankton to feed the coral. The off-reef floor is the shallow sea floor surrounding a reef. This zone occurs next to reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths and do not have such a floor. Usually sandy, the floor often supports seagrass meadows, which are important foraging areas for reef fish. The reef drop-off is, for its first 50 m, habitat for reef fish who find shelter on the cliff face and plankton in the water nearby. The drop-off zone primarily occurs around oceanic islands and atolls. The reef face is the zone above the reef floor or the reef drop-off. This zone is often the reef's most diverse area. Coral and calcareous algae provide complex habitats and areas that offer protection, such as cracks and crevices. Invertebrates and epiphytic algae provide much of the food for other organisms. ==Locations==

. Most corals live within this boundary. Note the cooler waters caused by upwelling on the southwest coast of Africa and off the coast of Peru. 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 km2 (109,800 sq mi), just under 0.1% of the oceans' surface area. 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 account for 7.6%. Although corals exist both in temperate and tropical waters, shallow-water reefs form only in a zone extending from approximately 30° N to 30° S of the equator. Tropical corals do not grow at depths of over . The optimum temperature for most coral reefs is , and few reefs exist in waters below . When the net production by reef-building corals no longer keeps pace with relative sea level, and the reef structure permanently drowns, a Darwin Point is reached. One such point exists at the northwestern end of the Hawaiian Archipelago; see Evolution of Hawaiian volcanoes#Coral atoll stage. However, reefs in the Persian Gulf have adapted to temperatures of in winter and in summer. Deep-water coral inhabits greater depths and colder temperatures at much higher latitudes, as far north as Norway. Although deep water corals can form reefs, little is known about them. The northernmost coral reef on Earth is located near Eilat, Israel. Coral reefs are rare along the west coasts of the Americas and Africa, due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas (the Humboldt, Benguela, and Canary Currents, respectively). Corals are seldom found along the coastline of South Asia—from the eastern tip of India (Chennai) to the Bangladesh and Myanmar borders • Blake Plateau has the world's largest known deep-water coral reef, comprising a 6.4 million-acre reef that stretches from Miami to Charleston, S. C. Its discovery was announced in January 2024. • Pulley Ridge—deepest photosynthetic coral reef, Florida • Numerous reefs around the Maldives • The Philippines coral reef area, the second-largest in Southeast Asia, is estimated at 26,000 square kilometres. They are populated by over 900 reef fish species and 400 scleractinian coral species, 12 of which are endemic. • The Raja Ampat Islands in Indonesia's Southwest Papua province offer the highest known marine diversity. • Bermuda is known for its northernmost coral reef system, located at . The presence of coral reefs at this high latitude is due to the proximity of the Gulf Stream. Bermuda coral species represent a subset of those found in the greater Caribbean. • The world's northernmost individual coral reef is located in the Finlayson Channel, in the inside passage of British Columbia, Canada. • The world's southernmost coral reef is at Lord Howe Island, in the Pacific Ocean off the east coast of Australia. ==Coral==

polyp anatomy When alive, corals are colonies of small animals embedded in calcium carbonate shells. Coral heads consist of accumulations of individual animals called polyps, arranged in diverse shapes. Polyps are usually tiny, but they can range in size from a pinhead to across. Reef-building or hermatypic corals live only in the photic zone (above 70 m), the depth to which sufficient sunlight penetrates the water. Zooxanthellae , the microscopic algae that live inside coral, give it colour and provide it with food through photosynthesis Coral polyps do not photosynthesize, but have a symbiotic relationship with microscopic algae (dinoflagellates) of the genus Symbiodinium, commonly referred to as zooxanthellae. These organisms live within the polyps' tissues and provide organic nutrients that nourish the polyp in the form of glucose, glycerol, and amino acids. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts. Clades B and C are found more frequently in deeper water, which may explain their higher vulnerability to increased temperatures. Terrestrial plants that receive less sunlight because they are found in the undergrowth are analogous to clades B, C, and D. Since clades B through D are found at deeper depths, they require an elevated light absorption rate to be able to synthesize as much energy. With elevated absorption rates at UV wavelengths, these phylotypes are more prone to coral bleaching than the shallow clade A. Clade D has been observed to be high temperature-tolerant, and has a higher rate of survival than clades B and C during modern bleaching events. the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral head's top upwards and outwards. 