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Coral reef at Nusa Lembongan, Bali, Indonesia Nusa Lembongan Reef.jpg
Coral reef at Nusa Lembongan, Bali, Indonesia
Pamalican island with surrounding reef, Sulu Sea, Philippines PamalicanAfterLiftOff.jpg
Pamalican island with surrounding reef, Sulu Sea, Philippines
A reef surrounding an islet Reef.jpg
A reef surrounding an islet
Reefs off Vanatinai Island in the Louisiade Archipelago Vanatinai, Louisiade Archipelago.jpg
Reefs off Vanatinai Island in the Louisiade Archipelago

A reef is a ridge or shoal of rock, coral, or similar relatively stable material lying beneath the surface of a natural body of water. [1] Many reefs result from natural, abiotic (non-living) processes such as deposition of sand or wave erosion planing down rock outcrops. However, reefs such as the coral reefs of tropical waters are formed by biotic (living) processes, dominated by corals and coralline algae. Artificial reefs, such as shipwrecks and other man-made underwater structures, may occur intentionally or as the result of an accident. These are sometimes designed to increase the physical complexity of featureless sand bottoms to attract a more diverse range of organisms. Reefs are often quite near to the surface, but not all definitions require this. [1]


Earth's largest coral reef system is the Great Barrier Reef in Australia, at a length of over 2,300 kilometres (1,400 miles).


Reefs may be classified in terms of their origin, geographical location, depth, and topography. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.


Part of Great Barrier Reef Great-Barrier-Reef-2018-Luka-Peternel.jpg
Part of Great Barrier Reef

There is a variety of biotic reef types, including oyster reefs and sponge reefs, but the most massive and widely distributed are tropical coral reefs. [1] Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcareous algae, especially, although not entirely, coralline algae.

Oyster larvae prefer to settle on adult oysters and thereby develop layers building upwards. These eventually form a fairly massive hard stony calcium carbonate structure on which other reef organisms like sponges and seaweeds can grow, and provide a habitat for mobile benthic organisms. [1]

These biotic reef types take on additional names depending upon how the reef lies in relation to the land, if any. Reef types include fringing reefs, barrier reefs, and atolls. A fringing reef is a reef that is attached to an island. Whereas, a barrier reef forms a calcareous barrier around an island, resulting in a lagoon between the shore and the reef. Conversely, an atoll is a ring reef with no land present.

The reef front, facing the ocean, is a high energy locale. Whereas, the internal lagoon will be at a lower energy with fine grained sediments.


Both mounds and reefs are considered to be varieties of organosedimentary buildups, which are sedimentary features, built by the interaction of organisms and their environment. These interactions have a synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. However, reefs are held up by a macroscopic skeletal framework, as what is seen on coral reefs. Corals and calcareous algae grow on top of one another, forming a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. [2]

Conversely, mounds lack a macroscopic skeletal framework. Instead, they are built by microorganisms or by organisms that also lack a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, United States, and in Shark Bay on the coast of Western Australia. [2] [3]

Cyanobacteria do not have skeletons, and individual organisms are microscopic. However, they can encourage the precipitation or accumulation of calcium carbonate to produce distinct sediment bodies in composition that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today. Stromatolites, for instance, are microbial mounds with a laminated internal structure. Whereas, bryozoans and crinoids, common contributors to marine sediments during the Mississippian period, produce a different kind of mound. Although bryozoans are small and crinoid skeletons disintegrate, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief. [2] [4]

The Proterozoic Belt Supergroup contains evidence of possible microbial mat and dome structures similar to stromatolite and chicken reef complexes. [2] [5]


Rocky reefs are underwater outcrops of rock projecting above the adjacent unconsolidated surface with varying relief. They can be found in depth ranges from intertidal to deep water, and provide a substrate for a large range of sessile benthic organisms, and shelter for a large range of mobile organisms. [6] They are often located in sub-tropical, temperate, and sub-polar latitudes.

On the other hand, biotic reefs are formed in tropical waters by live organisms such as algae, and the organisms responsible for building biotic reefs grow at 20-28 degrees celsius. Therefore, most are found between the Tropic of Capricorn and the Tropic of Cancer.


