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Deepwater Coral

Not all coral species live in tropical, shallow waters. There are many species of coral which exist and thrive far below the ocean's surface. Deep-water corals (DWC), like tropical corals, can be very large, three dimensional skeletal frameworks that host a variety of organisms. However, unlike tropical reefs, cold corals do not depend on light to survive and instead rely on extended polyps for food. Less is known about cold corals than tropical corals, and new cold-water corals are being discovered as more of the ocean is explored [1].


Deep water coral in the Gulf of Mexico

Deepwater corals grow in all the world’s ocean basins, including the waters of the United Kingdom, Australia, New Zealand, Canada, Ecuador, Japan, Norway, and the United States.[1] They can be found in waters as deep as 6,000m and as cold as -1ºC. [1] However, temperature is an influencing factor of coral distribution, and they are most commonly found in waters ranging from 4 to 12 C. [2] For this reason, DWC are also commonly referred to as cold water coral (CWC). Other factors influencing coral distribution include salinity and competitive interactions with other organisms like sponges and algae. [3] Cold corals, as they are also called, often are found on seamounts, ocean canyons, and continental shelves and slopes. [3] Corals require a hard surface on which to attach, which could be exposed rock, substrate, or even dead coral. They generally colonize areas where strong currents are found, as corals rely on the steady flow of water to supply them with food, disperse larvae, and to remove wastes and sediments.[3] For this reason, corals are often found specifically on seamounts due to the strong currents.


Beyond 100 m of ocean, not enough light filters through for photosynthesis to occur. [4] Due to the lack of sunlight in their environment, deep-water corals do not depend on symbiotic relationships with zooxanthallae as their shallow water counterparts do. Deep-water corals are completely heterotrophic out of necessity. Rather than relying primarily on photosynthesizing zooxanthallae for energy, DWC must filer feed through the use of extended polyps which capture food particles and microorganisms as they float by. Consequently, DWC rely heavily on ocean currents to bring them their food source and can be greatly impacted by shifts in currents [4]. However, recent research has indicated that there may be a link between chemosynthesizing bacteria and mircoorganisms of the deep sea and DWC. A 2008 study by Jensen S. et al. has uncovered the possibility of symbiotic relationships between deep-water coral and methanotropes, which are microorganisms that obtain energy from underwater methane seeps.[5]

Life Cycle

Due to the difficulties inherent in observation of deep-water coral, little is known about their life cycle. Structural similarities between deep and shallow water corals have led scientists to believe that DWC likely undergo a life cycle similar to tropical corals:

  • drifting medusa
  • attached coral polyp
  • mature coral structure
  • (rubble- the "afterlife" phase)


Leiopathes glaberrima

Leiopathes glaberrima

Black corals are the oldest known marine organisms.[6] They live in excess of 4,000 years. Black corals have dark skeletons and irregularly branching, tree-like structures. The coral polyps attach to the base of the skeleton, and their mouths are located at the other end, ringed in tentacles to catch food. .[6] Unlike tropical coral reefs, cold corals do not contain zooxanthellae within their tissues, so they must catch their food on passing currents. They are found in both the Pacific and Atlantic, though their complete distribution is unclear. .[6] Deep-water Black Coral is threatened by habitat degradation and also the trade of live corals for aquariums. .[6] While the trade is small, it is not sustainable as slow growing as black coral.

Lophelia pertusa

Lophelia pertusa

Lophelia pertusa are stony, deep-water corals mainly occurring mainly on the continental shelf and are one of the few species able to build a coral framework in the deep ocean.[2] They are rarely attached to soil substrata, but occur on soft ocean bottoms greater than 150m and even occasionally on oil industry structures where currents are strong.[7] The skeletons of Lophelia pertusa polyps occur in bush-like colonies that may join together to form a larger reef, joined together by their external calcareous skeletons. .[7] Individual polyps are white, pink, or yellowish, and contain up to 50 tentacles. Within the reef, stony corals are protected against other animals growing in the reef by a layer of mucus.[8] Even so, stony corals are a biodiversity hotspot and provide habitat to a variety of species at the edge of the continental shelf. They are threatened by bottom trawling and oil extraction. .[7] It is also though that deep, cold-water, stony corals are among the first marine organisms to be affect by ocean acidification. .[2]

