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A reef is a ridge or
shoal In oceanography, geomorphology, and Earth science, geoscience, a shoal is a natural submerged ridge, bank (geography), bank, or bar that consists of, or is covered by, sand or other unconsolidated material, and rises from the bed of a body ...
of rock,
coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
, or similar relatively stable material lying beneath the surface of a natural body of water. Many reefs result from natural,
abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them und ...
(non-living) processes such as deposition of sand or wave erosion planning down rock outcrops. However, reefs such as the
coral reef 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 group ...
s of tropical waters are formed by biotic (living) processes, dominated by corals and coralline algae.
Artificial reef An artificial reef (AR) is a human-created freshwater or marine benthic structure. Typically built in areas with a generally featureless bottom to promote Marine biology#Reefs, marine life, it may be intended to control #Erosion prevention, erosio ...
s, 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
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s. They provide shelter to various aquatic animals which help prevent extinction. Another reason reefs are put in place is for aquaculture, and fish farmers who are looking to improve their businesses sometimes invest in them. Reefs are often quite near to the surface, but not all definitions require this. Earth's largest coral reef system is the
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...
in Australia, at a length of over .


Etymology

The word "reef" traces its origins back to the
Old Norse Old Norse, also referred to as Old Nordic or Old Scandinavian, was a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants ...
word ''rif,'' meaning "rib" or "reef". ''Rif'' comes from the Proto-Germanic term ''ribją'' meaning "rib".


Classification

Reefs may be classified in terms of their origin, geographical location, depth, and
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
. For example a tropical coral fringing reef, or a temperate rocky intertidal reef.


Biotic

A variety of biotic reef types exists, including
oyster reef The term ''oyster reef'' refers to dense aggregations of oysters that form large colonial communities. Because oyster larvae need to settle on hard substrates, new oyster reefs may form on stone or other hard marine debris. Eventually the oyster ...
s and
sponge reef Sponge reefs are reefs produced by sea sponges. All modern sponge reefs are formed by hexactinellid sponges, which have an endoskeleton made of silica spicules and are often referred to as "glass sponges", while historically the non-spicule ...
s, but the most massive and widely distributed are tropical
coral reef 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 group ...
s. 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 Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...
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 Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...
and
seaweeds Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), '' Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
can grow, and provide a habitat for mobile benthic organisms. 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 reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows direc ...
s,
barrier reef 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. C ...
s, and
atoll 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 ...
s. 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 A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
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.


Mounds

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 The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
. However, reefs are held up by a macroscopic skeletal framework, as what is seen on coral reefs.
Corals Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
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. 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 Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
. Examples of
biostrome A reef knoll is a landform that comprises an immense pile of calcareous material that had previously accumulated on an ancient sea floor. Reef knolls are geological remnants of reefs and other organic concentrations of calcareous organisms. Reef ...
s formed by cyanobacteria occur in the
Great Salt Lake The Great Salt Lake is the largest saltwater lake in the Western Hemisphere and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah and has a substantial impact upon the local climate, partic ...
in
Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...
, United States, and in
Shark Bay Shark Bay () is a World Heritage Site in the Gascoyne region of Western Australia. The area is located approximately north of Perth, on the westernmost point of the Australian continent. UNESCO's listing of Shark Bay as a World Heritage S ...
on the coast of
Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...
. 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.
Stromatolite Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...
s, for instance, are microbial mounds with a laminated internal structure. Whereas, bryozoans and
crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...
s, 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. The
Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...
Belt Supergroup contains evidence of possible
microbial mat A microbial mat is a multi-layered sheet or biofilm of microbial colonies, composed of mainly bacteria and/or archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few surviv ...
and dome structures similar to stromatolite and chicken reef complexes.


Geologic

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. They are often located in sub-tropical, temperate, and sub-polar latitudes.


