deep water corals digogo paulo
March 28, 2023

Exploring the unknown world of deep-water corals 

Corals often conjure up visions of shallow water, white sands and tropical fish. But go a little deeper and you’ll find a very different organism 

Introducing deep-water corals. They don’t need the sun’s rays to survive, they can withstand near-freezing conditions and they are every bit as vital for our ocean’s health as their shallow-water relatives.  

Deep-water corals have been discovered in temperatures as low as -1ºC (30.2ºF), at depths of up to 6,000 m (20,000 ft). Despite growing in the dark, they are as diverse as tropical corals and they occur across the entire ocean, at all latitudes, even off the coast of Antarctica.  

Although their growth rate is slower, the coverage of deep-water corals likely exceeds that of tropical corals. Growing from single polyps no bigger than a grain of sand, the largest known deep-water coral reef is over 40 km long. At more than 4,000 years old, one colony in Hawaii contains the oldest marine organisms on record and as we are still exploring the deeper ocean, larger, older corals may still wait to be discovered. 

We know these corals are ecologically important, providing habitat, feeding grounds and nurseries for many species, including commercially important fish. And yet, we are likely to have lost vast spans of these habitats, destroyed by trawling and other disturbances of the sea floor, before we even knew of their presence.  

That’s why in May, we will hold a workshop focussing upon key questions on the conservation and restoration of deep-water corals. Part of our ongoing scoping studies, this latest workshop will identify gaps in our knowledge regarding deep-water corals and help us to develop recommendations on priority investment areas for R&D. 

Inaccessible, yet under threat  

Until recently, these corals were often hard for scientists to access due to the depths they inhabit. Now, thanks to advances in Remotely Operated Vehicles (ROVs) and submersibles, we are better equipped to map deep-water ecosystems, so new species are regularly being discovered and there is evidence that thriving ecosystems exist around these coral mounds. 

Living at depths means temperatures and salinity levels naturally remain constant, leaving corals and their inhabitants relatively sheltered by climate extremes on the surface. But while natural disturbances are few and far between, human interference has taken its toll.  

Total losses of deep-water corals to date are unknown, but most show signs of physical damage. Some deep-water corals in the Northeast Atlantic have been completely lost because of bottom trawling, which leaves deep scars of empty sand and mud along the seabed. Because these corals need to attach to something hard like rock or another coral skeleton in order to grow, trawling not only destroys existing coral, it also limits the potential for natural recovery.  

Deep-water corals are also vulnerable to rising temperatures, which can alter the flow of currents in the water. These currents stir up and carry the small plankton and organic particles that deep-water corals rely on as their main source of food, and without them they cannot survive.  

Other threats include cable and pipeline placement, bio-prospecting, waste dumping, pollution and the upcoming threats caused by releasing sequestered CO2 , mineral exploration and increased atmospheric CO2. Interventions are needed now, to ensure their ongoing survival.  

Protection, restoration and rehabilitation 

Closing fishing grounds to trawling and creating marine protected areas are key for conservation for deep-sea ecosystems. In some places it’s working well, with encouraging signs of coral regrowth and an increase in the number of species that rely on these reefs.  

But there is also evidence that simply ceasing destructive behaviour is not enough to ensure deep-water corals can regenerate everywhere. At least two separate studies have shown that there was no natural recovery of corals in the regions being monitored almost a decade after bottom-trawling activity ceased.  

To create the best results, these kind of conservation measures often need to be combined with active restoration. One method that’s been trialled is working with local fishers to return bycatch corals to their environment and a small number of species have seemingly been able to regenerate on their own once returned to the water.  

Others need careful collection and transplantation to survive, but even this does not work for all deep-water corals. The stress suffered during collection, from changes in temperature to the difficulty inherent in recreating their natural habitat once in the lab, can be too much for these delicate organisms.  

What does seem clear is that there is no “one size fits all” solution for deep-water coral conservation. But through a combination of protection, restoration and rehabilitation, and with further research and innovation, we can ensure we are giving ourselves the greatest chance of success. 

Hope for the future 

In the past, the difficulty in accessing remote deep-sea ecosystems and the associated costs, have contributed to the lack of research and active restoration. In theory, divers could transplant coral fragments using the same methods found in shallow coral reefs, but the risks posed by deep technical diving and the need for extra safety measures can make costs prohibitive. This is where improving the technology for remote sensing or remotely operated vehicles could be extremely helpful. 

ROVs offer a viable alternative, but existing technology can struggle with the delicacy required to attach fragile and sometimes flexible coral fragments to artificial reef structures, especially in strong currents. It is hoped that, in future with substantial technological development, ROVs or Autonomous Underwater Vehicles (AUVs) might offer a way to do this. Promising advances are already being made on the capability of AUVs to autonomously locate and manipulate objects and machine-learning approaches are also being used to help detect suitable sites for deep-water restoration projects. So, restoration may prove hard, but it isn’t impossible. Outcomes from the few restoration actions performed to date on deep-water corals have shown evidence of the feasibility of coral transplantation and artificial reef deployments, given the right funding and support. Combined with innovative technology there is real hope for the future.   

And that is where our scoping workshop will help. By bringing together the best minds and exploring the potential for testing novel and creative restoration techniques. we can identify priorities for our funding to accelerate our capacity to protect and preserve these unique and vulnerable corals. 


(c) Photo: Diogo Paulo