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. Typical shapes for coral species are named by their resemblance to terrestrial objects such as wrinkled brains, cabbages, table tops, antlers, wire strands, and pillars. These shapes can depend on the life history of the coral, like light exposure and wave action, and events such as breakages. Reproduction and attached to the ocean floor. But unlike plants, corals do not make their own food. Corals reproduce both sexually and asexually. An individual polyp uses both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesenteries, 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 species 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 across a reef simultaneously release eggs and sperm into the water en masse. Spawn disperse over a large area. The timing of spawning depends on the time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tide. The less water movement, the better the chance for fertilization. The release of eggs or planula usually occurs at night and is sometimes in phase with the lunar cycle (three to six days after a full moon). Image:Rhythms and Clocks in Marine Organisms Figure 1 Solar and Lunar Cycles ma150509.f1.jpg |thumb|center |400px | The emergence of complex rhythms from solar and lunar cycles in marine ecosystems. while at short distances chemical compounds become important. The larvae are vulnerable to predation and environmental conditions. The lucky few planulae that successfully attach to the substrate then compete for food and space. ==Gallery of reef-building corals==

Corals are the most prodigious reef-builders. However, many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner as corals. These include coralline algae, some sponges and bivalves. Reefs are always built by the combined efforts of these different phyla, with other organisms leading reef-building in other geological periods. Coralline algae Lithothamnion sp. Coralline algae are essential contributors to reef structure. Although their mineral deposition rates are much slower than corals, they are more tolerant of rough wave-action, and so help to create a protective crust over those parts of the reef subjected to the most significant forces by waves, such as the reef front facing the open ocean. They also strengthen the reef structure by depositing limestone in sheets over the reef surface. Furthermore, in locations unfavorable to the growth of corals, coralline algae can be the primary builders of an algal reef. Sponges Sponge reefs are reefs produced by sea sponges. Hexactinellid sponges are known to form reefs off the coast of British Columbia, southeast Alaska, and Washington state. Reefs discovered in Hecate Strait, British Columbia, have grown to up to 7 kilometres long and 20 metres high. Hexactinellid sponge reefs were first identified in the Middle Triassic (245–208 million years ago). The sponges reached their full extent in the late Jurassic (201–145 million years ago) when a discontinuous reef system 7,000 km long stretched across the northern Tethys and North Atlantic basins, but have since declined and were thought to be extinct until existing reefs were discovered in 1987–1988. Archaeocyatha, an extinct clade of sponges, were the planet's first reef-building animals and are an index fossil for the Lower Cambrian worldwide. Similarly, Stromatoporoidea was another extinct clade of reef-building sponges. Unlike corals, stromatoporoids usually settled on soft substrates, so their 'reefs' occupied only a single level rather than a multi-tiered vertical framework of built-up skeletons. Bivalves s (Crassostrea virginica) Oyster reefs are dense aggregations of oysters living in colonial communities. Other regionally specific names for these structures include oyster beds and oyster banks. Oyster larvae require a hard substrate or surface to attach to, which includes the shells of old or dead oysters. Thus, reefs can build up over time as new larvae settle on older individuals. Crassostrea virginica were once abundant in Chesapeake Bay and shorelines bordering the Atlantic coastal plain until the late nineteenth century. Ostrea angasi is a species of flat oyster that has also formed large reefs in South Australia. Hippuritida, an extinct order of bivalves known as rudists, were major reef-building organisms during the Cretaceous. By the mid-Cretaceous, rudists became the dominant tropical reef-builders, becoming more numerous than scleractinian corals. During this period, ocean temperatures and saline levels—which corals are sensitive to—were higher than they are today, which may have contributed to the success of rudist reefs. Gastropods Some gastropods, like family Vermetidae, are sessile and cement themselves to the substrate, contributing to the reef building. ==Darwin's paradox==

