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Deep Water Corals

Many people are familiar with the coral reefs that thrive in shallow, well lighted, clear tropical waters where myriad colorful hard and soft corals provide habitat "infrastructure" for numerous invertebrates and fishes. The corals provide protection and cover, sources of nutrition, and sites for reproduction. Corals, however, also grow in the deep, cold sea. Although the existence of some of these deep-sea coral thickets has been known for several centuries, initially from pieces of broken corals brought up with fishing gear, scientists know little about their distribution, biology, behavior, and function as essential habitats for fishes and invertebrates.

Some deep-water corals (also called cold-water corals) do not form reefs exactly like those in tropical waters. Often, they form colonial aggregations called patches, mounds, banks, bioherms, massifs, thickets or groves. These aggregations are often still referred to as “reefs.” While there are nearly as many species of deep–water corals as there are shallow-water species, only a few deep-water species develop “reefs.” Deep-water corals also provide crucial habitat and reproductive grounds for commercially important fisheries including sea bass, snapper, porgy, rock shrimp and calico shrimp, thus drawing the commercial fishing industry to these fragile areas.

Human activities constitute the most serious threat to these fragile corals. Destructive bottom fishing, as well as oil and gas exploration and exploitation have the potential to destroy large areas of coral habitat in a relatively short time. These activities create coral rubble, which is not a suitable habitat for fishes and invertebrates. In recent years, scientists have begun to study deep-water corals more closely, and some countries with deep-water corals in their territorial waters have begun to implement fishing restrictions in sensitive coral areas.

What are Deep-water Corals?

thicket of paragorgia corals
This thicket of Paragorgia corals was viewed by the deep-sea submersible Alvin at 1,043 m depth. Photo: Barbara Hecker

Three main groups of corals make up deep-water coral communities: hard (stony) corals of the Order Scleractinia, which form hard, ahermatypic reefs; black and horny corals of the Order Antipatharia; and soft corals of the order Alcyonacea, which includes the gorgonians (sea fans) (Williams, 2001). Deep-water corals are similar in some ways to the more familiar corals of shallow, tropical seas. Like their tropical equivalents, the hard corals develop sizeable reef structures that host rich and varied invertebrate and fish fauna. However, unlike their tropical cousins, which are typically found in waters above 70m depth and at temperatures between 23° and 29° C, deep-water corals live at depths just beneath the surface to the abyss (2000 m), where water temperatures may be as cold as 4° C and utter darkness prevails.

At these depths, corals lack zooxanthellae. These symbiotic algae provide food for many shallow-water corals through photosynthesis. They also assist in the formation of the calcareous skeleton, and give most tropical corals their coloration. By contrast, the polyps of deep-water corals appear to be suspension feeders. They capture and consume organic detritus and plankton that are transported by strong, deep-sea currents. These corals are commonly found along bathymetric highs such as seamounts, ridges, pinnacles and mounds (Southampton Oceanography Centre).

Deep-water corals range in size from small solitary colonies to large, branching tree-like structures, which appear as oases of teeming life surrounded by more barren bathymetry. The gorgonians (sea fans) also range from small individuals to those with tree-like dimensions. The gorgonian, Paragorgia arborea, may grow in excess of three meters in length (Watling, 2001). Growth rates of branching deep-water coral species, such as Lophelia and Oculina, range from ~ 1.0 - 2.5 cm/yr, whereas branching shallow-water corals, such as Acropora, may exceed 10-20 cm/yr. Using coral age-dating methods, scientists have estimated that some living deep-water corals date back at least 10,000 years (Mayer, 2001). However, little is known of their basic biology, including how they feed or their methods and timing of reproduction.

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General Distribution of Cold-water Corals

Deep-water corals are found globally, from coastal Antarctica to the Arctic Circle. In northern Atlantic waters, the principal coral species that contribute to reef formation are Lophelia pertusa, Oculina varicosa, Madrepora oculata, Desmophyllum cristagalli, Enallopsammia rostrata, Solenosmilia variabilis, and Goniocorella dumosa. Four of those genera (Lophelia, Desmophyllum, Solenosmilia, and Goniocorella) constitute the majority of known deep-water coral banks at depths of 400 to 700 m (Cairns and Stanley, 1982).

Madrepora oculata occurs as deep as 2,020 m and is one of a dozen species that occur globally and in all oceans, including the Subantarctic (Cairns, 1982). Colonies of Enallopsammia contribute to the framework of deep-water coral banks found at depths of 600 to 800 m in the Straits of Florida (Cairns and Stanley, 1982).

