Shallow-water coral bank in Chilean fjords, photo credit: Vreni Häussermann & G. Försterra

Cold- or deep-water corals build and maintain an ecosystem where the water is icy cold and the light dim or absent.

These corals and gorgonians are so-called ecosystem engineers, the 3-dimensional structures they build are called marine animal forests. Here many invertebrates find habitat, protection and food.

Exploring the Enigmatic World of Cold-Water Corals.

In the depths of the ocean, where the water is frigid and light barely penetrates, lies an unexpected wonderland of corals. Surprisingly, vibrant coral gardens thrive in these extreme conditions, reaching depths of up to 6,000 meters. Contrary to popular belief, the majority of coral reefs aren’t nestled in tropical shallows but rather in the deeper, colder realms of the ocean (1). These lesser-explored ecosystems, known as cold-water coral reefs (2, 3, 4.), have garnered increasing attention in recent years due to their fascinating biodiversity and their important role as nursery for many economically important fish. However, these ecosystems face numerous threats such as bottom trawling and ocean acidification caused by climate change.

Cold-water coral reef in Norway, photo credit: ROV team, GEOMAR.

Deep-sea corals and feather stars in 2.669 m off the coast of Florida, photo credit: NOAA.

Coral garden in 165 m depth off the coast of Alaska’s Aleutian Islands, photo credit: A. Lindner/NOAA.

Deep-sea coral garden off the west coast of Florida, photo credit: National Undersea Research Center at the University of North Carolina

Cold-water coral bank in a Northern Chilean fjord, 25 m, photo credit: Vreni Häussermann & G. Försterra.

Gorgonian coral in 28 m depth in a Central Patagonian channel, photo credit: G. Försterra.

Bottle-brush gorgonians in a Central Patagonian channel, 30 m, photo credit: Vreni Häussermann.

Hydrocoral reef in a channel with strong current in Madre de Dios Island, Central Patagonia, 25 m, photo credit: G. Försterra.

Unveiling the Secrets of Deep-Sea Animals

While the depths of the ocean pose challenges for research, some regions offer glimpses into these mysterious worlds. Fjords, such as those in Chilean Patagonia, provide unique opportunities to study cold-water coral communities up close. In some fjords of Northern Patagonia, corals that were previously only known from the continental slope have been discovered thriving at diving depths, adding to our understanding of these enigmatic ecosystems (5).

Unveiling the Secrets of Deep-Sea Animals

While the depths of the ocean pose challenges for research, some regions offer glimpses into these mysterious worlds. Fjords, such as those in Chilean Patagonia, provide unique opportunities to study cold-water coral communities up close. In some fjords of Northern Patagonia, corals that were previously only known from the continental slope have been discovered thriving at diving depths, adding to our understanding of these enigmatic ecosystems (5).

Cold-water coral bank in a Northern Chilean fjord, 25 m, photo credit: Vreni Häussermann & G. Försterra.

Gorgonian coral in 28 m depth in a Central Patagonian channel, photo credit: G. Försterra.

Bottle-brush gorgonians in a Central Patagonian channel, 30 m, photo credit: Vreni Häussermann.

Hydrocoral reef in a channel with strong current in Madre de Dios Island, Central Patagonia, 25 m, photo credit: G. Försterra.

Surviving in Extreme Environments

Corals are animals. Like shallow-water corals, deep-sea corals may exist as individual coral polyps, or as colonies containing many polyps of the same species. Many individuals or colonies can make up coral banks or coral reefs. Individual polyps are much larger than those of their tropical counterpart and can grow up to more than 40 cm in length and 7 cm in diameter. They live in water as cold as -1ºC and depth down to 6000 m. Unlike their shallow-water counterparts, deep-sea corals don’t rely on symbiotic relationships with algae which need sunlight for survival. Instead, they derive their energy and nutrients from capturing tiny organisms carried by ocean currents. Over the past two decades, advancements in technology have allowed

The Chilean fjord coral, Desmophyllum dianthus stained by endolithic algae, photo credit: Vreni Häussermann & G. Försterra.

Colony of Lophelia pertusa (Desmophyllum pertusum) coral polyps in the Gulf of Mexico, photo credit: NOAA.

Hydrocoral Errina antactica with opiurid  (gorgon’s head), Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Caryophyllia huinayensis with larvae in its tentacles, photo credit: Vreni Häussermann & G. Försterra.

Desmophyllum dianthus polyp, photo credit: Vreni Häussermann & G. Försterra.

Gorgonian garden (Primnoella chilensis) in 25 m Depth in Northern Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Different gorgonian species (Acanthogorgia and Muriceides) in Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Different colours of the deep-sea coral Thouarella koellikeri in 25 m depths in North Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Stunning diversity

To date, more than 3,300 species of cold-water corals are known, and new species are continuously discovered (6). They come in a great variety of colours – yellow, orange, red, purple and more. The shapes of these deep-water corals are highly variable including branched, fan-shaped, and feather-shaped. Their size equally varies from the size of a grain of rice, to tree-like colonies as tall as 10 m, and massive reefs that stretch for example from Norway to the coast of Africa.

