Imagine a coral reef and you probably picture colourful creatures living amongst rocky, plant-like structures in warm, shallow water. Not all corals are found in tropical environments, however.
Cold-water corals inhabit the ocean’s cold, dark depths. Like all corals, each individual is a colony of tiny, genetically-identical animals called polyps. Many species make limestone to surround their soft polyps in a hard exoskeleton, and some of these ‘stony corals’ form the foundations of reefs, creating habitats for marine life and hotspots of biodiversity.
The most widespread reef-building cold-water coral is Lophelia pertusa. This species has built the largest known reef, the Røst reef off the coast of Norway – 40km long and 3km wide, at a depth of 350 meters (1000 feet).
Because cold-water reefs are so far underwater, little is known about them, which is why the US National Oceanic and Atmospheric Administration has sponsored several expeditions to study Lophelia reef ecosystems in the Gulf of Mexico.
But by far the biggest mystery surrounding cold-water coral is how they form reefs in the first place.
Sticking together skeletons
In tropical waters, red algae grow on substrates and dead coral skeletons. Like stony corals, these ‘coralline algae‘ are rocky, and serve as the cement holding a reef’s bricks together. But coralline algae produce food through photosynthesis, which means they aren’t found at depths where sunlight can’t reach.
So how do cold-water corals form a reef?
Researchers led by marine biologist Sebastian Hennige of Heriot-Watt University, Scotland, have a clue that helps solve this mystery: cold-water corals stick their skeletons together.
Tropical corals sometimes combine skeletons. But they won’t fuse with friends, only family. Cold-water species, meanwhile, have a more liberal approach to relationships, something revealed by Lophelia‘s colors, which range from white to orange.
Diving down in JAGO, a small submersible vehicle for two, Hennige searched for Lophelia structures that appeared to come from separate colonies. It’s often impossible to tell whether different individuals have fused together, but the dives allowed him to get close enough to find orange and white skeletons stuck together, indicating they were distinct colonies.
After collecting samples with JAGO, DNA tests confirmed the different-coloured skeletons came from genetically distinct – and unrelated – individuals. Under the microscope, there were even subtle sutures where the corals had joined.
Coral fusion theories
Energy supply is one explanation for the differences between tropical and cold-water corals in their attitude to combining skeletons.
Most tropical species will form a symbiotic partnership with zooxanthellae, microscopic algae that carry out photosynthesis inside coral cells, exchanging food for nutrients. Cold-water species don’t have these internal food-making factories, which means they have to conserve energy.
In shallow-water ecosystems, photosynthetic organisms receive a plentiful supply of sunlight, power that passes through the food web so organisms are able to invest energy in competitive interactions that might lead to death and slow growth in the short term, but could win a greater share of resources in the long run. In the deep sea, the energy budget is tight, and confrontation is avoided.
Hennige and colleagues have proposed two theories for why Lophelia corals are able to fuse skeletons. One is that an individual won’t waste energy by responding aggressively when another coral enters its personal space. The second theory is individuals allow fusion to happen after recognising an ‘invader’ is from the same species, a bit like the way immune systems distinguishe between ‘self’ and foreign molecules.
Fusion between cold-water corals is thus a friendly takeover or a merger (maybe both).
However Lophelia goes about the business of forming reefs, the way colonies get together so freely is probably behind its success as a reef-building species – and its importance in engineering deep-sea habitats.
Photo: A school of Beryx fish and an orange basket star on a reef formed from the cold-water coral Lophelia pertusa in the Gulf of Mexico (Image: CC BY 2.0 Lophelia II 2010 Expedition, NOAA-OER/BOEMRE / https://flic.kr/p/93osk1)
View original article at: Deep-Sea Corals Get Friendly To Build Reefs