No sunlight? No symbiote? No problem!
Rita Steyn
Cold water corals survive (and thrive!) in the deep ocean where there is little to no light.
Coral reefs are some of the most vibrant and diverse communities in the ocean, and when most people picture them, they see crystal clear waters with an abundance of sunlight. But did you know, that in the depths of the ocean, where dappled sunlight gives way to deep-blue darkness, coral reefs of a different kind can be found that rival their shallow water counterparts in both beauty and diversity.
An ROV (Remote Operated Vehicle) inspects an underwater reef. Photo: Bachstroems_bilder.de (Shutterstock).
Submerge yourself on our deep dive into cold-water corals – and explore their habitat, diversity, and importance. Discover what happens when the lights go out, and the water gets cold, and deep. You will learn that scientists have been discussing cold-water corals in publications since 1870, but it was not until far more recently that our understanding, and our technology, improved exponentially. And there is still so much we do not know – for example, we understand little about how tiny coral babies get around, and how they find new places to settle. We also do not fully understand how location (geography), elevation (topography), and depth (bathymetry) correlate to form the ideal conditions required for these ecosystems to flourish. We do know however, how incredibly ecologically important these systems are, and how they face some of the very same threats that their shallow-water counterparts are subject to (spoiler alert: plastics). Not surprising when you consider that cold-water corals represent over half of all coral species worldwide.
Where tropical reefs are loud, brash, and bright; deep water corals are cool, dark, and quiet. Life happens slooowly here – just one of the many features of these cold-water reef systems that we are trying to understand. At roughly 200 meters (565 feet), sunlight has lost its significance in terms of powering any metabolic activity like photosynthesis (the process that harnesses light energy to make sugars). Once below 200 meters, in the twilight zone, sunlight decreases rapidly with depth. Although it may, under the right conditions, penetrate the water all the way down to 1000 meters (3280 feet - that’s really deep!), there is no photosynthesis in the twilight zone.
Cold-water reef systems are found at depths anywhere from 40 m to over 1000 m – firmly in the deep and dark. Sometimes referred to as the aphotic zone (literally meaning without light) if sunlight does not penetrate a habitat, it means organisms down there have to catch or scavenge their own food. This is one reason deep-water corals like heavy current – the fast flowing water brings not only more food, but allows more contact with suspended food particles, and a better catch rate.
Besides the lack of light, the middle-deep (mesopelagic) zone is also c-c-cold, about 2-4°C (35°F) on average, and largely unexplored. For hundreds of years, fishermen (and some biologists) have seen deep sea corals come up with their catch, but didn’t know anything about their biology, and in Nature 1 (1870) – the very first edition of the popular scientific journal – P. Martin Duncan asserted “there is not one species found in these deep seas which is “reef-building” in its habit or whose structures resemble those of the true reef forms.” But he was wrong about that!
A deep sea octopus (Muusoctopus profundorum), sea fan (species unknown), and crinoid (Ptilocrinus pinnatus) at a depth of 2700 m, in the Bering Sea. Photo: Victor Irvin (Shutterstock).
Deep sea reefs remained relatively unexplored both in real life and in the literature, and just carried on being a sunless, cold, marine mystery. I checked Google Scholar (the search engine for scientific publications) from that first publication in 1870 to the year 2000, and found 9370 results, compared to 2001 to 2020, which yields 17,100 results. What a huge difference! What could have been driving this exponential increase in the number of studies focused on deep water reefs? A combination of technology, curiosity, and sheer coincidence led to new and incredibly exciting cold-water coral discoveries…
Among some of the more notable of those discoveries in the last decade are multiple reefs found in the north Atlantic, off the coasts of Ireland, Scotland, Canada, Norway, and Greenland. According to Science Daily in 2014, Canadian researchers off the coast of Greenland nearly threw back the newly discovered corals after fragments had come up in some equipment that had been deployed to 900 meters! In Norway, reefs grow as tall as 30 m, and can be several kilometers long. The Darwin Mounds Marine Protected Area off the coast of Scotland is a Special Area of Conservation (SAC) focused solely on deep water habitat between 710 – 1129 meters. In 2017, the deepest cold-water coral reef in Irish waters was found (and filmed) at 1600 meters. These corals grow at a rate of 1-4 mm per year and are incredibly long lived – an adaptation observed in multiple deep-sea life forms. Some of the oldest cold-water corals in the world are estimated to be over 4000 years old!
A red sea fan - a woody coral type. Photo: Seashell World (Shutterstock).
Cold-water corals come in four major types: stony corals that build reefs, soft corals like sea pens that live between thickets and along reef fringes, sea fans that grow into large forests, and the long-lived bamboo corals with their many branched forms.
In shallow waters, corals live symbiotically (interdependently) with tiny algae powered by the sun. This relationship benefits both partners, and it means that shallow corals have an ever-present food source, for as long as the algae live in their tissues. Cold-water corals cannot depend on sunlight to power algae engines – instead, they must catch all their food themselves. Because of this, cold-water corals often have large polyps (the parts that look like cups with tentacles) in comparison to shallow water species, and are often brightly coloured in reds, yellows, purples, and all the hues in between. A reef with all these types combined is a veritable oasis of life in the deep sea – and much like tropical species, these cold water corals serve as nurseries for juvenile deep sea fish.
Deep-water ecosystems are especially vulnerable to bottom-trawling, the fishing practice that drags weighted nets along the seabed – decimating wide swaths of reefs, or even whole reefs, at a time. When a species is slow growing and long-lived, it takes centuries to restore that lost habitat, and everything else that once lived there has no habitat to live in or has been caught up in the net. Much like shallow-water corals, cold-water reefs provide snapshots of centuries past, climate records frozen in time in their skeletons. But cold-water corals are not susceptible to bleaching (the loss of symbiotic algae). Although they are still under pressure from climate change: ocean warming and ocean acidification. If that wasn’t enough (cue JAWS music) – there’s also…dun dun…dun dun…plastics! And microplastics! These can affect cold-water corals growth, feeding, and behaviour, especially the reef-building variety Lophelia.
Today, we know over 1300 species of cold-water corals are found in the north-eastern Atlantic alone, and over 3300 species all over the world in major ocean basins (Indian, Pacific, Atlantic, Southern), as well as the Mediterranean Sea. Found at depths ranging from 40 m to 1600 m, these corals, and their community associates (usually sponges, crustaceans, worms, sea stars, and molluscs) appear to like living on the edge of continental shelves and slopes, in ocean canyons, and atop seamounts. It is not clear how these corals disperse between suitable sites – as cold-water reefs are separated by hundreds or even thousands of meters. Surprisingly, cold-water reefs rival warm reefs in numbers of invertebrate species associated with the habitat. Fishes, well that is another story… Only 20-40 fish species are found in association with deep water reef ecosystems – which pales in comparison to the 4000 species found in some tropical reefs.
Sea pens (order Pennatulacea) in soft sediment. Photo: Greg Amptman (Shutterstock).
Once corals settle and begin to build a reef framework, they build suitable deep-water habitat for many other species to populate. Coral mounds up to 100 m tall remain some of the most unexplored habitat in the world - so imagine what else we might discover there! Just like their tropical, shallow coral cousins, these organisms are engineers of ecosystems – creating biodiversity hotspots in a barren and nutrient-poor deep-sea environment. This is perhaps the most important of all their functions – creating a corridor of life stretched out on the seabed - linking darkness with light, crushing depths teaming with life, and giving us access to a world as alien to us as outer space.