The habitat of Cold-water corals, extends to deeper, darker parts of the oceans than tropical corals, ranging from near the surface to the abyss, beyond 2,000 metres (6,600 ft) where water temperatures may be as cold as 4°C. Deep water corals belong to the Phylum Cnidaria and are most often stony corals, but also include black and horny corals and soft corals including the Gorgonians (sea fans). Like tropical corals, they provide habitat to other species, but deep water coral do not require zooxanthellae to survive.
While there are nearly as many species of deep–water corals as shallow-water species, only a few deep-water species develop traditional reefs. Instead, they form aggregations called patches, banks, bioherms, massifs, thickets or groves. These aggregations are often referred to as "reefs," but differ structurally and functionally. Deep sea reefs are sometimes referred to as "mounds," which more accurately describes the large calcium carbonate skeleton that is left behind as a reef grows and corals below die off, rather than the living habitat and refuge that deep sea corals provide for fish and invertebrates. Mounds may or may not contain living deep sea reefs.
Submarine communications cables and fishing methods such as bottom trawling tend to break corals apart and destroy reefs. The deep water habitat is designated as a United Kingdom Biodiversity Action Plan habitat.