In The Structure and Distribution of Coral Reefs, published in 1842, Darwin described how coral reefs were found in some tropical areas but not others, with no obvious cause. The largest and strongest corals grew in parts of the reef exposed to the most violent surf, and corals were weakened or absent where loose sediment accumulated. yet a coral reef can flourish like an "oasis in the desert". This has given rise to the ecosystem conundrum, sometimes called "Darwin's paradox": "How can such high production flourish in such nutrient poor conditions?" Coral reefs support over one-quarter of all marine species. This diversity results in complex food webs, with large predator fish eating smaller forage fish, which eat yet smaller zooplankton, and so on. However, all food webs ultimately depend on plants, which serve as primary producers. Coral reefs typically produce 5–10 grams of carbon per square meter per day (gC·m−2·day−1) biomass. One reason for the unusual clarity of tropical waters is their nutrient deficiency and drifting plankton. Further, the sun shines year-round in the tropics, warming the surface layer, making it less dense than subsurface layers. The warmer water is separated from deeper, 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. In most parts of the ocean, there is little exchange between these layers. Organisms that die in aquatic environments generally sink to the bottom, where they decompose, which 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 do not return directly to the surface. Plants form the base of the food chain and need sunlight and nutrients to grow. In the ocean, these plants are mainly microscopic phytoplankton which drift in the water column. They need sunlight for photosynthesis, which powers carbon fixation, so they are found only relatively near the surface, but they also need nutrients. Phytoplankton rapidly use nutrients in the surface waters, and in the tropics, these nutrients are not usually replaced because of the thermocline. Explanations Around coral reefs, lagoons fill in with material eroded from the reef and the island. They become havens for marine life, protecting them from waves and storms. Most importantly, reefs recycle nutrients, which happens much less in the open ocean. In coral reefs and lagoons, producers include phytoplankton, seaweed, and coralline algae, especially small types called turf algae, which transfer nutrients to corals. Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from water. Many corals extend their tentacles at night to catch zooplankton that pass near. Zooplankton provide the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element. and pigmented proteins (reds, blues, greens, etc.) produced by the corals themselves. Sponges live in the crevices of reefs. They are efficient filter feeders, and in the Red Sea they consume about 60% of the phytoplankton that drifts by. Sponges eventually excrete nutrients in a form that corals can use. The roughness of coral surfaces is key to coral survival in agitated waters. Normally, a boundary layer of still water forms around a submerged object, acting as a barrier. Waves breaking on the extremely rough edges of corals disrupt the boundary layer, allowing the corals access to passing nutrients. Turbulent water thereby promotes reef growth. Without access to nutrients brought by rough coral surfaces, even the most effective recycling would not suffice. Deep nutrient-rich water entering coral reefs through isolated events may have significant effects on temperature and nutrient systems. This water movement disrupts the relatively stable thermocline that usually exists between warm shallow water and deeper colder water. Temperature regimes on coral reefs in the Bahamas and Florida are highly variable, spanning temporal scales from minutes to seasons and spatial scales across depths. Water can pass through coral reefs in various ways, including current rings, surface waves, internal waves, and tidal changes. Movement is generally created by tides and wind. As tides interact with varying bathymetry and wind mixes with surface water, internal waves are created. An internal wave is a gravity wave that moves along density stratification within the ocean. When a water parcel encounters a different density, it oscillates and creates internal waves. While internal waves generally have a lower frequency than surface waves, they often form as a single wave that breaks into multiple waves as it hits a slope and moves upward. This vertical breakup of internal waves causes significant diapycnal mixing and turbulence. Internal waves can act as nutrient pumps, bringing plankton and cool nutrient-rich water to the