Two Important Deep-water Corals

Two of the more significant deep-sea coral species are Lophelia pertusa and Oculina varicosa. These species form extensive deep-water communities that attract commercially important species of fishes, making them susceptible to destructive bottom trawling practices (Reed, 2002a). Increased sedimentation places additional stress on corals.

Oil and gas exploration structures and activities, particularly in the North Sea and adjacent areas, also damage Lophelia communities. Subsequent oil and gas production activities may also introduce noxious substances into these areas.

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Lophelia pertusa Distribution

Global distribution of Lophelia pertusa corals.
Global distribution of Lophelia pertusa. Image: Southampton Oceanography Centre, UK.

Lophelia pertusa is the most common aggregate-forming deep-water coral. Typically, it is found at depths between 200 and 1,000 m in the northeast Atlantic, the Mediterranean Sea, along the mid-Atlantic Ridge, the West African and Brazilian coasts, and along the eastern shores of North America (e.g., Nova Scotia, Blake Plateau, Florida Straits, Gulf of Mexico) as well as in parts of the Indian and Pacific Oceans. Like tropical coral reefs, Lophelia communities support diverse marine life, such as sponges, polychaete worms, mollusks, crustaceans, brittle stars, starfish, sea urchins, bryozoans, sea spiders, fishes, and many other vertebrate and invertebrate species.

Lophelia has been found most frequently on the northern European continental shelves between 200 and 1000 m, where temperatures range from 4° to 12°C, but it has also been found at depths greater than 2,000 m. Once a colonial patch is established, it can spread over a broad area by growing on dead and broken pieces of coral (coral rubble). Lophelia has a linear extension of the polyps of about 10 mm per year. The reef structure has been estimated to grow about 1 mm per year (Fossa, 2002). Scientists have also found Lophelia colonies on oil installations in the North Sea (Bell and Smith, 1999).

Lophelia pertusa can occur in a variety of structures and forms. DNA-based sequencing tests conducted at the University of Southampton Oceanography Centre, UK, have indicated that different morphological varieties of Lophelia all belong to the same species (Rogers et al., on-line).

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Noted Lophelia Areas

The world's largest known deep-water Lophelia coral complex is the Røst Reef. It lies in depths between 300 and 400 m west of Røst Island in the Lofoten archipelago, Norway. Discovered during a routine survey in May 2002, the reef is still largely intact. It covers an area approximately 40 km long by 3 km wide.

Relatively close by is the Sula Reef, located on the Sula Ridge, west of Trondheim on the mid-Norwegian Shelf, at 200 to 300 m depth. It is estimated to be 13 km long, 700 m wide, and up to 35 m high (Bellona Foundation, 2001), an area one-tenth the size of the 100 km2 Røst Reef.

Discovered and mapped in 2002, Norway's 1,000-year-old and 2-km-long Tisler Reef lies in the Skagerrak -- the submarine border between Norway and Sweden at a depth of 74 to 155 m. The Tisler Reef contains the world’s only known yellow Lophelia pertusa corals. At present, Norway is the only European nation to enact laws protecting its Lophelia reefs from trawling, pollution, and oil and gas exploration. However, the European Commission has introduced an interim ban on trawling in the Darwin Mounds area, west of Scotland, in August 2003. A permanent ban on trawling is expected to follow.

Elsewhere in the northeastern Atlantic, Lophelia is found around the Faroe Islands, an island group between the Norwegian Sea and the Northeast Atlantic Ocean. At depths from 200 to 500 m, Lophelia is chiefly on the Rockall Bank and on the shelf break north and west of Scotland (Tyler-Walters, 2003).

One of the most researched deep-water coral areas in the United Kingdom are the Darwin Mounds. The mounds were discovered in 1998 by the Atlantic Frontier Environmental Network (AFEN) while conducting large-scale regional surveys of the sea floor north of Scotland. They discovered two areas of hundreds of sand and cold-water coral mounds at depths of about 1,000 m, in the northeast corner of the Rockall Trough, approximately 185 km northwest of the northwest tip of Scotland. Named after the research vessel Charles Darwin, the Darwin Mounds have been extensively mapped using low-frequency side-scan sonar. They cover an area of approximately 100 km2 and consist of two main fields -- the Darwin Mounds East, with about 75 mounds, and the Darwin Mounds West, with about 150 mounds. Other mounds are scattered in adjacent areas. Each mound is about 100 m in diameter and 5 m high.

