When people think of coral reefs, they typically picture warm, clear waters with brightly colored corals and fishes. But other corals live in deep, dark, cold waters, often far from shore in remote locations. These varieties are just as ecologically important as their shallow water counterparts. They also are just as vulnerable to human activities like fishing and energy production.
Earlier this year I was part of a research expedition conducted by the Deep Search project, which is studying little-known deep-sea ecosystems off the southeast U.S. coast. We were exploring areas that had been mapped and surveyed by the U.S. National Oceanic and Atmospheric Administration’s research ship Okeanos.
In an area 160 miles off South Carolina we deployed Alvin, a three-person research submersible, to explore some features revealed during the mapping. What the scientists aboard Alvin found was a huge “forest” of coldwater corals. I went down on the second dive in this area and saw another dense coral ecosystem. These were just two features in a series that covered about 85 miles, in water nearly 2,000 feet deep. This unexpected find shows how much we still have to learn about life on the ocean floor.
Life in the dark
Deep corals are found in all of the world’s oceans. They grow in rocky habitats on the seafloor as it slopes down into the deep oceans, on seamounts (underwater mountains), and in submarine canyons. Most are found at depths greater than 650 feet (200 meters), but where surface waters are very cold, they can grow at much shallower depths.
Shallow corals get much of their energy from sunlight that filters down into the water. Like plants on land, tiny algae that live within the corals’ polyps use sunlight to make energy, which they transfer to the coral polyps. Deep-sea species grow below the sunlit zone, so they feed on organic material and zooplankton, delivered to them by strong currents.
In both deep and shallow waters, stony corals – which create hard skeletons – are the reef builders, while others such assoft corals add to reef diversity. Just five deep-sea stony coral species create reefs like the one we found in August.
The most widely distributed and well-studied is Lophelia pertusa, a branching stony coral that begins life as a tiny larva, settles on hard substrate and grows into a bushy colony. As the colony grows, its outside branches block the flow of water that delivers food and oxygen to inner branches and washes away waste. Without flow, the inner branches die and weaken, then break apart, and the outer live branches overgrow the dead skeleton.
This sequence of growth, death, collapse, and overgrowth continues for thousands of years, creating reefs that can be hundreds of feet tall. These massive, complex structures provide habitat for diverse and abundant assemblages of invertebrates and fishes, some of which are economically valuable.
Other important types include gorgonians and black corals, often called “tree corals.” These species can grow very large and form dense “coral gardens” in rocky, current-swept areas. Small invertebrates and fishes use their branches for shelter, feeding and nursery habitat.
Probing the deep oceans
Organisms that live in deep, cold waters grow slowly, mature late and have long lifespans. Deep-sea black corals are among the oldest animals on earth: One specimen has been dated at 4,265 years old. As they grow, corals incorporate ocean elements into their skeletons. This makes them archives of ocean conditions that long predate human records. They also can provide valuable insights into the likely effects of future changes in the oceans.