Coral species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

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).

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.

[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.

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.

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.

[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.

In scleractinian corals, "centers of calcification" and fibers are clearly distinct structures differing with respect to both morphology and chemical compositions of the crystalline units.

[32] Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment.

[45] Other factors that affect the rhythmicity of organisms in marine habitats include salinity, mechanical forces, and pressure or magnetic field changes.

Once released, gametes fertilize at the water's surface and form a microscopic larva called a planula, typically pink and elliptical in shape.

[54] The larvae often need a biological cue to induce settlement such as specific crustose coralline algae species or microbial biofilms.

Coral microbiomes have been examined in a variety of studies, which demonstrate how oceanic environmental variations, most notably temperature, light, and inorganic nutrients, affect the abundance and performance of the microalgal symbionts, as well as calcification and physiology of the host.

[62][63][64] Studies have also suggested that resident bacteria, archaea, and fungi additionally contribute to nutrient and organic matter cycling within the coral, with viruses also possibly playing a role in structuring the composition of these members, thus providing one of the first glimpses at a multi-domain marine animal symbiosis.

The coral can live with and without zooxanthellae (algal symbionts), making it an ideal model organism to study microbial community interactions associated with symbiotic state.

[81] Coral reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, molluscs, crustaceans, and many other animals.

The currently ubiquitous stony corals, Scleractinia, appeared in the Middle Triassic to fill the niche vacated by the extinct rugose and tabulate orders and is not closely related to the earlier forms.

[105] Submarine springs found along the coast of Mexico's Yucatán Peninsula produce water with a naturally low pH (relatively high acidity) providing conditions similar to those expected to become widespread as the oceans absorb carbon dioxide.

[110] The growth rings allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past climatic and environmental changes using geochemical techniques.

[111] Certain species form communities called microatolls, which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive.

Corals are shallow, colonial organisms that integrate oxygen and trace elements into their skeletal aragonite (polymorph of calcite) crystalline structures as they grow.

Time can be attributed to coral geochemistry anomalies by correlating strontium/calcium minimums with sea surface temperature (SST) maximums to data collected from NINO 3.4 SSTA.

[120] The global moisture budget is primarily being influenced by tropical sea surface temperatures from the position of the Intertropical Convergence Zone (ITCZ).

These activities can damage coral but international projects such as Green Fins that encourage dive and snorkel centres to follow a Code of Conduct have been proven to mitigate these risks.

In medicine, chemical compounds from corals can potentially be used to treat cancer, neurological diseases, inflammation including arthritis, pain, bone loss, high blood pressure and for other therapeutic uses.

[135] Coral Calx, known as Praval Bhasma in Sanskrit, is widely used in traditional system of Indian medicine as a supplement in the treatment of a variety of bone metabolic disorders associated with calcium deficiency.

[136] In classical times ingestion of pulverized coral, which consists mainly of the weak base calcium carbonate, was recommended for calming stomach ulcers by Galen and Dioscorides.

[138] Ancient (fossil) coral limestone, notably including the Coral Rag Formation of the hills around Oxford (England), was once used as a building stone, and can be seen in some of the oldest buildings in that city including the Saxon tower of St Michael at the Northgate, St. George's Tower of Oxford Castle, and the medieval walls of the city.

[141] The total economic value of coral reef services in the United States – including fisheries, tourism, and coastal protection – is more than $3.4 billion a year.