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.

A coral "group" is a colony of very many genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in height. A set of tentacles surround a central mouth opening. Each polyp excretes an exoskeleton near the base. Over many generations, the colony thus creates a skeleton characteristic of the species which can measure up to several meters in size. Individual colonies grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously overnight, often around a full moon. Fertilized eggs form planulae, a mobile early form of the coral polyp which, when mature, settles to form a new colony.

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Although some corals are able to catch plankton and small fish using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates of the genus Symbiodinium that live within their tissues. These are commonly known as zooxanthellae and give the coral color. Such corals require sunlight and grow in clear, shallow water, typically at depths less than 60 metres (200 feet; 33 fathoms), but corals in the genus Leptoseris has been found as deep as 172 metres (564 feet; 94 fathoms).[1] Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the Great Barrier Reef off the coast of Australia. These corals are increasingly at risk of bleaching events where polyps expel the zooxanthellae in response to stress such as high water temperature or toxins.

Other corals do not rely on zooxanthellae and can live globally in much deeper water, such as the cold-water genus Lophelia which can survive as deep as 3,300 metres (10,800 feet; 1,800 fathoms).[2] Some have been found as far north as the Darwin Mounds, northwest of Cape Wrath, Scotland, and others off the coast of Washington state and the Aleutian Islands.

The classification of corals has been discussed for millennia, owing to having similarities to both plants and animals. Aristotle's pupil Theophrastus described the red coral, korallion, in his book on stones, implying it was a mineral, but he described it as a deep-sea plant in his Enquiries on Plants, where he also mentions large stony plants that reveal bright flowers when under water in the Gulf of Heroes.[3] Pliny the Elder stated boldly that several sea creatures including sea nettles and sponges "are neither animals nor plants, but are possessed of a third nature (tertia natura)".[4] Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535 book On the French and Latin Names of the Fishes of the Marseilles Region; it is popularly but wrongly supposed that Aristotle created the term.[4] Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal.[4] The Babylonian Talmud refers to coral among a list of types of trees, and the 11th-century French commentator Rashi describes it as "a type of tree ( ) that grows underwater that goes by the (French) name "coral."[5]

The Persian polymath Al-Biruni (d.1048) classified sponges and corals as animals, arguing that they respond to touch.[6] Nevertheless, people believed corals to be plants until the eighteenth century when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal.[7]

Presently, corals are classified as species of animals within the sub-classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria.[8] Hexacorallia includes the stony corals and these groups have polyps that generally have a 6-fold symmetry. Octocorallia includes blue coral and soft corals and species of Octocorallia have polyps with an eightfold symmetry, each polyp having eight tentacles and eight mesenteries. The group of corals is paraphyletic because the sea anemones are also in the sub-class Hexacorallia.

The delineation of coral species is challenging as hypotheses based on morphological traits contradict hypotheses formed via molecular tree-based processes.[9] As of 2020, there are 2175 identified separate coral species, 237 of which are currently endangered,[10] making distinguishing corals to be the utmost of importance in efforts to curb extinction.[9] Adaptation and delineation continues to occur in species of coral[11] in order to combat the dangers posed by the climate crisis. Corals are colonial modular organisms formed by asexually produced and genetically identical modules called polyps. Polyps are connected by living tissue to produce the full organism.[12] The living tissue allows for inter module communication (interaction between each polyp),[12] which appears in colony morphologies produced by corals, and is one of the main identifying characteristics for a species of coral.[12]

There are two main classifications for corals: hard coral (scleractinian and stony coral)[13] which form reefs by a calcium carbonate base, with polyps that bear six stiff tentacles,[14] and soft coral (Alcyonacea and ahermatypic coral)[13] which are pliable and formed by a colony of polyps with eight feather-like tentacles.[14] These two classifications arose from differentiation in gene expressions in their branch tips[12] and bases that arose through developmental signaling pathways such as Hox, Hedgehog, Wnt, BMP etc.

Scientists typically select Acropora as research models since they are the most diverse genus of hard coral, having over 120 species.[12] Most species within this genus have polyps which are dimorphic:[15] axial polyps grow rapidly and have lighter coloration, while radial polyps are small and are darker in coloration.[12][16] In the Acropora genus, gamete synthesis and photosynthesis occur at the basal[17] polyps, growth occurs mainly at the radial polyps. Growth at the site of the radial polyps encompasses two processes: asexual reproduction via mitotic cell proliferation,[12] and skeleton deposition of the calcium carbonate via extra cellular matrix (EMC) proteins acting as differentially expressed (DE) signaling genes[12] between both branch tips and bases. These processes lead to colony differentiation, which is the most accurate distinguisher between coral species.[9] In the Acropora genus, colony differentiation through up-regulation and down-regulation of DEs.[12]

Systematic studies of soft coral species have faced challenges due to a lack of taxonomic knowledge.[9] Researchers have not found enough variability within the genus to confidently delineate similar species, due to a low rate in mutation of mitochondrial DNA.[18]

Environmental factors, such as the rise of temperatures and acid levels in our oceans account for some speciation of corals in the form of species lost.[12] Various coral species have heat shock proteins (HSP) that are also in the category of DE across species.[12] These HSPs help corals combat the increased temperatures they are facing which lead to protein denaturing, growth loss, and eventually coral death.[12] Approximately 33% of coral species are on the International Union for Conservation of Nature's endangered species list and at risk of species loss.[19] Ocean acidification (falling pH levels in the oceans) is threatening the continued species growth and differentiation of corals.[12] Mutation rates of Vibrio shilonii, the reef pathogen responsible for coral bleaching, heavily outweigh the typical reproduction rates of coral colonies when pH levels fall.[20] Thus, corals are unable to mutate their HSPs and other climate change preventative genes to combat the increase in temperature and decrease in pH at a competitive rate to these pathogens responsible for coral bleaching,[20] resulting in species loss.

For most of their life corals are sessile animals of colonies of genetically identical polyps. Each polyp varies from millimeters to centimeters in diameter, and colonies can be formed from many millions of individual polyps. Stony coral, also known as hard coral, polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism. This is deposited by the polyps and by the coenosarc, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are markedly variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.[21]

The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner layer as the endoderm. Between ectoderm and endoderm is a supporting layer of gelatinous substance termed mesoglea, secreted by the cell layers of the body wall.[22] The mesoglea can contain skeletal elements derived from cells migrated from the ectoderm.

The sac-like body built up in this way is attached to a hard surface, which in hard corals are cup-shaped depressions in the skeleton known as corallites. At the center of the upper end of the sac lies the only opening called the mouth, surrounded by a circle of tentacles which resemble glove fingers. The tentacles are organs which serve both for tactile sense and for the capture of food.[22] Polyps extend their tentacles, particularly at night, often containing coiled stinging cells (cnidocytes) which pierce, poison and firmly hold living prey paralyzing or killing them. Polyp prey includes plankton such as copepods and fish larvae. Longitudinal muscular fibers formed from the cells of the ectoderm allow tentacles to contract to convey the food to the mouth. Similarly, circularly disposed muscular fibres formed from the endoderm permit tentacles to be protracted or thrust out once they are contracted.[22] In both stony and soft corals, the polyps can be retracted by contracting muscle fibres, with stony corals relying on their hard skeleton and cnidocytes for defense. Soft corals generally secrete terpenoid toxins to ward off predators.[21] 152ee80cbc

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