Cold-water corals belong to the Phylum Cnidaria within the Class Anthozoa, five major taxa of these corals include:
The Octocarallia are soft corals including Gorgonian sea fans, Precious coral and Bamboo coral (Roberts et al. 2006). There are estimated to be 2325 species within Octocarallia the deepest occurring at 6620 metres spread from the Arctic to Antarctic. They can form huge colonies, an example of this being Paragorgia arborea found in New Zealand reaching 10 metres in height (Roberts et al. 2009). They posses calcium carbonate scelerities inside their polyps and have a flexible internalbranch axis. All are colonial except a solitary genus Taiaroa found only in New Zealand (Roberts et al. 2009). The colonies of soft corals provide a habitat for lots of associated epifauna including invertebrates like Crustacea. They are gonochoric and use both external and internal fertilisation (Roberts et al. 2009).
Scleractinia are stony corals: soft coral polyps encrusted by a calcium carbonate shell. 622 out of a total of 711 azooxanthellae scleractinian corals occur in water deeper than 50 metres (Roberts et al. 2009). They occur from Antarctica to the Bering Sea but are absent from high Arctic regions (Roberts et al. 2009). They can occur at depths of 6328 metres as discovered in the Aleutian Trench (Keller 1976) but the majority live on continental slopes between 200-1000 metres (Roberts et al. 2009). Twenty-six percent of scleractinian corals are colonial reef building species, the remainder are solitary corals (Cairns 2007). Most of the framework building species occur in the family Caryophylliidae under which common species include Lophelia pertusa and Solenosmila variabilis is often found associated with Lophelia. Lophelia pertusa produces bush like colonies consisting of thousands of polyps creating a large reef framework (Roberts et al. 2009).
Antipatharia are the Black corals; they have been found at depths of 8600 metres distributed off Antarctica, the Arctic circle and Greenland. They form branching bushy colonies and unbranched whip like structures (Roberts et al. 2009). Leiopathesis is an example of a species with high longevity within this group of corals, living for over 2000 years (Roberts et al. 2009). They form colonies that are united by a fleshy coenosarc with spicules or embedded sand particles to build structures.
Stylasteridae are not key species in forming deep water reef frameworks but they do contribute to structure with large Calcium Carbonate skeletons. They are found occurring in Antarctica, the Beiring Sea and Northern Iceland. They occur in deep water, the deepest found at 2789 metres (Roberts et al. 2009). They are colonial corals supported by colorful calcite or argonitic skeltons (Cairns & MacIntyre 1992). Most are gonochoric with internal fertilisation, larvae develop with ampullarstructures embedded in the skeleton surface and are released into the water column through an efferent pore on the parent polyp (Roberts et al. 2009).