Fossil brain coral (Diploria) at the Windley Key Fossil Reef Geological State Park. US Quarter near top for scale. Fossil Reef Windley Key 1.jpg
Fossil brain coral (Diploria) at the Windley Key Fossil Reef Geological State Park. US Quarter near top for scale.

Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about the location in Earth's history. In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which may serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits. [7]

Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic since the Ordovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae (Rhodophyta), and molluscs (especially the rudist bivalves during the Cretaceous Period) have created massive structures at various times.

During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs. [8] Other groups, such as the Bryozoa, have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia, arose after the Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups). They became increasingly important reef builders throughout the Mesozoic Era. [9] They may have arisen from a rugose coral ancestor.

Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite. [10] However, there are some unusual examples of well-preserved aragonitic rugose corals in the Late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian). [2]


An artificial reef is a human-created underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, [11] or improve surfing. [12]

Many reefs are built using objects that were built for other purposes, for example by sinking oil rigs (through the Rigs-to-Reefs program), scuttling ships, or by deploying rubble or construction debris. Other artificial reefs are purpose built (e.g. the reef balls) from PVC or concrete. Shipwrecks become artificial reefs on the seafloor. Regardless of construction method, artificial reefs generally provide stable hard surfaces where algae and invertebrates such as barnacles, corals, and oysters attach; the accumulation of attached marine life in turn provides intricate structure and food for assemblages of fish.

See also

Related Research Articles

<span class="mw-page-title-main">Atoll</span> Ring-shaped coral reef

An atoll is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most of the approximately 440 atolls in the world are in the Pacific Ocean.

<span class="mw-page-title-main">Coral</span> Marine invertebrates of the class Anthozoa.

|} Corals are colonial marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

<span class="mw-page-title-main">Coral reef</span> Outcrop of rock in the sea formed by the growth and deposit of stony coral skeletons

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

<span class="mw-page-title-main">Scleractinia</span> Order of Hexacorallia which produce a massive stony skeleton

Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles. Although some species are solitary, most are colonial. The founding polyp settles and starts to secrete calcium carbonate to protect its soft body. Solitary corals can be as much as 25 cm (10 in) across but in colonial species the polyps are usually only a few millimetres in diameter. These polyps reproduce asexually by budding, but remain attached to each other, forming a multi-polyp colony of clones with a common skeleton, which may be up to several metres in diameter or height according to species.

<span class="mw-page-title-main">Coralline algae</span> Order of algae (Corallinales)

Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, and limpets and chitons feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène ("coralligenous"). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens may form relatively smooth compact balls to warty or fruticose thalli.

<span class="mw-page-title-main">Bioerosion</span> Erosion of hard substrates by living organisms

Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping. On dry land, bioerosion is typically performed by pioneer plants or plant-like organisms such as lichen, and mostly chemical or mechanical in nature.

<span class="mw-page-title-main">Calcareous</span> Adjective meaning mostly or partly composed of calcium carbonate

Calcareous is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.

<span class="mw-page-title-main">Carbonate platform</span> Sedimentary body with topographic relief composed of autochthonous calcareous deposits

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha or extinct cnidaria were important reef builders.

<span class="mw-page-title-main">Stromatoporoidea</span> Extinct clade of sponges

Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Middle Ordovician to the Late Devonian. They can be characterized by their densely layered calcite skeletons lacking spicules. Stromatoporoids were among the most abundant and important reef-builders of their time, living close together in flat biostromes or elevated bioherms on soft tropical carbonate platforms.

<span class="mw-page-title-main">Chazy Formation</span>

The Chazy Reef Formation is a mid-Ordovician limestone deposit in northeastern North America.

<span class="mw-page-title-main">Fire coral</span> Genus of hydrozoans

Fire corals (Millepora) are a genus of colonial marine organisms that exhibit physical characteristics similar to that of coral. The name coral is somewhat misleading, as fire corals are not true corals but are instead more closely related to Hydra and other hydrozoans, making them hydrocorals. They make up the only genus in the monotypic family Milleporidae.

