Black Corals of the Deep

:  Black corals often resemble bushes or trees. Their living tissue can be one of several colors. It’s the skeleton that is black.

Black corals, like this one growing on the Manning Seamount off the New England coast, often resemble bushes or trees. Contrary to its name, the living tissue of black coral can be one of several colors. It’s the skeleton that is black. See more pictures of coral in our Deep-sea Corals article.

(Mountains in the Sea Research Team; the IFE Crew; and NOAA)

Tropical reefs are the cities of shallow waters. Corals of various colors, shapes, and sizes provide nooks and crannies for fishes to hide in and towering structures for sea stars and crabs to climb. These are the corals people are familiar with—the ones they see on snorkel excursions and when diving in scuba gear.  But beyond the shallows, corals thrive in places void of light. Here, they do not rely on symbiotic algae for their food, but rather filter the water using outstretched tentacles on the branches of their skeletons. Many of these deep-sea corals are species unfamiliar to many of us: black corals. 
Black corals live in every ocean, from the shallow waters near the surface to the abyssal plains where it is darker than midnight. They thrive on pebbly seafloors where light is scarce. Unlike the hard stony corals that make up shallow reefs, black corals are built with softer chitin, the same material as the outer skeletons of insects. When they die, they do not leave behind hard calcium carbonate structures like shallow water corals but eventually decay. Despite this, black corals still create important physical habitat for other creatures to thrive in. About 2,500 species were found living on a single black coral colony. Also, like other corals, black corals are animals that contain hundreds of individual polyps, the soft tissue part of the animal that can emerge from the hard skeleton with outstretched tentacles. 

Preview a robotic arm holds a coral specimen

A deep-sea black coral specimen (Hexapathes bikofskii) is collected by a robotic arm.

(Courtesy of Jeremy Horowitz)

Most shallow water corals rely on symbiotic algae called zooxanthellae to convert sunlight into energy. Black corals instead eat floating particles in the water column with their polyps. This is one of the reasons why black corals are more tree-like than shallow water hard corals—they grow vertically and then branch to maximize their ability to obtain food. Species that live in the deep sea have had to evolve in interesting ways to survive where there are so little nutrients and hard seafloor is scarce. For example, some grow to form wind tunnels, which funnel nutrients into their outstretched polyps. Others modify how they attach to the seafloor by growing a hook to anchor themselves in soft sediment.
For much of recent history scientists have questioned the origin of these deep-sea denizens. Are they relatives of the shallow-water corals that migrated to the deep, or did all corals begin at depth and then slowly migrate up toward the coast? New research by Smithsonian scientists led by Jeremy Horowitz sheds light on this unknown. By comparing the DNA sequences of 83 species of black coral, he’s found that the black corals likely started from a single species that lived between the shallows and the deep sea around 600 million years ago. Genetic relationships between species can help tell scientists when in history they diverged from a common ancestor.
Horowitz’s research shows that the Leiopathidae were the first lineage of black corals to diverge from the other corals. These corals live at depths between 1,300 and 2,600 feet (400 to 800 m) and are tall and tree-like. Many are brilliantly colored. The ancestors of the first black corals began to diversify in the Silurian Period (437 million years ago), a time when jawless fishes and trilobites were common throughout the ocean. These early black corals thrived in the continental slope, the area where the ocean floor gradually slants down from the shallow coast waters to the deep, abyssal plane. Filter feeding became a popular mode of feeding as the ocean was filled with small creatures called zooplankton that drifted throughout the open water. Black corals took advantage of this abundance and thrived. 
Then in the Jurassic (252-66 million years ago) the ocean’s waters turned acidic as volcanic activity spewed carbon dioxide into the atmosphere and warmed the planet. Shallow-water corals with delicate calcium carbonate skeletons were unable to grow in these acidic waters and black corals likely took advantage of the open habitat and lack of competition. 

Preview A feather like coral with two lobsters

A single colony of the black coral Bathypathes sp. forms habitat for two squat lobsters and a fish at 400 meters (1,312 feet) depth in Roatan, Honduras.


It would be another 120 million years before black corals would expand to the deepest depths of the sea. This time the black corals were invading an area with few sources of food. Unlike the continental shelf that is filled with algae, animals, and food scraps raining from the nearby sea surface, the depths of the ocean are cold and relatively lifeless by comparison. At these depths the black corals needed to adapt to survive. They began to grow in specific branching patterns that scientists refer to as pinnulate branching in order to increase surface area and enhance the corals’ ability to filter-feed. The corals in the Schizopathes genus also developed hooks at the base of their stems to hold onto extremely small pebbles or even silty mud. 
Understanding the evolutionary history of the black corals provides more than the answer to a simple curiosity—it will be important in the future as deep-sea habitats become threatened by climate change and become target areas for human development. Deep-sea mining, fishing, offshore oil drilling, and the construction of offshore wind farms are all a threat to coral communities. Many of these coral species are particular about where they can survive, with distances between suitable habitats ranging up to several miles. Loss of even one habitat can be devastating for a deep-sea population. These corals also take thousands of years to grow, meaning loss of only a few individuals can decimate the rest of the ecosystem.
As we continue to explore the deep sea and unravel its mysteries, perhaps in the process we will also gain a newfound appreciation for that alien world. Black corals are only a tip of the iceberg when it comes to bizarre and beautiful creatures—there are so many amazing discoveries yet to be found.

November 2023