Stunning diversity

To date, more than 3,300 species of cold-water corals are known, and new species are continuously discovered (6). They come in a great variety of colours – yellow, orange, red, purple and more. The shapes of these deep-water corals are highly variable including branched, fan-shaped, and feather-shaped. Their size equally varies from the size of a grain of rice, to tree-like colonies as tall as 10 m, and massive reefs that stretch for example from Norway to the coast of Africa.

Hydrocoral Errina antactica with opiurid  (gorgon’s head), Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Caryophyllia huinayensis with larvae in its tentacles, photo credit: Vreni Häussermann & G. Försterra.

Desmophyllum dianthus polyp, photo credit: Vreni Häussermann & G. Försterra.

Gorgonian garden (Primnoella chilensis) in 25 m Depth in Northern Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Different gorgonian species (Acanthogorgia and Muriceides) in Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Different colours of the deep-sea coral Thouarella koellikeri in 25 m depths in North Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Oldest Organisms in the Sea?

Not only are deep-sea corals more diverse than ocean scientists ever imagined, they are also amazingly old. A gold coral (Gerardia sp.) found off the coast of Hawaii was about 2,742 years old and a black coral (Leiopathes sp.) about 4,265 years old. These coral colonies are the oldest recorded marine organisms. 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..

Black coal (Leiopathes) from off the coast of Hawaii estimated to be 4265 years old, photo credit: Texas A&M University.

ROV collecting gold corals (Gerardia) collected off Hawaii estimated to be 2742 years old, photo credit: NOAA.

Global distribution of cold-water corals.

Ophiurid  Astrotoma agassizi on gorgonian Thouarella brucei in Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Lophelia thicket with lobster, photo credit: NOAA.

A dead hydrocoral is used by a multitude of invertebrates as substrate in Central Patagonia, photo credit: : Vreni Häussermann & G. Försterra.

Homes for other creatures around the globe

The distribution of deep-sea corals spans the globe, from the waters of Canada, the UK (5 min. documentary narrated by Sir David Attenborough), Ecuador, and New Zealand to the icy depths of Antarctica. They live on continental shelves and slopes, in deep-sea canyons, and seamounts. In some regions, they can also be found in shallow waters, e.g. in fjords of Chile (7) and Norway. Deep-sea corals are so-called “ecosystem engineers” and play a crucial role building habitats (called marine animal forests: 8) for a myriad of marine life, including many commercially important fish, shrimps, and crabs. The corals offer food, places to hide from predators and nurseries for juveniles, and a solid surface where invertebrates can take hold. Moreover, these ecosystems hold potential for the discovery of new medicines, with some organisms found in deep-sea coral habitats showing promising bioactive properties.

Homes for other creatures around the globe

The distribution of deep-sea corals spans the globe, from the waters of Canada, the UK (5 min. documentary narrated by Sir David Attenborough), Ecuador, and New Zealand to the icy depths of Antarctica. They live on continental shelves and slopes, in deep-sea canyons, and seamounts. In some regions, they can also be found in shallow waters, e.g. in fjords of Chile (7) and Norway. Deep-sea corals are so-called “ecosystem engineers” and play a crucial role building habitats (called marine animal forests: 8) for a myriad of marine life, including many commercially important fish, shrimps, and crabs. The corals offer food, places to hide from predators and nurseries for juveniles, and a solid surface where invertebrates can take hold. Moreover, these ecosystems hold potential for the discovery of new medicines, with some organisms found in deep-sea coral habitats showing promising bioactive properties.

Global distribution of cold-water corals.

Ophiurid  Astrotoma agassizi on gorgonian Thouarella brucei in Central Patagonia, photo credit: Vreni Häussermann & G. Försterra.

Lophelia thicket with lobster, photo credit: NOAA.

A dead hydrocoral is used by a multitude of invertebrates as substrate in Central Patagonia, photo credit: : Vreni Häussermann & G. Försterra.

Guardians of Earth’s History Insights from Ancient Coral Archives

Deep-sea corals also serve as archives of Earth’s climate history, with their skeletons providing valuable insights into past ocean conditions. By studying these ancient structures, scientists can decipher the effects of climate change and ocean acidification over time. The layers of coral growth, akin to tree rings, offer a timeline of environmental changes, enabling researchers to understand the long-term impacts of human activities on marine ecosystems.

Sea floor before and after bottom trawling, photo credfit: Howard Wood.

Oil spill, photo credit: NOAA.

Ocean acidification, resulting from climate change, is dissolving shells and skeletons of deep-sea animals, photo credit: NOAA.

