The basic unit of the adult is the polyp; this consists of a cylindrical column topped by a disc with a central mouth surrounded by tentacles.

Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual.

Members of Anthozoa possess cnidocytes, a feature shared among other cnidarians such as the jellyfish, box jellies and parasitic Myxozoa and Polypodiozoa.

Internal tensions pull the mouth into a slit-shape, and the ends of the slit lead into two grooves in the pharynx wall called siphonoglyphs.

Another form of weapon is the similarly armed acontia (threadlike defensive organs) which can be extruded through apertures in the column wall.

[15] Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like supports internally.

[2] Most anthozoans supplement their predation by incorporating into their tissues certain unicellular, photosynthetic organisms known as zooxanthellae (or zoochlorellae in a few instances); many fulfil the bulk of their nutritional requirements in this way.

The behaviour of the anthozoan can also be affected, with it choosing to settle in a well lit spot, and competing with its neighbours for light to allow photosynthesis to take place.

[4] With longitudinal, transverse and radial muscles, polyps are able to elongate and shorten, bend and twist, inflate and deflate, and extend and contract their tentacles.

Contraction is achieved by pumping fluid out of the coelenteron, and reflation by drawing it in, a task performed by the siphonoglyphs in the pharynx which are lined with beating cilia.

[2] Gas exchange and excretion is accomplished by diffusion through the tentacles and internal and external body wall, aided by the movement of fluid being wafted along these surfaces by cilia.

[2] Anthozoans exhibit great powers of regeneration; lost parts swiftly regrow and the sea anemone Aiptasia pallida can be vivisected in the laboratory and then returned to the aquarium where it will heal.

[2] Sea anemones for example can crawl across a surface leaving behind them detached pieces of the pedal disc which develop into new clonal individuals.

[19] Transverse fission is less common, but occurs in Anthopleura stellula and Gonactinia prolifera, with a rudimentary band of tentacles appearing on the column before the sea anemone tears itself apart.

[2] To make fertilisation more likely, corals emit vast numbers of gametes, and many species synchronise their release in relation to the time of day and the phase of the moon.

[22] The zygote develops into a planula larva which swims by means of cilia and forms part of the plankton for a while before settling on the seabed and metamorphosing into a juvenile polyp.

[2] The planulae of the stony coral Pocillopora damicornis, for example, have lipid-rich yolks and remain viable for as long as 100 days before needing to settle.

In 1989, the invasive crown-of-thorns starfish (Acanthaster planci) caused havoc in American Samoa, killing 90% of the corals in the reefs.

In the Atlantic Ocean, the cold-water coral Lophelia pertusa forms extensive deep-water reefs which support many other species.

The mollusc Simnialena marferula is only found on the sea whip Leptogorgia virgulata, is coloured like it and has sequestered its defensive chemicals, and the nudibranch Tritonia wellsi is another obligate symbiont, its feathery gills resembling the tentacles of the polyps.

The sea slug Aeolidia papillosa feeds on the aggregating anemone (Anthopleura elegantissima), accumulating the nematocysts for its own protection.

[34][35] Coral reefs and shallow marine environments are threatened, not only by natural events and increased sea temperatures, but also by such man-made problems as pollution, sedimentation and destructive fishing practices.

These may directly kill or injure marine life, or may encourage the growth of algae that smother native species, or form algal blooms with wide-ranging effects.

Oil spills at sea can contaminate reefs, and also affect the eggs and larva of marine life drifting near the surface.

These ecosystems have been little studied, but in the perpetual darkness and cold temperatures, animals grow and mature slowly and there are relatively fewer fish worth catching than in the sunlit waters above.

To what extent deep-water coral reefs provide a safe nursery area for juvenile fish has not been established, but they may be important for many cold-water species.