 Two images of the deep sea coral Lophelia pertusa
Two images of the deep sea coral Lophelia pertusa. The image on the left shows the complex growth structure of a small colony. On the right is a closeup of individual polyps. Photo: John Reed, 2002a.

The tops of the mounds are covered with Lophelia corals and coral rubble, which attract other marine life. Side-scan sonar images show that the mounds appear to be sand volcanoes, each with a unique feature -- a “tail.” The tails are up to several hundred m long, all oriented downstream (ICES, 2001a). The tails and the mounds are uniquely characterized by large congregations of deep-sea organisms called xenophyophores (Syringammina fragilissima), which are giant unicellular organisms that can grow up to 25 cm in diameter (ICES, 2002). Scientists are uncertain why these interesting organisms congregate in these areas. In addition, the Lophelia of the Darwin Mounds are growing on sand rather than hard substrate, an unusual condition unique to this area. Usually, coral larvae almost always settle and grow on hard substrates, such as dead coral skeletons or rock.

Lophelia corals exist in Irish waters as well (Rogers, 1999). The Porcupine Seabight, the southern end of the Rockall Bank, and the shelf to the northwest of Donegal all exhibit large, mound-like Lophelia structures. One of them, the Theresa Mound, is particularly noted for its Lophelia pertusa and Madrepora oculata colonies. Lophelia reefs are also found along the U.S. East Coast at depths of 500 to 850 m along the base of the Florida-Hatteras slope. South of Cape Lookout, NC, rising from the flat sea bed of the Blake Plateau, is a band of ridges capped with thickets of Lophelia. These are the northernmost East Coast Lophelia pertusa growths. The coral mounds and ridges here rise as much as 150 m from the plateau plain. These Lophelia communities lie in unprotected areas of potential oil and gas exploration and cable-laying operations, rendering them vulnerable to future threats (Sulak and Ross, 2001).

Finally, Lophelia is known to exist around the Bay of Biscay, the Canary Islands, Portugal, Madeira, the Azores, and the western basin of the Mediterranean Sea. (ICES, 2001a).

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Location of the Oculina Banks Coral Reef Reserve
Chart of the Deep-water Oculina Coral Banks Marine Protected Area (MPA). The shaded area is the entire Oculina Habitat Area of Particular Concern (HAPC); the Experimental Oculina Research Reserve (EORR) is the smaller inset box. Recent dive sites from 2001 to 2003 include: 1) Cape Canaveral, 2) Cocoa Beach, 3) Eau Gallie, 4) Malabar, 5) Sebastian, and 6) Chapmans Lumps/ Jeff's Reef. (Courtesy of: Dr. John K. Reed, Harbor Branch Oceanographic Institution)

Oculina varicosa Distribution

Oculina varicosa is a branching ivory coral that forms giant but slow-growing, bushy thickets on pinnacles up to 30 m in height. The Oculina Banks, so named because they consist mostly of Oculina varicosa, exist in 50 to 100 m of water along the continental shelf edge about 26 to 50 km off of Florida's central east coast.

Discovered in 1975 by scientists from the Harbor Branch Oceanographic Institution conducting surveys of the continental shelf,Oculina thickets grow on a series of pinnacles and ridges extending from Fort Pierce to Daytona, Florida (Avent et al, 1977; Reed, 1981; Reed, 2000a,b).

Like the Lophelia thickets, the Oculina Banks host a wide array of macroinvertebrates and fishes. They also are significant spawning grounds for commercially important species of food fishes including gag, scamp, red grouper, speckled hind, black sea bass, red porgy, rock shrimp, and the calico scallop (Koenig et al., on-line).

Threats to Lophelia and Oculina Corals

Both Lophelia and Oculina corals face uncertain futures. Until recently, Lophelia habitats remained undisturbed by human activity. However, as traditional fish stocks are depleted from northern European waters, bottom trawling has moved into deeper waters, where the gear has affected the coral beds. The towed nets break up the reef structure, kill the coral polyps and expose the reef to sediment by altering the hydrodynamic and sedimentary processes around the reef (Hall-Spencer et al., 2002).

The fishes and invertebrates that depend on the coral structure lose their habitat and move out of the area. Damage may range from a decrease in the size of the coral habitat with a corresponding decrease in the abundance and biodiversity of the associated invertebrate and fish species, to the complete destruction of the coral habitat. The trawls also may resuspend sediments that, in turn, may smother corals growing downstream of the current. In addition, oil and gas exploration and extraction operations have begun to move into these deep-water areas, further threatening the resident corals.

Two images of the deep sea coral Oculina varicosa
Two images of the deep-sea coral Oculina varicosa. The image on the left shows the complex growth structure of a small colony. On the right is a closeup of an individual branch. Photo: John Reed, 2002a.

