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Corals are marine invertebrates in class Anthozoa of phylum Cnidaria typically living 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 "head" is a colony of myriad genetically identical polyps. Each polyp is a spineless 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 thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.

Although some corals can catch small fish and plankton, using stinging cells on their tentacles, like those in sea anemone and jellyfish, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae called zooxanthellae that live within the coral's tissue. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft). Corals can be 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. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft). Examples live on the Darwin Mounds located north-west of Cape Wrath, Scotland. Corals have also been found off the coast of the U.S. in Washington State and the Aleutian Islands in Alaska.

In biology, an organism is any contiguous living system (such as animal, fungus, micro-organism, or plant). In at least some form, all types of organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole.

An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell.

All organisms living on Earth are divided into the eukaryotes and prokaryotes based on the presence or absence of true nuclei in their cells. The prokaryotes represent two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus, and contain additional membrane-bound compartmentalization called organelles (such as mitochondria in animals and plastids in plants, both generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms that are eukaryotes.

Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine waters cover two-thirds of the surface of the Earth. Such places are considered ecosystems because the plant life supports the animal life and vice-versa. See food chains.

Marine ecosystems are very important for the overall health of both marine and terrestrial environments. According to the World Resource Center, coastal habitats alone account for approximately 1/3 of all marine biological productivity, and estuarine ecosystems (i.e., salt marshes, seagrasses, mangrove forests) are among the most productive regions on the planet. In addition, other marine ecosystems such as coral reefs, provide food and shelter to the highest levels of marine diversity in the world.

Marine ecosystems usually have a large biodiversity and are therefore thought to have a good resistance against invasive species. However, exceptions have been observed, and the mechanisms responsible in determining the success of an invasion are not yet clear.

Underwater photography is the process of taking photographs while under water. It is usually done while scuba diving, but can be done while diving on surface supply, snorkeling, swimming, from a submersible or remotely operated underwater vehicle, or from automated cameras lowered from the surface.

Underwater photography can also be categorised as an art form and a method for recording data.

Successful underwater imaging is usually done with specialized equipment and techniques. However, it offers exciting and rare photographic opportunities. Animals such as fish and marine mammals are common subjects, but photographers also pursue shipwrecks, submerged cave systems, underwater "landscapes", invertebrates, seaweeds, geological features, and portraits of fellow divers.