With darkness prevailing the depths of the oceans and pressures greater than you or I could ever survive, one would probably consider the deep dark world to be quite a lifeless void. Science, however, has proven that this is not the case and at over 2,000 m from the surface, faced with pressures of around 200 times the pressure on land you may be faced with an ‘ancient’ coral; the black coral (Leiopathes sp.).

What are corals?

The black coral Leiopathes spp. It is clear here to see the orange colouring of the polyps of this ‘black coral’ (source).
The black coral Leiopathes sp. It is clear here to see the orange colouring of the polyps of this ‘black coral’ (source).

Corals are part of the phylum Cnidaria; a phylum which also includes jellyfish and sea anenomes among other organisms too. The Cnidarians are characterised by their ability to produce an ‘explosive’ cell called the cnida that is used to stun and capture its prey. Coral are widely distributed in the world’s oceans from  deep cool waters to warmer tropical waters. There are some species of coral that rely on small photosynthetic organisms called zooxanthellae; these species of coral are mostly found in shallow tropical waters on reefs as the zooxanthellae require an adequate amount of light in order to photosynthesise. However, most species of coral can actually be found in the much cooler, darker waters of the deep sea. The black corals are part of the order Antipatharia; an order which contains 235 species, including Leiopathes sp. This order is characterised by its black chitonous skeleton, which is where it gets the name ‘black corals’. However, the polyps of healthy corals are usually a vibrant colour such as orange.

Life in the deep sea

Leiopathes provides a home to some species. Here it can be seen with squat lobsters (source).
Leiopathes provides a home to other organisms. Here it can be seen with squat lobsters (source).

Light in the ocean is absorbed at a relatively shallow depth with around 80% of solar energy absorbed with the first 10 m from the surface. In fact, the ‘euphotic zone’ ends at around 200 m deep as very little light reaches past this point, so little that photosynthesis can no longer take place. At depths below 1000 m, we reach the ‘aphotic zone‘, a zone where sunlight does not reach at all and the only light seen is from bioluminescent organisms; it is this zone in which Leiopathes spp can be found. Not only is Leiopathes spp exposed to complete darkness in the deep sea, but they are also exposed to increasing pressures and at 2000 m down, Leiopathes spp would be exposed to 200 times the pressure that animals living at sea level would encounter.

How old?

It may be possible that black corals may actually hold the secret to old age. Some corals have annular growth rings like those of trees and a radiocarbon analysis was performed on these growth bands by Roark et al (2009) on two deep sea corals from the waters of Hawaii to determine the age of the coral. One of these corals was the black coral Leiopathes sp; three live samples of this coral were obtained for studying. Due to lack of records on species of Leiopathes, the researchers were not entirely sure which species exactly were sampled, however more recently the species has been identified as Leiopathes annosa. The results of the study by Roark et al showed that the coral, Leiopathes annosa, can live to a remarkable age with the oldest sample aged around 4,265 years old; the other samples were aged around 350 years old and 2,300 years old, respectively. The scientists also found that the coral are also incredibly slow growing. These coral grow less than 5 micrometres every year; this is less than the thickness of a human hair.

Humans and the future …

This is a pendant made from black coral that can easily be bought online (source).
This is a pendant made from black coral that can easily be bought online (source).

As with many organisms and ecosystems, Leiopathes sp. coral are vulnerable to anthropogenic activities. Because these coral have a distinctive black skeleton, they are much desired for the jewelry industry for use as pendants etc ; the estimated value of the Hawaiian black coral fishery in 2001 was $15 million. These coral also have a cultural heritage in some communities and are believed to possess healing properties so they are brought to the surface and crushed up for medicinal purposes. They are also indirectly impacted by the fishing industry; they are often caught as bycatches in deep sea dredging and bottom trawling. Little research has been undertaken to study the impacts of removing these coral from the ocean, but with such a slow growth rate and long life, it would not be surprising if removing these coral would have a large impact on local ecosystems. The annual growth bands of the coral can give an indicator of how the local environment was at the time. For example, the thickness of the bands can indicate the temperature when the ring was formed (more information here), so maybe by studying these corals it may be possible to be able to create an image of how the climate of the oceans has changed over the 4,000 years the coral has been alive.


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