It may be the last place you’d expect to find corals—up to 6,000 m (20,000 ft) below the ocean’s surface, where the water is icy cold and the light dim or absent. Yet believe it or not, lush coral gardens thrive here. In fact, scientists have discovered nearly as many species of deep-sea corals (also known as cold-water corals) as shallow-water species.
Like shallow-water corals, deep-sea corals may exist as individual coral polyps, as diversely-shaped colonies containing many polyps of the same species, and as reefs with many colonies made up of one or more species. Unlike shallow-water corals, however, deep-sea corals don’t need sunlight. They obtain the energy and nutrients they need to survive by trapping tiny organisms in passing currents.
Within the last 20 years scientists, aided by technological advances, have uncovered one surprise after another about deep-sea corals.
Corals Without Sunlight?
Around the World
Because they don’t depend on warm water or sunlight, deep-sea corals are able to live in many different places around the world. They are far more widely distributed than scientists previously imagined—living even in waters as cold as -1ºC (30.2ºF).
For example, deep-sea corals occur in the waters of the United Kingdom, Australia, New Zealand, Canada, Ecuador, Japan, Norway, and the United States. Scientists have even found deep-sea corals off the coast of Antarctica. They grow in all the world’s ocean basins, where they form deep-water havens on continental shelves and slopes, in ocean canyons, and on tall seamounts.
How many Species?
For a long time, because deep-sea corals were so inaccessible, no one had any idea how many species existed. Now, by studying specimens collected on research expeditions, ocean scientists are starting to come up with a count. They study the corals’ shape and form (morphology) to identify species that are known to be found in other locations and species that are new to science. Because so many species of deep-sea corals look alike, marine researchers may do DNA testing to confirm the results.
So, how many species of deep-sea corals are there? It is still too soon to say because new species are continually being discovered. To date, however, more than 3,300 species of deep-sea corals have been identified. And the numbers keep climbing.
Oldest Organisms in the Sea?
Not only are deep-sea corals more diverse than ocean scientists ever imagined, they are also amazingly old. According to scientific estimates, one particular colony of gold coral (Gerardia sp.) found off the coast of Hawaii was about 2,742 years old. Marine researchers determined that another deep-sea coral colony in Hawaii—this one a black coral (Leiopathes sp.)—was about 4,265 years old. These coral colonies are the oldest marine organisms on record.
Due to the continuous regeneration of new polyps, some deep-sea coral reefs have been actively growing for as long as 40,000 years. And there may very well be even older deep-sea coral reefs or colonies out there—in Hawaii or elsewhere.
Diversity & Natural History
Deep-sea corals come in a virtual paint box of colors—yellow, orange, red, purple, and more. Their shapes are equally varied and include branching, fan-shaped, and feather-shaped forms, to name a few.
When it comes to size, the range among deep-sea corals is tremendous. Scientists have discovered single polyps as small as a grain of rice, tree-like coral colonies that tower as tall as 10 m (35 ft), and massive coral reefs that stretch for 40 km (25 mi). But the ocean is a vast realm. There may be even bigger deep-sea corals out there still to be discovered.
Homes for Other Creatures
In the ocean’s vast expanse, deep-sea corals provide a precious commodity—habitats for marine life. Invertebrates like worms, starfish, and lobsters as well as vertebrates like fishes depend on deep-sea corals. The corals offer food, places to hide from predators, nurseries for juveniles, and a solid surface where invertebrates can take hold.
Among the diverse species that depend on deep-sea corals are ones that are commercially important to humans—including shrimp, crabs, groupers, rockfish, and snappers. Off Alaska’s Aleutian Islands, 85 percent of commercially important fish species are associated with deep-sea coral habitats.
Help for Humans
Some organisms that live in deep-sea coral habitats produce chemicals that have enormous potential for use as new medicines. For example, scientists recently discovered that two sponges that grow in deep-sea coral ecosystems have compounds with anti-inflammatory and anti-viral properties. A compound from another deep-water sponge, Discodermia dissoluta, displays potent anti-tumor activity against human lung and breast cancer cells. And who knows what other potential lifesaving compounds lie within coral reefs deep below the ocean’s surface?
Because deep-sea corals are found worldwide and live for so long, their skeletons provide important clues to the history of Earth’s climate. As deep-sea corals grow, they form layers or bands—similar to tree rings. The chemical composition of the bands reflects the changing ocean conditions under which the corals formed. By measuring and examining the thickness of each band, marine scientists can estimate how much the corals grew during a given time period. This information sheds light on what ocean conditions existed during that period. By conducting more complex analyses of deep-sea corals, ocean scientists can gather valuable information about changes in water temperature, nutrients, and ocean circulation over time.