<span class="mw-page-title-main">Rhodolith</span> Calcareous marine nodules composed of crustose red algae

Rhodoliths are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic growth habit has been attained by a number of unrelated coralline red algae, organisms that deposit calcium carbonate within their cell walls to form hard structures or nodules that resemble beds of coral.

<span class="mw-page-title-main">Thrombolite</span> Clotted accretionary structures formed in shallow water

Thrombolites are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria.

<i>Madrepora oculata</i> Species of coral

Madrepora oculata, also called zigzag coral, is a stony coral that is found worldwide outside of the polar regions, growing in deep water at depths of 50 to at least 1500 meters. It was first described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae. It is one of only 12 species of coral that are found worldwide, including in Subantarctic oceans. In some areas, such as in the Mediterranean Sea and the Northeast Atlantic Ocean, it dominates communities of coral. Due to their similar distribution and taxonomic relationship, M. oculata is often experimentally compared to related deep sea coral, Lophelia pertusa.

Fossils of many types of water-dwelling animals from the Devonian period are found in deposits in the U.S. state of Michigan. Among the more commonly occurring specimens are bryozoans, corals, crinoids, and brachiopods. Also found, but not so commonly, are armored fish called placoderms, snails, sharks, stromatolites, trilobites and blastoids.

<span class="mw-page-title-main">Mesophotic coral reef</span>

A mesophotic coral reef or mesophotic coral ecosystem (MCE), originally from the Latin word meso (meaning middle) and photic (meaning light), is characterized by the presence of both light-dependent coral and algae, and organisms that can be found in water with low light penetration. Mesophotic coral ecosystems occur at depths beyond those typically associated with coral reefs as the mesophotic ranges from brightly lit to some areas where light does not reach. Mesophotic coral ecosystem (MCEs) is a new, widely-adopted term used to refer to mesophotic coral reefs, as opposed to other similar terms like "deep coral reef communities" and "twilight zone", since those terms sometimes are confused due to their unclear, interchangeable nature. Many species of fish and corals are endemic to the MCEs making these ecosystems a crucial component in maintaining global diversity. Recently, there has been increased focus on the MCEs as these reefs are a crucial part of the coral reef systems serving as a potential refuge area for shallow coral reef taxa such as coral and sponges. Advances in recent technologies such as remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs) have enabled humans to conduct further research on these ecosystems and monitor these marine environments.

A Waulsortian mudmound is a calcareous geographical feature found in Early Carboniferous strata of Central Europe. It is a type of fossil-rich bioconstruction, formed by microbial activity in deep tropical waters during the mid-Dinantian. Mud mounds are a type of high-relief biologically-mediated seafloor deposit, similar to a reef. Unlike a true reef, a mud mound is composed almost entirely of uncemented mud-sized matrix grains, and the isolated fossils do not form a significant skeletal framework.

<span class="mw-page-title-main">Ocean acidification in the Great Barrier Reef</span> Threat to the reef which reduces the viability and strength of reef-building corals

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean. The more humanity consumes fossil fuels, the more the ocean absorbs released CO₂, furthering ocean acidification.

<span class="mw-page-title-main">Marine biogenic calcification</span> Shell formation mechanism

Marine biogenic calcification is the production of calcium carbonate by organisms in the global ocean.

<i>Ricordea yuma</i> Species of coral

Ricordea yuma is a species of coral in the family Ricordeidae, order Corallimorpharia; This order of corals do not produce the distinctive calcification of the closely related Scleractinian, or reef building corals. Ricordea yuma are found on the sea floor in relatively shallow, tropical or subtropical ocean environments. Distinctive features include a large mouth disk that takes up most of the organism, and brightly colored tentacles. Ricordea yuma can reproduce both sexually, and asexually by budding a new coral with replicated elements from the mother coral. This may be one mechanism of how they are able to spread and overtake areas rapidly; They have been observed being competitively successful at monopolizing areas by excluding reef-building coral species, after a disturbance in the substrate.


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