Why Deep Sea Corals Are Hard to Study and Protect

Deep sea corals are difficult to study because they live in extreme environments that are expensive and technically challenging to reach, and they are difficult to protect because their slow growth, long lifespans, and vulnerability to bottom-contact fishing make recovery from damage extremely slow. Researchers must use deep-sea submersibles, remotely operated vehicles (ROVs), and specialized sampling gear to observe them, and even then, only a tiny fraction of the ocean floor has been surveyed. Protection efforts are complicated by the fact that these corals grow in areas that are often outside national jurisdiction, making international cooperation essential but difficult to achieve.

What Are Deep Sea Corals?

Deep sea corals are not a single species but a diverse group of cnidarians that live in cold, dark waters below 200 meters (656 feet) and often much deeper. Unlike their tropical, sunlit relatives, deep sea corals do not rely on symbiotic algae for food; they are filter feeders that capture plankton and organic particles from the water. They include stony corals (order Scleractinia), which build hard calcium carbonate skeletons, as well as soft corals, black corals, and sea pens. Some of these corals form vast reef structures, such as the cold-water coral reefs found off the coasts of Norway, Ireland, and Canada, while others grow as solitary polyps or small colonies.

These corals are found in every ocean, from the Arctic to the Antarctic, and at depths ranging from 200 meters to over 6,000 meters (20,000 feet). They thrive in temperatures between 4°C and 12°C (39°F to 54°F), which is why they are often called cold-water corals. Some species are known to live for thousands of years, making them among the longest-lived animals on Earth. For example, the deep sea coral Leiopathes (a black coral) has been dated to over 4,000 years old.

Why Are They So Hard to Reach?

The primary challenge in studying deep sea corals is simply getting to them. The ocean depths are dark, under immense pressure, and far from land. Most research vessels are equipped with ROVs or autonomous underwater vehicles (AUVs) that can cost tens of thousands of dollars per day to operate. A single dive can last only a few hours, and the area covered is often just a few hundred square meters. In contrast, the deep ocean floor covers about 60% of Earth’s surface, so the scale of exploration is minuscule relative to the area that remains unmapped.

Sampling deep sea corals is also technically demanding. Standard trawling nets, which are used to sample fish, would destroy fragile coral structures. Instead, researchers use remotely operated arms to carefully collect individual colonies or use coring devices to extract sediment cores. Even with these tools, samples are often damaged or incomplete, and the process is slow. For example, a single ROV dive might collect only a handful of coral specimens, each of which requires careful documentation of its exact location, depth, and surrounding habitat.

What We Don’t Know About Them

Our knowledge of deep sea corals is still in its infancy. Scientists estimate that only a small percentage of deep sea coral species have been formally described. Many specimens collected during expeditions remain in museum collections, awaiting identification. The lack of basic taxonomic information makes it difficult to assess biodiversity and to know which species are rare or endangered.

We also have limited understanding of their biology and ecology. For example, the reproductive cycles of most deep sea corals are unknown. Some species may reproduce only once every few years, and their larvae may disperse over vast distances or settle very close to the parent colony. Without this knowledge, it is impossible to predict how quickly a damaged reef might recover or whether it can recover at all.

Furthermore, the genetic connectivity between populations is poorly understood. Deep sea corals can be found on seamounts, continental slopes, and mid-ocean ridges, but we do not know if these populations are isolated or if they are connected by ocean currents that carry larvae. This information is critical for designing marine protected areas (MPAs) that can effectively safeguard these species.

Threats: Fishing, Mining, and Climate

The most immediate and well-documented threat to deep sea corals is bottom trawling. Fishing vessels drag heavy nets across the seafloor to catch fish and shrimp, and in doing so, they crush and dislodge coral colonies. A single trawl pass can destroy a reef that has been growing for thousands of years. Trawling also resuspends sediment, which can smother corals and block their feeding structures.

Deep sea mining is an emerging threat. The seabed contains nodules rich in metals like manganese, nickel, and copper, which are of interest to mining companies. These mining operations would remove large areas of the seafloor, directly destroying coral habitat. Even the sediment plumes from mining could affect corals many kilometers away.