Madrepora oculata

Madrepora oculata

  • Along with L. pertusa, it is one of the two main base corals of the Atlantic Ocean. Can be found at depths of 200-1,000m. [5]


Just as with tropical coral reefs, deep water corals are important for a number of reasons, mainly due to their integral role in the functioning of other systems. They provide critical ecological services that have value both to marine species and to humans. Deepwater coral reefs provide critical habitat to a disproportionate number of marine species. There is high species diversity around cold corals as well as high endemism.[3] Cold corals are essential as nurseries to deep water juvenile fish and the framework of these corals create sub-habitats for a variety of marine organisms, including both micro and macro fauna.[3] [5]

Cold corals are specifically important commercially, providing deep-sea habitats to many commercial fish species, such as groupers, snappers, and a variety of shellfish.[9] Off the Aleutian Islands, 85% of commercial fisheries are associated with cold corals.[1]

Corals are also an important resource for new medicines. Some of the chemicals that corals produce have potential to be used in fields such as oncology to create new medicines for the treatment of cancer. One such compound found in the deep-water sponge Discoderma dissolute has shown powerful anti-tumor activity against human lung and breast cancer cells. Two other deep-water sponges have been shown to contain anti-inflammatory and anti-viral properties. [1]

Another important use of coral is found in paleoclimatology. Since deep water corals have such slow growth rates (centuries) and impressive longevities (thousands of years), they provide important clues about past ocean temperatures and chemistry.[10] By using cores drilled from massive DWC species such as boulder and brain coral, climatologists can read the shifts in the earths climate through changes in band size and color- much like one would examine tree rings for indications of past storms.[1] [11]

The ecological services provided by deepwater corals are just as diverse as shallow reefs.



As in any ecosystem, deep-water coral experiences negative effects from other members of the benthic plant and animal communities. The major natural factor which negaticely impact DWC growth is bioerosion.


Swiss cheese.png
  • DWC experiences bioerosion much like shallow reefs do. However, in both areas, the main contributors are different:
  • parrotfish in shallow coral -- grazing
  • sponges in DWC- boring [12]
  • In general, grazing decreases with increasing depth, but boring of sponges does not- It may actually intensify since there are more sponges in the benthic zones of DWC reefs.


Many of the most devastating threats facing deep-water coral reefs are posed by humans. These threats stem from particular practices such as unsustainable fishing methods and also from lifestyle choices which affect the planet as a whole.

Climate Change

While global temperature fluctuation might not be as big a problem to these stoic survivors of the deep, side effects of temperature changes are an issue. Climate change on a global often has direct impacts on ocean currents. Rising sea levels along with rising temperatures can cause currents to divert from their normal direction, which can be problematic for DWC.

Since they are a stationary species, many deep-water corals depend heavily on ocean currents to bring them food particles on which to filter feed. When currents shift, a given DWC reef may not receive food particles anymore and the reef can starve. Furthermore, DWC are impacted by shifts in ocean currents even more so than shallow reef species because they do not have the option of living on energy from photosynthesis of zooxanthellae. Their entire diet is dependent on ocean currents alone.[1]

Ocean Acidification

Ocean acidification is the process by which the ocean takes in excess carbon dioxide from the atmosphere. This changes the chemistry of the seawater, making it more acidic and dangerous to many animals which rely on calcium carbonate shells. Coral are also negatively impacted by hyper-acidic ocean water. Their calcium carbonate skeletons are weakened by the higher acidity, leaving them more fragile and vulnerable to ocean currents and physical disturbances. Ocean acidity also stunts the growth of coral skeletons, which can be detrimental to DWC. Since they grow more slowly than shallow water coral (SWC), any interruption in growth requires a longer period of recovery (decades or centuries), if the coral recovers at all.[1]

Smaller-Scale Anthropogenic Threats
Other threats to DWC that are not as far-reaching include oil and gas exploration, especially in the event of oil spills, and retail sale of corals as souvenirs. The Deep Water Horizon (BP) oil spill off the Gulf of Mexico was particularly damaging to DWC and creatures that live in the sediments of the seafloor.[1]

Bottom Trawling

Bottom trawling.png

One of the greatest threats to deep-water coral is the affect of bottom trawling. Bottom trawling is a method of deep sea fishing in which a vessel drags a heavy weighted net through the water, which scrapes along the ocean floor. It has been shown to level the topography of the sea floor and destroy many deep sea habitats.[1] Bottom trawling directly affects reefs by catching coral in fishing nets and removing the coral framework. It also destroys the coral itself by shattering the fragile and easily broken skeletons. If enough coral is removed or damaged, the reef can reach a size where it is not longer sexually viable and does not have enough members to reproduce. Recruitment of larvae becomes sporadic. This is particularly significant because cold corals are slow growing and can take thousands of years to fully develop.[3] Many species of deep sea fish also grow more slowly. For example, the orange roughy (a type of deep sea perch) takes 30 years to reach sexual maturity and can live up to 125 years. Fish that mature at such slow growth rates are overwhelmed by commercial fishing- especially when fishing pressures are compounded by the destruction of their habitats. [13]

Bottom trawling has increased in recent years due to the overfishing of shallow waters. However, bottom trawling can be counterproductive in that by destroying deep-water reefs, fishermen also destroy critical habitat for many commercial fish species. The current gear used in these deep-water fisheries has shown an increased negative affect on coral species, characterized by heavy coral by-catch, in which coral polyps are easily caught in the netting and removed. However, as coral reefs have been removed, the incidence of by-catch has decreased.[3]



Legislative efforts have thus far been centered on conservation rather than restoration. This is accomplished primarily through establishing Marine Protected Areas (MPAs) and enacting restrictions on fishing equipment that lessen or ban harmful practices such as bottom trawling. Both methods have met with with some success. In 2006, the General Fisheries Commission of the Mediterranean created the first legal gategory of "Deep-sea fisheries restricted areas" with the prohibition of dredging and trawling in the DWC banks of Santa Maria.[14] Likewise, in 2013, Chile became the first country to protect all of its seamounts from bottom trawling. [1] Legislation has also allowed for significant recovery of the Darwin Mounds off of the north west coast of Scotland, which were first discovered in 1998. Subsequent studies of 1999 and 2000 found that many of the mounds were being damaged by trawling. In response, the area was closed in 2003 to all bottom contact fisheries via an emergency procedure under the EU Common Fisheries Policy.[15]

However, such responsive legislation is not usual and has its shortcomings. Current legal measures do not offer protection to coral rubble- what is left in the aftermath of most trawling zones. Recent research by Bongiorni et al suggests that coral rubble, which is generally regarded as a lost cause, can actually still sustain life and even increase species diversity by offering a habitat to higher order consumers. Also, these rubble areas could serve as buffer zones between the edges of established MPAs and areas of high fishing activity, which would give DWC systems a better chance of recovery.[14]

Challenges with Conservation

One of the largest problems with DWC conservation efforts is lack of research. This is mainly due to the inherent difficulty of studying these corals at such great depths. During the past 15 years, scientists have made use if new technologies such as Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) to carry out deep sea research and mapping. Though advancements in these technologies have allowed for more deep sea research, the open ocean floor is an area still vastly unexplored. Within this unknown territory lies the possibility of undiscovered DWC reefs. [1] [15]

Due to this lack of data, scientists are left the task of predicting the quantity of DWC in the world. Without a more exact count of existing reefs, scientists cannot know the extent of damage that human activities such as trawling are causing globally. They can only compare damaged reefs against their former selves or nearby reefs, making it difficult to prove the global impacts and urgency of DWC degradation or to recommend specific courses of action. For these reasons, legislation has so far only been targeted at specific localized sites where the damage can be clearly quantified and recored. [15] This means that only responsive legislation is being instituted. In order to put proactive and preventative legal measures in place, scientists need much more data on the location, distribution, and current states of DWC reefs.

Another challenge to legislation and conservation is that many of the DWC reefs that are known exist in international waters, on deep-water seamounts and along tectonic plate boundaries and methane seeps. [5] In these areas, only international legislation would be effective, since laws of one country would be ignored and exploited by unregulated competitors (mainly in the commercial fishing industry). However, because it requires agreement from all countries regarding a shared resource, this type of legislation is the most difficult to enact.

Further Links

Deepwater corals are comparable to trees

"The State of Deep Coral Ecosystems of the United States"

"Deep water corals may be key to restoring damaged reefs"


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