Corals are marine invertebrates in the class Anthozoa of phylum Cnidaria.

They typically live in compact colonies of many identical individual polyps. The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

A coral group is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony creates a large skeleton that is characteristic of the species.

Although some corals can catch small fish and plankton, using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues. 

These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres.

Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland.

Other corals do not rely on zooxanthellae and can live in much deeper water, with the cold-water genus lophelia surviving as deep as 3,000 metres. 

Feeding

Coral polyps also feed on a variety of small organisms, from microscopic zooplankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts.

These cells carry venom which they rapidly release in response to contact with another organism. A dormant nematocyst discharges in response to nearby prey touching the trigger (cnidocil). A flap (operculum) opens, and its stinging apparatus fires the barb into the prey. The venom is injected through the hollow filament to immobilise the prey; the tentacles then manoeuvre the prey to the mouth.

The tentacles then contract to bring the prey into the stomach. Once the prey is digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle. They can scavenge drifting organic molecules and dissolved organic molecules.

Each coral polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length.

As part of normal growth an exoskeleton is excreted near the base of each polyp. 

Over many generations, the colony creates a large skeleton that is characteristic of the species.

Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia.

Four hundred coral species, both hard corals and soft corals inhabit the reef. The majority of these spawn gametes, breeding in mass spawning events that are triggered by the rising sea temperatures of spring and summer, the lunar cycle, and the diurnal cycle. 

While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures. The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability.