Some background - What is a coral?

When I worked as a biologist and guide on dive and snorkel trips on the Great Barrier Reef, I used to ask people on the tours I guided if they thought a coral (referring to the reef-building tropical corals) were an animal, a plant or a rock. Most people answered animal. My answer was that you could say it is a little bit of all three.

The animal part or the corals scientifically belong to the phylum Cnidaria and the class Anthozoa. Other members on cnidarians are sea anemones, jellyfish and sea pens. One things that distinguishes cnidarians is that they have a stinging cells called nematocysts. The nematocyst works kind of like a speargun and is used by the animal to capture prey as it is shot out by the stinging cells when something triggers it. For anyone who has ever got stung by a jellyfish, congrats! You have experienced the effect of these cnidocysts first hand.

The individual corals are called polyps. Some corals are solitary meaning that one coral is only one polyp, whereas other species can form huge colonies and create the complex structures that make up the reefs.

When you see a tropical coral reef the first thing that strikes you are all the colours. The coral colours are caused by a tiny single-celled micro algae (dinoflagellate) that lives in symbiosis with the coral polyps. This is the plant part of the coral.

As these microalgae, called zooxanthellae, use the sunlight to photosynthesis, they pass on a lot of the energy to the corals. In fact, the coral receives most of its necessary energy needs from its symbiotic partner. So even thought the coral polyps have the ability to capture food, it is the zooxanthellae that takes care of most of the energy production.

Coral diversity

Corals are generally divided up into either soft corals or hard corals. Hard corals secrete a calcium carbonate skeleton and create the incredible complex hard structures that make up coral reefs. As the corals grow or die, they leave behind this hard scaffolding which becomes the rock part of the corals.

Soft corals are instead kind of fleshy and can bend. There are two main types of soft corals. The true soft corals, which don’t have an internal skeleton, and the gorgonians, also called sea fans, which have a skeleton-like internal structure made up by a protein which allows them to stand all and sometimes grow really big. Both groups have so called ‘spicules’ in their tissues, which are calcified spine-like structures that provide support and some protection for the soft corals. 

Because tropical reef corals largely depend on their symbiotic relationship with zooxanthellae algae, they are also dependent on sunlight. Tropical corals also require a water temperature above  about 18°C to survive and build the hard reefs structures (some differences depending on species but most generally need temperatures above this). Therefore these corals thrive in tropical warm waters close to the surface where there is enough sunlight for photosynthesis, called the photic zone. So corals are usually limited to depth less than 40m with most of most diversity around the first 20m from the surface.

This is how many texts describes where you can find corals. And even if it is true, it certainly isn’t the whole picture. Because corals and coral reefs can actually be found in pretty much all oceans on the planet, and down to depths well past 2000m. The deepest known coral species was found below 6000m!

What is a deep water coral?

Unlike tropical corals, cold- and deep water corals don’t have a symbiotic algae living in their tissues. This is probably not so surprising as not much sunlight can penetrate down to these depths. Instead they feed entirely on food particles they catch from the water using their tentacles. Since they lack the zooxanthellae algae, they also lack the vibrant colors seen on tropical species. The exposed coral skeleton of the deepwater corals stands eerily bright white, yet just like their warm water relatives, the reefs house an incredible diversity of life, dependent on the complexity of the reefs for many stages of their life stages. This also included many commercially important species such as cod (Gadhus morhua), rockfish (Sebastes sp.) and pollock (Polachius virens). Another big difference to the tropical corals reefs where coral diversity is high within the reef, is that deep water reefs tend to only be dominated by one or maybe a couple of different species. That does not mean that the total diversity of existing cold water species is low.

In fact, cold and deep water corals are more diverse than one might think. More than half of the approximately 5100 species of corals actually live in waters deeper than 50m. Some being solitary species and some capable of constructing gigantic continuous reef structures covering several square kilometres. The largest deepwater reefs are made by a coral called Lophelia pertusa.

Lophelia pertusa

The most common cold and deep water reef-building coral is called Lophelia pertusa. A white hard coral, with fairly large tentacles, loving it in water temperatures somewhere between 4-12°C.

Lophelia reefs are found between 40m down to more than 2000m and the largest known reef, called Røst Reef, lies off the coast of Lofoten in northern Norway. It was discovered in 2002 and stretches a length of about 43km (27 miles), with a width of up to 6.9km (4.3 miles). A reef made up of pretty much only one species of coral is a huge contrast to tropical reefs often made by hundreds of coral species.

The Lophelia corals may only grow a few millimeters every year so the corals on the reefs may have developed over many hundreds or even thousands of years. 

Project plans 2020/2021

Together with researchers from Gothenburg University, the University of Edingburgh and commercial diver Oliver Stubbs and his team we are setting out to document part of the Scandinavian Lophelia reefs. Our focus will be on the southern Scandinavian reefs located around the border between Sweden and Norway. A team of commercial divers led by Oliver Stubbs will dive down to the reef at Tisler, to document and collect important scientific data and to create 3D map of the reef. It is a task that requires detailed planning and safety measures as the divers prepares to go down below 110m.

Project delayed due to Covid-19. Please stay patient. Full video will come.