Coral growth rings, photo credit: Smithsonian institution.

3D map if the ocean floor, photo credit: US Bureau of Ocean Energy Management.

Remotely-operated vehicle (ROV) collecting coral samples, photo credit: NOAA.

Building 3D maps of the ocean floor

Technological advancements, such as multibeam sonar and underwater vehicles, have revolutionized our ability to explore and study deep-sea coral habitats. These tools allow scientists to create detailed 3D maps of the ocean floor, pinpointing areas where deep-sea corals might thrive. Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) enable researchers to venture into the depths, capturing close-up images and collecting samples for further study.

Building 3D maps of the ocean floor

Technological advancements, such as multibeam sonar and underwater vehicles, have revolutionized our ability to explore and study deep-sea coral habitats. These tools allow scientists to create detailed 3D maps of the ocean floor, pinpointing areas where deep-sea corals might thrive. Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) enable researchers to venture into the depths, capturing close-up images and collecting samples for further study.

Coral growth rings, photo credit: Smithsonian institution.

3D map if the ocean floor, photo credit: US Bureau of Ocean Energy Management.

Remotely-operated vehicle (ROV) collecting coral samples, photo credit: NOAA.

Threats to Fragile Ecosystems

Despite their resilience, deep-sea corals face numerous threats (9, 10), including damage from fishing gear, oil and gas exploration, and climate change-induced ocean acidification. Excess amounts of carbon dioxide from the atmosphere is absorbed by the ocean and changes its chemistry, causing slower growth and weaker skeletons in corals and other animals which rely on calcareous skeletons. Bottom trawling poses another significant risk to deep-sea corals: decades of this destructive fishing practice has resulted in habitat destruction by smoothing out the sea floor. Additionally, accidents such as oil spills can have devastating effects on deep-sea coral communities, leading to long-term damage and slow recovery.

Sea floor before and after bottom trawling, photo credfit: Howard Wood.

Oil spill, photo credit: NOAA.

Ocean acidification, resulting from climate change, is dissolving shells and skeletons of deep-sea animals, photo credit: NOAA.

How can we help cold-water corals?

More and more people are recognizing the critical importance of preserving deep-sea corals, also for the benefit of many fisheries since the habitats constructed by these ecosystem engineers are home to the juveniles of many economically important species. Conservation efforts are necessary to protect these fragile ecosystems, including the establishment of marine protected areas and sustainable fishing practices. However, individual actions are also crucial: by reducing carbon emissions, making informed seafood choices, and supporting organizations dedicated to coral protection, we can all contribute to preserving these invaluable treasures of the ocean.

References

How can we help cold-water corals?

More and more people are recognizing the critical importance of preserving deep-sea corals, also for the benefit of many fisheries since the habitats constructed by these ecosystem engineers are home to the juveniles of many economically important species. Conservation efforts are necessary to protect these fragile ecosystems, including the establishment of marine protected areas and sustainable fishing practices. However, individual actions are also crucial: by reducing carbon emissions, making informed seafood choices, and supporting organizations dedicated to coral protection, we can all contribute to preserving these invaluable treasures of the ocean.

  1. Roberts JM, Wheeler A, Freiwald A, & Cairns, SD (2009): Cold-Water Corals: The Biology and Geology of Deep-Sea Coral Habitats. Cambridge University Press.
    2. Cordes E & Mienis F (eds)(2023): Cold-water coral reefs of the world. Springer, 350 pp.
    3. Freiwald A, et al. (2004). Cold-water Coral Reefs. Out of sight – no longer out of mind. UNEP-WCMC, Cambridge, UK.
    4. Roberts JM et al. (2006): Reefs of the Deep: The Biology and Geology of Cold-Water Coral Ecosystems. Science 312, 543-547.
    5. Mongabay (2023): Home to rare corals, a Chilean fjord declines in spite of protection.
    6. Henry LA, Roberts JM (2017). Global Biodiversity in Cold-Water Coral Reef Ecosystems. In: Rossi S, Bramanti L, Gori A, Orejas C (eds) Marine Animal Forests. Springer, Cham.
    7. Häussermann V, et al (2021): Species That Fly at a Higher Game: Patterns of Deep–Water Emergence Along the Chilean Coast, Including a Global Review of the Phenomenon. Front. Mar. Sci. 8:688316
    8. Rossi S, Bramanti L, Gori A, Orejas C (eds) (2017): Marine Animal Forests. The ecology of benthic biodiversity hotspots, Springer. 1366pp.
    9. Roberts JM et al. (2016). Cold-Water Corals in an Era of Rapid Global Change: Are These the Deep Ocean’s Most Vulnerable Ecosystems? In book: The Cnidaria, Past, Present and Future
    10. JM Roberts & SD Cairns (2014): Cold-water corals in a changing ocean. Current Opinion in Environmental Sustainability, 7: 118-126.
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