surface. The irregular structure of coral reef bathymetry may enhance mixing, producing pockets of cooler water and variable nutrient levels. Arrival of cool, nutrient-rich water from depths due to internal waves and tidal bores has been linked to growth rates of suspension feeders and benthic algae as well as plankton and larval organisms. The seaweed Codium isthmocladum reacts to deep water nutrient sources because their tissues have different concentrations of nutrients dependent upon depth. Coral reefs often depend on surrounding habitats, such as seagrass meadows and mangrove forests, for nutrients. Seagrass and mangroves supply dead plants and animals that are rich in nitrogen and 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 in which the mangroves and seagrass can root. ==Biodiversity==

Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of organisms. Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and sea grass protect the coral from large influxes of silt, fresh water and pollutants. This level of environmental variety benefits many coral reef animals, which, for example, may feed on seagrass and use the reefs for protection or breeding. Reefs are home to a variety of animals, including fish, seabirds, sponges, cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp, spiny lobsters and crabs), mollusks (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 species feed directly on corals, while others graze on algae on the reef. Different species may regularly inhabit the same hideouts in a reef at different times of day. Nighttime predators such as cardinalfish and squirrelfish hide during the day, while damselfish, surgeonfish, triggerfish, wrasses and parrotfish hide from eels and sharks. Coral reefs also have a very high degree of microorganism diversity compared to other environments. Algae Reefs are chronically at risk of algal encroachment. Overfishing and excess nutrient supply from onshore can enable algae to outcompete and kill the coral. Increased nutrient levels can be a result of sewage or chemical fertilizer runoff. Runoff can carry nitrogen and phosphorus, which promote excess algae growth. Algae can sometimes out-compete the coral for space. The algae can then smother the coral by decreasing the oxygen supply available to the reef. Decreased oxygen levels can slow down calcification rates, weakening the coral and leaving it more susceptible to disease and degradation. Algae inhabit a large percentage of surveyed coral locations. The algal population consists of turf algae, coralline algae and macro algae. Some sea urchins (such as Diadema antillarum) eat these algae and could thus decrease the risk of algal encroachment. Sponges Sea sponges are an essential component of coral reef communities. There are 420 species of sponges in coral reefs from Indonesia, 486 species in coral reefs from Indian waters, and 1500 species in the Great Barrier Reef from Australia. Sponges occupy an important role as detritivores in coral reef food webs by recycling detritus to higher trophic levels through their sponge loop. For example, several sponge species can convert dissolved organic matter (DOM) derived from corals and algae into sponge detritus, which serves as food for species incapable of directly consuming DOM. Sponges with photosynthesizing endosymbionts also produce up to three times more oxygen, as well as more organic matter than they consume. Such contributions to their habitats' resources are significant along Australia's Great Barrier Reef but relatively minor in the Caribbean. Aside from providing nutrition, sponges also offer microhabitats to various invertebrates and some fish species. Healthy reefs can produce up to 35 tons of fish per square kilometre each year, but damaged reefs produce much less. Researchers are investigating the use of native collector urchins, Tripneustes gratilla, for their potential as biocontrol agents to mitigate the spread of invasive algae species on coral reefs. Nudibranchia and sea anemones eat sponges. Several 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. Animals boring into the rock include sponges, bivalve mollusks, and sipunculans. Those settling on the reef include many other species, particularly crustaceans and polychaete worms. Each seabird species has 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 19th century. Other Sea snakes feed exclusively on fish and their eggs. Marine birds, such as herons, gannets, pelicans and boobies, feed on reef fish. Some land-based reptiles intermittently associate with reefs, such as monitor lizards, the marine crocodile and semiaquatic snakes, such as Laticauda colubrina. Sea turtles, particularly hawksbill sea turtles, feed on sponges. File:Prionurus laticlavius.jpg|Schooling reef fish File:Caribbean reef squid.jpg|Caribbean reef squid File:Stenopus hispidus (high res).jpg|Banded coral shrimp File:Triaenodon obesus moc.jpg|Whitetip reef shark File:Green turtle John Pennekamp.jpg|Green turtle File:Red sea-reef 3627.jpg|Giant clam File:Nephtheidae komodo.jpg|Soft coral, cup coral, sponges, and ascidians File:Laticauda colubrina (Wakatobi).jpg|Banded sea krait File:Latiaxis wormaldi 002.jpg|The shell of Latiaxis wormaldi, a coral snail ==Ecosystem services==

