Black coral in Ribadeo (Spain). Photo by Ana Ferreiro

What are black corals?

When we think about corals, the first images that pop into our heads are coral reefs in clear tropical waters, full of color and life. But this is just one of the different environments where corals can live. Stony corals, which create reefs with their calcium carbonate skeletons, are symbiotic organisms that need algae to live. These algae feed them by photosynthesis, which is why they need to stay just a few meters below the surface, where there is sufficient light. This is not the case for black corals. Black corals feed by filtering organic matter in the water, such as plankton. They do not need light. These corals can be found in every ocean, from the tropics all the way to the poles, living beneath the ice sheet of the Antarctica coast. Though some of them can live in shallow waters, they are uncommon there. On the surface, they compete for space with other organisms such as stony corals and seaweed. It is in the depths where they flourish. Most of them are usually found below 50m (164 ft) and can go all thew way down, thousands of meters deep.

A global map showing the spots where black coral have been found, with the darker areas showing a higher number of observations.

Source: Catalog of Life

Black corals create a hard skeleton made of layers of proteins and chitin, unlike the calcium carbonate skeleton created by stony corals. This polymer, chitin, is the same that forms the exoskeleton of insects and cell walls of fungi. The skeleton is the reason why black corals earned their name. While black corals may look white or colored on the outside, depending on the color of the polyps, their skeleton is black.


What are the threats black corals are facing?

This skeleton is also the reason why it has been harvested for centuries. Harvesting black corals for jewelry was one of the main threats to it, particularly to the shallower populations. There were abundant beds of black corals in Cozumel (Mexico) that drastically decreased in the 20th century, after having been harvested from the 1960s until 1995. Despite decades of protection in the area, this population has sadly not yet recovered. It is a long living organism that can take decades to grow a few centimeters. And we have been using it to create rings and necklaces!

The threats to it keep increasing nowadays. They usually live at great depths, in an environment that barely changes for centuries, with steady conditions such as temperature or salinity. Climate change is affecting these parameters. They do not bleach, since they do not have algae, but temperature changes can influence their physiology. Climate change can also affect them indirectly by decreasing sensitive species of plankton and therefore reducing the availability of their food or by affecting the currents that they rely on to nourish and disperse.

On top of all that, we have the impact of commercial fishing on deep-water corals. Trawling nets are dragged along the seabed, destroying everything that is in their way. In fact, just a few months ago on the 5th of June 2020, the United States announced that they will allow commercial fishing in the Northeast Canyons and Seamounts Marine National Monument. This area contains 58 species of deep-water corals, including black corals, that provide a habitat for the associated fauna. They create an ecosystem, just like shallow water corals do, but in over 1000m (3280 ft) of depth. These millennia-old abyssal reefs could disappear in a matter of hours, wiped out by trawling nets.


Why should we study black coral and similar species?

Unfortunately, we lack information about the status of the different species of black corals living down there and the same happens with every other species that inhabits these deep-water environments. We know more about the moon than about the depths of the ocean. There are thousands of incredible species adapted to live there, without any light, under an enormous amount of pressure and dealing with extreme temperatures. These species could therefore become extinct before we even got the chance to discover them.

Black Corals in Ribadeo. Photo by Carlos Gil

But this is not only tragic because of the impact on the environment. We need these species. Biodiversity provides us with scientific advances such as new medical treatments. The PCR technique that is used to diagnose COVID-19 was developed thanks to a bacterium that grows in the Yellowstone geysers. We cannot know where these game changing discoveries are. The cure for Alzheimer could be in an undiscovered coral, or a lichen growing in the tundra. The adaptations that make these organisms able to live in extreme environments, that make them so unique, can lead to astonishing discoveries.
We need to protect the ocean, including the parts we do not even fully know and understand yet, and limit the mass extinction that has already started. But we also need to support and encourage research, not just for “practical” matters. We need to explore and study every corner of the earth. The bottom of the ocean is one of the biggest mysteries left on this planet but it is facing the same threats as the rest of the world. Who knows what is down there, waiting to be discovered?

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