Erika Gress is a marine scientist with a passion for the mysterious and often overlooked corals of the Order Antipatharia, commonly known as black corals. Erika works across shallow (0-30 meters), mesophotic (30-150 meters), and deep sea (more than 200 meters) ecosystems as black corals can be found at almost all ocean depths. Her work explores how black corals are related to each other (phylogenomics), how they are classified (taxonomy) and how they live and interact with their environment (ecology). In this Q&A, Erika shares what makes black corals so special and the role they play in deep-sea environments.
© Digital DEPTH/Erika GressWhat is a black coral, what makes them unique and where are they typically found?
The common name ‘black corals’ derives from the colouration of the coral skeletons, which are either black or dark brown, but the tiny animals that form the coral colonies, known as polyps, can have any colouration apart from blue and black. They can be found throughout the global ocean and at almost all depths, from 1 to 9,000 meters, making them the deepest corals we have ever found. Black corals generally require a hard surface to attach to and can be found across deep-sea habitats including seamounts and abyssal plains, where they are associated with polymetallic nodules. Some species have been estimated to be around 7,000 years old, which makes them one of the longest-lived animals on the planet.
Tell us about the moment you first encountered deep-sea black corals – what drew you to study them?
I first saw black corals in the deep sea while conducting surveys around Roatán Honduras, a Caribbean island, on board the deep-sea submarine Idabel in 2017. I had seen them before while scuba diving in shallow and mesophotic reefs, but I remember been impressed by the large size of some of the colonies in the deep sea, I thought they were all going to be smaller than at shallower depths. I was also mesmerised by how colourful they can be, making the deep sea a colourful place, rather than an all-dark ecosystem, as is usually imagined by most people.
How are deep-sea black corals identified? What features distinguish them from other deep-sea corals?
Although black corals look very different to their close relatives – the hard corals of the order Scleractinia that build shallow reefs – they do look very similar to soft corals and gorgonians, such as sea fans and sea whips (Class Octocorallia). To distinguish between soft and black corals we look closely at their physical traits and features, known as morphological characteristics. One key difference is the number of tentacles on each polyp: black corals have six, while soft corals have eight. Soft corals often have no real skeleton or only flexible support structures, while black corals have a tough, thorny skeleton made of special proteins (scleroproteins).
Black corals are known to be some of the longest-living organisms on the planet. How do researchers determine their age and what’s the oldest specimen ever recorded?
Using radiocarbon dating* some colonies have been estimated to be about 2,400 years old. Based on the growth rate and size of these, other colonies of the same or closely related species were estimated to be about 7,000 years old – the longest living colonial animal we know of!
© Digital DEPTH/Erika GressWhat is the ecological and scientific significance of black corals in deep-sea ecosystems? What roles do they play?
The main ecological role of black corals is habitat provision and a single colony can support many different invertebrates and fish. However, they have different types of associations with different marine fauna: some species of shark, squid and octopus lay eggs on their branches, while some fish use them as feeding grounds.
What are the biggest threats facing black corals today, and how can deep-sea research and conservation help protect them?
In shallow and mesophotic reefs black corals are threatened by the jewellery industry (for which many colonies are harvested), and by overgrowth of cyanobacteria (a bacteria that looks like algae), often a consequence of nutrient runoff from land-based sources such as sewage discharge and agriculture. While in the deep sea they are affected by the mining industry and bottom trawling fisheries, which can remove entire colonies from the seafloor. Only by avoiding/banning these activities in the deep sea, can we prevent their depletion.
* Radiocarbon dating is a technique used by scientists to determine out how old something is. All living things absorb carbon from the environment, including a form called carbon-14 which is slightly radioactive. Carbon-14 breaks down slowly over time at a known rate, and the remaining amount of carbon-14 provides an estimate of an organism’s age. Black corals grow by adding layers to their skeletons, much like rings in a tree trunk and by extracting a small sample from the base of the coral, the oldest part, scientists can measure the carbon-14 levels to determine how long the coral has been growing.
Credit: Digital DEPTH/Erika Gress