Coral reefs deliver ecosystem services to tourism, fisheries, and coastline protection. The global economic value of coral reefs has been estimated to be between US$29.8 billion and $375 billion per year. About 500 million people benefit from ecosystem services provided by coral reefs. The economic cost of destroying one square kilometre of coral reef over 25 years has been estimated at between $137,000 and $1,200,000. To improve the management of coastal coral reefs, the World Resources Institute (WRI) developed and published tools to calculate the value of coral reef-related tourism, shoreline protection, and fisheries, partnering with five Caribbean countries. As of April 2011, published working papers covered St. Lucia, Tobago, Belize, and the Dominican Republic. The WRI was "making sure that the study results support improved coastal policies and management planning". The Belize study estimated the value of reef and mangrove services at $395–559 million annually. Bermuda's coral reefs provide economic benefits to the Island worth, on average, $722 million per year, based on six key ecosystem services, according to Sarkis et al. (2010). Shoreline protection . The Bay Islands are part of the Mesoamerican coral reef system. Due to this, the authorities have made huge investments for its preservation. Coral reefs protect shorelines by absorbing wave energy, and many small islands would not exist without reefs. Coral reefs can reduce wave energy by 97%, helping to prevent loss of life and property damage. Coastlines protected by coral reefs are also more stable in terms of erosion than those without. Reefs can attenuate waves as well as, or better than, artificial structures designed for coastal defence, such as breakwaters. An estimated 197 million people who live both below 10 m elevation and within 50 km of a reef consequently may receive risk reduction benefits from reefs. Restoring reefs is significantly cheaper than building artificial breakwaters in tropical environments. Expected damages from flooding would double, and costs from frequent storms would triple without the topmost meter of reefs. For 100-year storm events, flood damages would increase by 91% to $US 272 billion without the top meter. Fisheries About six million tons of fish are taken each year from coral reefs. Well-managed reefs have an average annual yield of 15 tons of seafood per square kilometre. Southeast Asia's coral reef fisheries alone yield about $2.4 billion in seafood annually. ==Threats==

off Yap, Micronesia in Australia {{ external media Since their emergence 485 million years ago, coral reefs have faced many threats, including disease, predation, invasive species, bioerosion by grazing fish, algal blooms, and geologic hazards. Recent human activities present new threats. From 2009 to 2018, coral reefs worldwide declined 14%. Human activities that threaten coral include coral mining, bottom trawling, and the digging of canals and accesses into islands and bays, all of which can damage marine ecosystems if not done sustainably. Other localized threats include blast fishing, overfishing, coral overmining, and marine pollution, including use of the banned anti-fouling biocide tributyltin; although absent in developed countries, these activities continue in places with few environmental protections or poor regulatory enforcement. Chemicals in sunscreens may awaken latent viral infections in zooxanthellae However, concentrating tourism activities via offshore platforms has been shown to limit the spread of coral disease by tourists. Greenhouse gas emissions present a broader threat through sea temperature rise and sea level rise, resulting in widespread coral bleaching and loss of coral cover. Climate change causes more frequent and more severe storms, also changes ocean circulation patterns, which can destroy coral reefs.Ocean acidification also affects corals by decreasing calcification rates and increasing dissolution rates, although corals can adapt their calcifying fluids to changes in seawater pH and carbonate levels to mitigate the impact. Volcanic and human-made aerosol pollution can modulate regional sea surface temperatures. In 2011, two researchers suggested that "extant marine invertebrates face the same synergistic effects of multiple stressors" that occurred during the end-Permian extinction. That genus "with poorly buffered respiratory physiology and calcareous shells", such as corals, was particularly vulnerable. Corals respond to stress by "bleaching", or expelling their colorful zooxanthellate endosymbionts. Corals with Clade C zooxanthellae are generally vulnerable to heat-induced bleaching, whereas corals with the hardier Clade A or D are generally resistant, as are tougher coral genera like Porites and Montipora. ==Protection==