Scientists estimate that within the Norwegian Exclusive Economic Zone, 30 to 50 percent of the total coral area of the Norwegian shelf has been damaged or destroyed by trawling (Fossa, 2002). Scientists from the International Council for the Exploration of the Sea, the main provider of scientific advice to the European Commission on fisheries and environmental issues in the northeast Atlantic, have recommended that to protect the remaining deep-water coral groves, all of Europe’s deep corals must be accurately mapped and then closed to fishing trawlers (ICES, 2001b).

In 1999, the first complete mapping of the Sula Reef was carried out by the Norwegian Hydrographic Society, which used the latest available multibeam echosounder equipment to record both depth and backscatter data. The mapping was the product of joint cooperation between the Geological Survey of Norway and the Institute of Marine Research. That same year, the Norwegian Ministry of Fisheries issued regulations for the protection of the Lophelia reefs. An area of 1,000 km2 at Sula, including the large reef, is now closed to bottom trawling. In 2000, an additional area, about 600 km2 was closed. The Røst Reef, an area of about 300 km2, was closed to bottom trawling in 2002.

Florida's Oculina Banks, once teeming with commercially important fish, now appear to be severely depleted of fish stocks (MPA, 2002). Much of the Oculina coral has been reduced to rubble, probably the result of a combination of destructive bottom trawling and natural causes like bioerosion and episodic die-offs. Regardless of the cause, the Oculina Banks now support a drastically reduced fishery because most of the significant spawning grounds have been destroyed (Reed, 2000a,b).

An beautiful and unusual species of Lophelia coral
This beautiful species of Madrepora oculata coral was collected off the coast of South Carolina.

Efforts to protect the Oculina Banks began in 1980, when scientists from Harbor Branch Oceanographic Institution initiated a call to implement protective measures for the area.  In 1984, the South Atlantic Fishery Management Council established the 92-square mile Oculina Bank Habitat Area of Particular Concern (HAPC) in order to protect the fragile coral from damage by bottom-tending fishing gear, including bottom trawls, bottom longlines, dredges, and fish traps. Subsequent management measures provided further protection by prohibiting anchoring, trawling for rock shrimp, and by requiring vessel monitoring systems (VMS) on rock shrimp vessels.

In 1994, the 92-square mile OHAPC was declared the Oculina Experimental Closed Area, and possession of or fishing for snapper/grouper species was prohibited for a period of 10 years to allow for scientific studies. Having an area where deepwater species such as snowy grouper and speckled hind can grow and reproduce without fishing pressure provides a unique opportunity for study. The SAFMC has extended the closure indefinitely with periodic review for further protection and research.  The former Oculina Bank HAPC was expanded in 2000 to include an additional 300 square miles.

The Oculina Banks remain a hot spot for research and efforts to rehabilitate the coral (MPA, 2002). Scientists recently deployed concrete “reef balls” in the area in an attempt to attract fish and provide substrate for coral attachment, settlement and growth. They are cautiously optimistic about their initial restorative efforts in the reserve (MPA, 2002).

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References and Additional Readings

Atlantic Coral Ecosystem Study (ACES), 2000. Consortium of European scientists completed an environmental assessment of the status of Europe’s deep-water corals, available on-line at: http://www.cool-corals.de

Avent, R.M., M.E. King, and R.M. Gore. 1977. Topographic and faunal studies of shelf-edge prominences off the central eastern Florida coast. Int. Revue ges.Hydrobiol. 62:185-208.

Bell, N. and J. Smith. 1999. Coral growing on North Sea oil rigs. Nature 402:601.

Bellona Foundation. 2001. Coral reefs in Norwegian Waters. Available on-line at http://www.bellona.no/en/b3/biodiversity/
coral_reefs/12784.html.


Cairns, S.D. 1982. Antarctic and Subantarctic Scleractinia. Antarctic Research Series 34: 74 pp.

Cairns, S. and G. Stanley. 1982. Ahermatypic coral banks: Living and fossil counterparts. Proceedings of the Fourth International Coral Reef Symposium, Manila (1981), 1: 611-618.

Fossa, J.H., P.B. Mortensen, and D.M. Furevic. 2002. The deep-water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts. Hydrobiologia 417: 1-12. Available on-line at: http://www.imr.no/dokumenter/fossa.pdf

Hall-Spencer, J. et al. 2002. Trawling damage to Northeast Atlantic ancient coral reefs. Proc. R. Soc. London B.

