Most ocean lovers will be familiar with corals. Tropical coral reefs present an incredible palette of colour and an array of shapes; from branching and spiky to massive brain-like stony corals as well as tree-like gorgonians and the hazardous fire corals. All are alive with the movement of fish and a marvellous array of other animals like sea slugs, sea stars, lobsters, crabs, and colourful shrimp. 

It may come as a surprise that there are more species of corals in waters deeper than 50 metres than there are in the warm, shallow waters of the tropics. In the cold, dark waters of the North Atlantic, the branching white (or sometimes pink) stony coral (Desmophyllum pertusum) can form large complex reefs with estimates of over 1,800 other species living with it.

Many fish species, including commercial fished ones, live in these Atlantic cold-water coral reefs as well as a startling array of invertebrates such as sponges, worms, squat lobsters and brittle stars. In some cases, these species have evolved intimate relationships with other species such as the large, segmented worm (Eunice norvegica) which not only ferociously defends the coral (it has big jaws) but secretes a parchment tube which the corals can settle on or grow over, adding to reef complexity and in return, protecting the worm. 

The deep sea is also a habitat for coral gardens. These are dense stands of a variety of different types of coral, including gorgonians or octocorals, black corals, sea pens, and stony corals. Unlike cold-water coral reefs, which are often formed from one or two framework-building species, coral gardens can be occupied by a colourful array of these different coral species. Like their deep-reef counterparts, coral gardens can host an amazing array of other invertebrates, which use their elevated and branching forms as platforms to reach food flowing over the seafloor. They can also form essential fish habitat such as in Alaska and off the eastern coast of Canada and in some cases are used by sharks to lay their eggs.

Because they live in cold, dark water, where food supplies are quite limited, corals can be long-lived. Desmophyllum pertusum can live for more than one hundred years. There are records of some cold-water coral reefs surviving for more than 10,000 years. Individual black corals have been aged at more than 4,000 years old.

Unfortunately though, the fragility and slow growth of these corals can render them highly vulnerable to human impacts. Bottom fishing, especially trawling, has had devastating impacts on deep-sea corals, especially on habitats such as seamounts. As we learnt during the Deep-Water Horizon blow out, cold water corals can also be killed by oil pollution and the use of dispersants used to treat it. Deep-sea mining has the potential to harm corals through removal of the nodules they attach to and from the effects of plumes of sediment caused by the mining activity itself. Finally, climate change can also alter the environmental conditions of the deep sea effecting coral survival. Recovery from impacts such as deep-sea trawling has, in some localities not been seen over decades of observation.

Recovery may take hundreds of years or even millenia.