Marine protected areas (MPAs) are areas designated because they provide various kinds of protection to ocean and/or estuarine areas. They are intended to promote responsible fishery management and habitat protection. MPAs can also encompass social and biological objectives, including reef restoration, aesthetics, biodiversity, and economic benefits. The effectiveness of MPAs is still debated. For example, a study investigating the success of a small number of MPAs in Indonesia, the Philippines, and Papua New Guinea found no significant differences between the MPAs and unprotected sites. Furthermore, in some cases they can generate local conflict, due to a lack of community participation, clashing views of the government and fisheries, effectiveness of the area and funding. In some situations, as in the Phoenix Islands Protected Area, MPAs provide revenue to locals. The level of income provided is similar to the income they would have generated without controls. Protecting networks of diverse and healthy reefs, not only climate refugia, helps ensure the greatest chance of genetic diversity, which is critical for coral to adapt to new climates. A variety of conservation methods applied across marine and terrestrial threatened ecosystems makes coral adaption more likely and effective. 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. Australia compiled a Coral Reef Resilience Action Plan. This plan consists of adaptive management strategies, including reducing carbon footprint. A public awareness plan provides education on the "rainforests of the sea" and how people can reduce carbon emissions. 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 no net or spear fishing. Both biomass and individual fish sizes are significantly larger than in places where fishing is unrestricted. Increased atmospheric CO2 levels contribute to ocean acidification, which in turn damages coral reefs. To help combat ocean acidification, several countries have enacted laws to reduce greenhouse gas emissions, such as carbon dioxide. Many land use laws aim to reduce CO2 emissions by limiting deforestation. Deforestation can release significant amounts of CO2 unless sequestered through active follow-up forestry programs. Deforestation can also cause erosion, which flows into the ocean, contributing to ocean acidification. Incentives are used to reduce vehicle miles traveled, thereby reducing carbon emissions into the atmosphere and lowering dissolved CO2 in the ocean. State and federal governments also regulate land activities that affect coastal erosion. High-end satellite technology can monitor reef conditions. The United States Clean Water Act puts pressure on state governments to monitor and limit run-off of polluted water. ==Restoration==

Coral reef restoration has grown in prominence over the past several decades because of the unprecedented reef die-offs around the planet. Coral stressors can include pollution, warming ocean temperatures, extreme weather events, and overfishing. With the deterioration of global reefs, fish nurseries, biodiversity, coastal development, livelihoods, and natural beauty, these are under threat. Fortunately, researchers have taken it upon themselves to develop a new field, coral restoration, in the 1970s–1980s Coral farming Coral aquaculture, also known as coral farming or coral gardening, is showing promise as a potentially effective tool for restoring coral reefs. The "gardening" process bypasses the early growth stages of corals when they are most at risk of dying. Coral seeds are grown in nurseries, then replanted on the reef. Coral is farmed by coral farmers whose interests range from reef conservation to increased income. Due to its straightforward process and substantial evidence of the technique having a significant effect on coral reef growth, coral nurseries became the most widespread and arguably the most effective method for coral restoration. Coral gardens take advantage of a coral's natural ability to fragment and continue to grow if the fragments can anchor themselves onto new substrates. This method was first tested by Baruch Rinkevich in 1995 which found success at the time. By today's standards, coral farming has evolved into various forms, but it still aims to cultivate corals. Consequently, coral farming quickly replaced previously used transplantation methods, which involved physically moving sections or entire coral colonies to a new area. Creating substrates Efforts to expand the size and number of coral reefs generally involve supplying substrate to allow more corals to find a home. Substrate materials include discarded vehicle tires, scuttled ships, subway cars, and formed concrete, such as reef balls. Reefs