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.
Scientists have discovered that deep-sea corals live very different lives than their shallow water relatives. For example, shallow water corals depend on algae that live in their tissues for much of their energy. They can only grow in the upper photic zone, where enough sunlight penetrates to the seafloor to allow photosynthesis, and so they are restricted to shallow tropical waters on the continental shelves and the margins of islands.
Deep-sea corals, on the other hand, are not limited by sunlight. Because they get all of their energy from feeding on other organisms, they can live in the darkness of the aphotic zone, along the outer continental shelves, down the slope, and even into the abyssal zone.
Freed from the need for strong sunlight, deep-sea corals are common and widespread. Found in every ocean, from Antarctica to the Arctic Circle, they thrive in water as deep as 6,000 meters and as cold as 4° Celsius. Food is scarce, temperatures are low, and pressures are high on the deep-sea floor. Under such extreme conditions, individual corals grow very slowly-- only a few millimeters to centimeters a year. Some appear to be remarkably long-lived with life spans of several thousand years.
All corals are invertebrates, fleshy cylinders topped with a crown of tentacles. The coral animal, called a polyp, is soft and small but it secretes a hard external skeleton made of limestone. The flexible polyp can squeeze itself down into the hollow of this skeleton for protection.
Deep-sea corals use their tentacles to gather food, snatching at plankton and small bits of organic debris that drift past. Their individual skeletons merge together slowly raising the entire group away from the muddy bottom into clear water as they grow.
Some corals are highly colonial, growing together into congested reefs and mounds. Others are more solitary, living in open groves of tree- and fan-like structures.
These hard and structurally complex coral mounds and thickets provide a distinctive habitat on the otherwise featureless and muddy open ocean floor. Sometimes called “the rainforests of the deep,” they shelter and nourish a highly diverse ecosystem. Some animals – sponges, anemones, and small fish – grow on and hide within the protection of the coral framework. Others – starfish and sea urchins, octopi, larger fish, and sharks - come in search of prey. A few - sharks and some bottom fishes - return as adults to lay their eggs.
Among the most important reef building deep-sea corals are members of the genus Lophelia, found in great numbers throughout the Atlantic Ocean. Lophelia banks hugging the edge of the continental shelf off of North Carolina have been the focus of several expeditions conducted by the National Oceanographic and Atmospheric Administration, or NOAA.
Lophelia is very slow growing, stony coral, forming dense thickets on the tops and sides of ridges. The living coral is pure white and somewhat delicate. Frequently, branches break off, snapped from the outside by currents or chewed off from within by coral eating sponges and microbes. Dead branches accumulate in gray piles several meters deep and hundreds of meters long beneath and around the living reefs.
The reefs and their rubble piles grow on high spots on the seafloor, which may rise as much as 150 meters above the surrounding sea floor. Here, strong currents sweep away mud, giving coral polyps a patch of bare rock to attach to sweeping in plankton and giving corals and other filter feeders a steady food supply.
Eddies peel off the main currents and concentrate plankton and organic matter within and around the reef, much to the delight of a variety of filter feeding organisms. As the corals grow and intertwine and are coated with sponges and other encrusting animals, the reef shape becomes increasingly complex and irregular.
Once established, Lophelia reefs are pockets of high abundance and biodiversity in the otherwise barren deep-sea floor. Several types of crabs and lobsters are abundant, luring predatory fishes to the deep reefs. Other invertebrates, including brittle stars and sea anemones, also populate the coral reef and rubble in high numbers.
Discovery of the unusual lifestyles of deep-sea corals has aroused enormous excitement and curiosity in oceanographers and marine biologists. However, there are also purely practical reasons to explore the deep reef ecosystem. Scientists believe the history of our climate and the future of global fisheries both lie hidden in these remote coral fields.
Deep-sea coral reefs and groves appear to act as nurseries for the young of many species, including a number of commercially valuable animals. Varieties of sea bass, snapper, grouper, scallops, and shrimp, for example, rely on the shelter and readily available food provided by the coral habitat when they are too small to survive the rigors of the open ocean.
Scientists are still working to understand the full role of the deep coral habitat in nurturing creatures important to commercial fisheries and to the larger open ocean ecosystem. Their efforts are becoming increasingly urgent as evidence mounts that the reefs are no longer protected by their depth and distance from shore.
Bottom trawling, and to a lesser extent oil exploration and undersea cable laying, are becoming more common on the outer continental shelves and slopes. These activities drag equipment across the bottom of the ocean floor uprooting corals, breaking apart reef frameworks, and kicking up clouds of sediment that smother polyps.
Scientists are also very interested in what corals reveal about sudden and severe climate change. Atmospheric temperature and precipitation patterns reflect and reinforce the circulation of oceanic water masses. Water movements in turn control the chemistry and temperature of the deep sea.
Deep-sea corals grow in the path of some of the most vital deep oceanic currents. When a coral polyp secretes its stony home, water conditions are recorded in the rock’s composition. The coral skeleton and its chemical record survive long after the animal dies. By sampling reef layers of different ages, scientists can compile a history of deep-sea circulation spanning tens to hundreds of thousands of years. They hope to combine data from reefs around the world into a record of the speed, processes, and consequences of global climate change.
Early in 2004, over 1,000 marine scientists petitioned the United Nations and governments around the world to protect the deep corals. Many of them are also are working with fishermen to help develop and test less destructive fishing methods. They hope that increased knowledge of deep-coral distribution and ecology will help them identify the most important reef areas and encourage the creation of coral protection zones.