Structures

Ancient reefs buried within
stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
sections are of considerable interest to
geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...
s because they provide paleo-environmental information about the location in
Earth's history The natural history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by consta ...
. In addition, reef structures within a sequence of
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s provide a discontinuity which may serve as a trap or conduit for
fossil fuel A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geolog ...
s or mineralizing fluids to form
petroleum Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring un ...
or
ore deposit Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...
s. Corals, including some major extinct groups
Rugosa The Rugosa or rugose corals are an extinct Class (biology), class of solitary and Colony (biology), colonial corals that were abundant in Middle Ordovician to Late Permian seas. Solitary rugosans (e.g., ''Caninia (genus), Caninia'', ''Lopho ...
and Tabulata, have been important reef builders through much of the
Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...
since the
Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...
Period. However, other organism groups, such as calcifying algae, especially members of the red algae (
Rhodophyta Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species within over 900 genera amidst ongoing taxonomic revisions. ...
), and molluscs (especially the rudist bivalves during the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
Period) have created massive structures at various times. During the
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
Period, the conical or tubular skeletons of
Archaeocyatha Archaeocyatha (), 'ancient cups') is a taxon of extinct, Sessility (zoology), sessile, reef-building Marine (ocean), marine Sponge, sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is believed that the cent ...
, an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa, have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the
Scleractinia 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 mo ...
, arose after the
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
that wiped out the earlier rugose corals (as well as many other groups). They became increasingly important reef builders throughout the
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
and have a different symmetry from that of the scleractinian corals, whose skeletons are
aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...
. However, there are some unusual examples of well-preserved aragonitic rugose corals in the
Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...
. 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).


Artificial

An artificial reef is a human-created underwater structure, typically built to promote
marine life Marine life, sea life or ocean life is the collective ecological communities that encompass all aquatic animals, aquatic plant, plants, algae, marine fungi, fungi, marine protists, protists, single-celled marine microorganisms, microorganisms ...
in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of
trawling Trawling is an industrial method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch di ...
nets, or improve
surfing Surfing is a surface water sport in which an individual, a surfer (or two in tandem surfing), uses a board to ride on the forward section, or face, of a moving wave of water, which usually carries the surfer towards the shore. Waves suita ...
. 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 Rubble is broken stone, of irregular size, shape and texture; undressed especially as a filling-in. Rubble naturally found in the soil is known also as 'brash' (compare cornbrash)."Rubble" def. 2., "Brash n. 2. def. 1. ''Oxford English Dictionar ...
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 Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
and invertebrates such as
barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...
s, corals, and
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...
s attach; the accumulation of attached marine life in turn provides intricate structure and food for assemblages of fish.


See also

* * * *


References


Sources

* Shears N.T. (2007) Biogeography, community structure and biological habitat types of subtidal reefs on the South Island West Coast, New Zealand. ''Science for Conservation 281''. p 53. Department of Conservation, New Zealand

* "General Information on Reefs." General Information on Reefs – Reef & Ocean Ecology Lab. Accessed February 1, 2024. https://www.reefoceanlab.org.au/resources/general-information-on-reefs/#:~:text=Rocky%20reefs%20are%20more%20typical,many%20parts%20of%20New%20Zealand. * "Coral Reefs ~ Marinebio Conservation Society." MarineBio Conservation Society, November 10, 2023. https://www.marinebio.org/creatures/coral-reefs/#:~:text=Organisms%20responsible%20for%20building%20tropical,and%20the%20Tropic%20of%20Cancer.


External links


''Reef Rescue'' - Smithsonian Ocean Portal

Coral Reefs of the Tropics
: facts, photos and movies from
The Nature Conservancy The Nature Conservancy (TNC) is a global environmental organization headquartered in Arlington, Virginia, United States. it works via affiliates or branches in 79 countries and territories, as well as across every state in the US. Founded in ...

NOAA Photo Library

Reef Environmental Education Foundation

NOS Data Explorer
- A portal to obtain NOAA National Ocean Service data


Atolls – Distribution, Development and Architecture
{{Authority control * Stratigraphy Coastal geography Coastal and oceanic landforms