grow unaided on marine structures such as oil rigs. In large restoration projects, propagated hermatypic coral on substrate can be secured with metal pins, superglue, or milliput. Needle and thread can also attach A-hermatype coral to the substrate. Biorock is a substrate produced by a patented process that runs low-voltage electrical currents through seawater to cause dissolved minerals to precipitate onto steel structures. The resultant white carbonate (aragonite) is the same mineral that makes up natural coral reefs. Corals rapidly colonize and grow on these coated structures. The electrical currents also accelerate the formation and growth of both chemical limestone rock and the skeletons of corals and other shell-bearing organisms, such as oysters. The vicinity of the anode and cathode provides a high pH environment, which inhibits the growth of competing filamentous and fleshy algae. The increased growth rates depend entirely on accretion activity. Under the influence of an electric field, corals exhibit increased growth rates, sizes, and densities. Simply having many structures on the ocean floor is not enough to form coral reefs. Restoration projects must consider the complexity of the substrates they are creating for future reefs. Researchers conducted an experiment near Ticao Island in the Philippines in 2013 where several substrates in varying complexities were laid in the nearby degraded reefs. Large complexity consisted of plots with both human-made substrates (smooth and rough rocks) and a surrounding fence; medium consisted of only the human-made substrates; and small had neither the fence nor substrates. After one month, researchers found a positive correlation between structural complexity and larval recruitment rates. Warming oceans are forcing corals to adapt to unprecedented temperatures. Those that do not have a tolerance for the elevated temperatures experience coral bleaching and eventually mortality. There is already research aimed at creating genetically modified corals that can withstand a warming ocean. Madeleine J. H. van Oppen, James K. Oliver, Hollie M. Putnam, and Ruth D. Gates described four levels of human intervention for genetically modifying corals, each increasing in intensity. These methods focus on altering the genetics of the zooxanthellae within coral rather than the alternative. The first method is to induce acclimatization of the first generation of corals. Historically, algae growth was controlled by herbivorous fish and sea urchins. Parrotfish are a prime example of reef caretakers. Consequently, these two species can be considered keystone species in reef environments due to their role in protecting reefs. Before the 1980s, Jamaica's reefs were thriving and well cared for; however, this all changed after Hurricane Allen occurred in 1980 and an unknown disease spread across the Caribbean. In the wake of these events, massive damage was caused to both the reefs and the sea urchin population across Jamaican reefs and into the Caribbean Sea. As little as 2% of the original sea urchin population survived the disease. Parrotfish and other herbivorous fish were few in numbers because of decades of overfishing and bycatch at the time. Microfragmentation and fusion In 2014, Christopher Page, Erinn Muller, and David Vaughan from the International Center for Coral Reef Research & Restoration at Mote Marine Laboratory in Summerland Key, Florida developed a new technology called "microfragmentation", in which they use a specialized diamond band saw to cut corals into 1 cm2 fragments instead of 6 cm2 to advance the growth of brain, boulder, and star corals. Corals Orbicella faveolata and Montastraea cavernosa were outplanted off the Florida's shores in several microfragment arrays. After two years, O. faveolata had grown 6.5x its original size while M. cavernosa had grown nearly twice its size. Under conventional means, both corals would have required decades to reach the same size. It is suspected that if predation events had not occurred near the beginning of the experiment O. faveolata would have grown at least ten times its original size. By using this method, Mote Marine Laboratory successfully generated 25,000 corals within a single year, subsequently transplanting 10,000 of them into the Florida Keys. Shortly after, they discovered that these microfragments fused with other microfragments from the same parent coral. Typically, corals that are not from the same parent fight and kill nearby corals in an attempt to survive and expand. This new technology, known as "fusion," has been shown to grow coral heads in just 2 years, rather than the typical 25–75 years. After fusion occurs, the reef will act as a single organism rather than several independent reefs. Currently, no published research has been conducted on this method. ==See also==

• Coral Reef Protection: What Are Coral Reefs?. US EPA. • • • ==External links==