Climate change also poses a long-term risk. Ocean acidification, caused by increased carbon dioxide in the atmosphere, reduces the availability of carbonate ions, which are needed by stony corals to build their skeletons. In cold, deep waters, the saturation state of calcium carbonate is already low, and further acidification could make it impossible for corals to grow. Additionally, changes in ocean temperature and currents could affect the food supply that deep sea corals depend on.

Why Protection Is So Complicated

Protecting deep sea corals is difficult because they often live in areas beyond national jurisdiction (the so-called high seas), which cover about 60% of the ocean. No single country has the authority to regulate activities in these areas, and international agreements like the United Nations Convention on the Law of the Sea (UNCLOS) have only recently begun to address biodiversity conservation on the high seas. In 2023, the UN adopted the High Seas Treaty, which provides a legal framework for creating marine protected areas in international waters, but its implementation is still years away.

Even within national waters, protection is often patchy. Many countries have established MPAs that include deep sea coral habitats, but their enforcement is challenging. Monitoring vast, remote ocean areas is expensive, and illegal fishing can go undetected. Moreover, some MPAs allow certain types of fishing or mining, so the level of protection varies widely.

Another complication is the slow growth and reproduction of deep sea corals. If a reef is damaged, it may take centuries or even millennia to recover. This means that even a single disturbance event can have long-lasting consequences. For example, a trawler that passes over a reef once may cause damage that persists for generations, making it difficult to demonstrate that protection measures are effective in the short term.

How Researchers and Managers Are Responding

Despite the challenges, scientists and conservationists are making progress. Advanced mapping technologies, such as multibeam sonar, allow researchers to identify potential coral habitats from the surface, reducing the need for time-consuming dives. Environmental DNA (eDNA) analysis can detect the presence of corals from water samples, providing a low-cost way to survey large areas.

In terms of protection, the establishment of regional fisheries management organizations (RFMOs) has led to the closure of some areas to bottom trawling where deep sea corals are known to occur. For example, the North Atlantic Fisheries Organization (NAFO) has closed several seamounts and coral areas to bottom fishing. The High Seas Treaty, once ratified, will provide a mechanism for creating a network of high seas MPAs, which could protect coral habitats in international waters.

There are also efforts to restore damaged reefs, although these are still experimental. Fragments of coral can be transplanted to damaged areas, but the success rate is uncertain, and the cost is high. Given the vast scale of the deep ocean, restoration is unlikely to be a solution for widespread damage; prevention is the only practical approach.

Frequently Asked Questions

Can deep sea corals live in the dark?

Yes, deep sea corals do not need sunlight because they do not have symbiotic algae. They feed by capturing plankton and organic particles from the water with their tentacles. Some species also absorb dissolved organic matter directly from the water.

How old are deep sea corals?

Some deep sea corals are extraordinarily old. Black corals and some species of stony corals have been dated to be over 4,000 years old. Their slow growth rates, often just a few millimeters per year, contribute to their longevity.

Are deep sea corals affected by ocean acidification?

Yes, ocean acidification is a major threat. As carbon dioxide dissolves in seawater, it lowers the pH and reduces the concentration of carbonate ions. Stony corals need carbonate ions to build their calcium carbonate skeletons. In deep, cold waters, the carbonate saturation is already low, so further acidification could severely impair coral growth.

Why can’t we just transplant deep sea corals to protect them?

Transplanting deep sea corals is extremely difficult and rarely successful. The corals are adapted to specific depths, temperatures, and currents, and they are very fragile. Bringing them to the surface for transplantation would kill them. Even in situ, moving corals can damage them, and the success rate of such efforts is very low.

What can individuals do to help protect deep sea corals?

Individuals can support organizations that advocate for marine conservation and sustainable fishing practices. Reducing your carbon footprint helps mitigate ocean acidification and warming. Also, choosing seafood that is certified as sustainably sourced can reduce the demand for bottom-trawled fish, which is a major threat to deep sea corals.

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