Hoskin, C.M., J.K. Reed, and D.H. Mook. 1987. Sediments from a living shelf-edge reef and adjacent area off central eastern Florida. Pp. 42-77, In: F, JMR,Maurrasse (ed.), Symposium on south Florida geology. Miami Geological Society. Memoirs 3.

International Council for Exploration of the Sea (ICES). 2001a. Marine World, article on deep-sea coral. Available on-line at http://www.ices.dk/marineworld/deepseacoral.asp

International Council for Exploration of the Sea (ICES). 2001b. Close Europe’s cold-water coral reefs to fishing. Available on-line at http://www.ices.dk/aboutus/pressrelease/coral.asp

International Council for Exploration of the Sea (ICES). 2001c: Extract of the report of the Advisory Committee on Ecosystems 2002 on identification of areas where cold-water corals may be affected by fishing.

International Council for Exploration of the Sea (ICES). 2002. Advisory Committee on Ecosystems. Report of the study group on mapping the occurrence of cold-water corals: 1-17.

Koenig, C., C.F. Coleman, and S. Brooke. Coral restoration in the Experimental Oculina Research Reserve of the South Atlantic. Available on-line at
http://www.bio.fsu.edu/coleman_lab/oculina_research.php

Marine Protected Areas of the United States (MPA). 2002. Experimental Oculina Research Reserve. Available on-line at http://mpa.gov/

Mayer, T. 2001. 2000 Years Under the Sea. Available on-line at http://exn.ca/Stories/2001/10/16/52.asp

National Oceanic and Atmospheric Administration. 2000. Final Rule, Amendment 4 to the Fishery Management Plan for Coral, Coral Reefs, and Live/Hard Bottom Habitats of the South Atlantic Regions (Coral FMP). Federal Register, Vol. 65, No. 115, June 14, 2000.

National Oceanic and Atmospheric Administration. 1982. Fishery Management Plan for Coral and Coral Reefs of the Gulf of Mexico and South Atlantic. Gulf of Mexico and South Atlantic Fishery Management Councils.

Reed, J.K. 1981. In Situ growth rates of the scleractinian coral Oculina varicosa occurring with zooxanthellae on 6-m reefs and without on 80-m banks. pp. 201-206. in: W.J. Richards (ed.), Proceedings of Marine Recreational Fisheries Symposium.

Reed, J.K. 2002a. Comparison of deep-water coral reefs and lithoherms off southeastern U.S.A. Hydrobiologia 471: 43-55.

Reed, J.K. 2002b. Deep-water Oculina reefs of Florida: biology, impacts and management. Hydrobiologia 471:43-55.

Reed, J.K. and C.M. Hoskin. 1984. Studies of geological and biological processes at the shelf-edge with use of submersibles. In: Undersea research and technology – Scientific application and future needs – abstracts, Symposia Series for Undersea Research, Vol. 2, NOAA.

Reed, J.K., R.H. Gore, L.E. Scotto and K.A. Wilson. 1982. Community composition, structure,aerial and trophic relationships of decapods associated with shallow and deep-water Oculina varicosa coral reefs. Bull. Mar. Sci. 32:761-786.

Rogers, A.D. 1999. The biology of Lophelia pertusa (Linnaeus 1758) and other deep-water reef-forming corals and impacts from human activities. International Review of Hydrobiology 84: 315–406.

Rogers, A., M. Le Goff, and B. Stockley. Ecology of deep-water stoney corals (the Darwin Mounds). Available on-line at http://www.soton.ac.uk/~merg/corals.htm

Southampton Oceanographic Centre. Coral ecosystems in deep water. Available on-line at
http://www.soc.soton.ac.uk/GDD/DEEPSEAS/
coralecosystems.html


Sulak, K. and S. Ross. 2001. A profile of the Lophelia reefs. Available on the NOAA Ocean Explorer Web site at http://oceanexplorer.noaa.gov/explorations/islands01/
background/islands/sup10_lophelia.html
.

Tyler-Walters, H. 2003. Lophelia reefs. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme. Plymouth: Marine Biological Association of the United Kingdom. Available on-line at http://www.marlin.ac.uk/biotopes/bio_basicinfo_COR.Lop.htm


Watling, L. 2001. Deep sea coral. Available on the NOAA Ocean Explorer Web site at http://oceanexplorer.noaa.gov/explorations/deepeast01/
background/corals/corals.html
.


Williams, G.C. 2001. Octocoral Research Center Web Site. Available on-line at
http://www.calacademy.org/research/
izg/orc_home.html


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