Technology & Exploration
Using a technique known as multibeam sonar, ocean scientists working at the surface can now create 3-D maps of the ocean floor below. These detailed images enable scientists to locate areas where deep-sea corals might be found and to learn more about the conditions under which these corals live.
The scientists use an instrument called a multibeam echosounder, which is encased in a watertight shell mounted to the hull of a ship or underwater vehicle. When activated, it pulses sound into the water. The time it takes for the sound to travel to the ocean floor and return is interpreted as depth. Computer software reads the data and turns it into a colorful topographic map showing the contours of the ocean floor.
Exploring the Ocean's Deep Realms
A new generation of research vessels and underwater vehicles has empowered ocean scientists to gather information about even the most remote deep-sea coral habitats.
Some underwater vehicles, like the Pisces V submersible, transport scientists themselves to the ocean depths, where they can observe deep-sea corals firsthand. Remotely Operated Vehicles (ROVs), like the one seen here, and Autonomous Underwater Vehicles (AUVs) let scientists see and study those places they can’t normally reach. Specialized underwater cameras capture close-up images of deep-sea coral ecosystems, and finely tuned robotic arms collect samples that are identified and preserved for additional study.
Deep-sea corals may grow in remote locations. But they are not beyond the reach of human activities such as fishing. In fact, fishing efforts have increasingly focused on deeper habitats, intensifying the impact on deep-sea corals. Damage from fishing gear, especially equipment that trawls the sea bottom, poses a major threat to deep-sea corals. Decades of bottom trawling has been shown to actually smooth out the sea-floor, changing the ecosystem entirely, in addition to destroying deep-sea coral.
Oil and gas exploration and development also lead to coral reef destruction, particularly when accidents occur. Deep-sea coral were damaged by the Deep Water Horizon oil spill in the Gulf of Mexico. Climate change and, especially, ocean acidification, are additional problems. Ocean acidification occurs when the ocean absorbs excess carbon dioxide in the air, changing the ocean’s chemistry and causing slower growth and weaker skeletons in corals.
Because deep-sea corals grow so slowly, they can take decades or even centuries to recover from threats like these—if they recover at all.
How You Can Help
More and more people are beginning to recognize the critical importance of preserving deep-sea corals. In waters under national jurisdiction as well as in the high seas, these fragile ecosystems are beginning to receive some attention.
One effective way of protecting deep-sea coral habitats from physical damage caused by fishing gear is by establishing marine protected areas (MPAs) that restrict use of specific fishing gear types. Several such areas have already been created in Alaska and off the west and southeastern coasts of the continental United States. In 2013, Chile became the first country to protect all seamounts from bottom trawling. Unfortunately, however, MPAs cannot protect deep-sea corals from the damage caused by environmental changes such as ocean acidification.
There are many important reasons to preserve deep-sea corals—including the rich biodiversity of the habitats they create, their critical ecological role, and the valuable history of Earth’s ancient climate recorded within them. If you want to contribute to the effort to protect these unique and fragile habitats, here are some things you can do.
●Don’t buy jewelry or curios made from deep-sea corals.
●Make seafood choices that support wise and sustainable fishing practices.
●Reduce the amount of carbon you add to the air.
●Support organizations that are working for coral protection.
Explore the deep-sea corals page with a class, then dive into one of these classroom activities, each corresponding with sections on this page.
Coral reefs are vibrant ecosystems teeming with color and life. Most grow in the warm sunlit waters of tropical seas. Beautiful and accessible, shallow water corals are beloved by the public and well known to scientists. In contrast, deep-sea corals are generally unknown and unappreciated. Living in the icy darkness of the abyss, these creatures are difficult and expensive to study. For most of the 20th century, deep-sea corals could only be studied when fishermen accidentally pulled a broken specimen to the surface. In the last few years, scientists have used sophisticated submersible and underwater sensing technologies to penetrate the corals’ remote realm.
Grade Level: 7-8
Focus: Biology and Ecology ofLopheliaCorals (Life Science)
Description: Students will describe the general biology and morphology of Lophelia corals, explain how these corals contribute to the development of complex communities, identify ways in which these corals are threatened by human activities, and discuss ways in which Lophelia communities are important to humans.
More information on our corals and coral reefs page.
NOAA's Coral Reef Information System (CoRIS).
Information on cold water coral reef systems around the world.
Book: Cold-Water Corals. Murray, J.M. and A. Wheeler, A. Freiwald and S. Cairns. New York: Cambridge University Press, 2009.