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  • [♩INTRO]

  • When you think about coral reefs, you probably imagine a sparkling tropical oasis

  • that you can easily see while snorkeling or diving.

  • But that's just part of the picture when it comes to corals.

  • Reefs can be found as deep as 8000 meters.

  • In many ways, the corals of the deep are similar to shallower corals,

  • but they also form their own weird, wonderful ecosystems.

  • And while they might seem really far away, they're not immune

  • to what happens above, nor are we outside their sphere of influence.

  • We're just beginning to understand how important deep reefs are

  • to the ocean and beyond.

  • But already, we know that we rely on the health and wellbeing

  • of all coral ecosystems, whether we can see them or not.

  • And unfortunately, all corals are struggling as a result of human activitieseven those

  • tucked hundreds of meters below the surface.

  • No matter where you are in the world, there's probably a deep reef nearby

  • even if you're in Antarctica!

  • And the animals that build them have a lot in common

  • with their shallow water cousins.

  • Like, both shallow and deep corals are foundational species which create

  • ecosystems and provide vital habitat for a huge variety of creatures.

  • Also, in both cases, what we'd call a singlecoralis actually a colony

  • of identical anemone-like animals called polyps!

  • .

  • The biggest difference between deep sea and shallow water corals

  • has to do with algae.

  • Shallow water corals all have symbiotic relationships

  • with single celled algae known as zooxanthellae.

  • These algae live inside the coral's tissues.

  • And in exchange for a safe place to live and access to plentiful sunlight,

  • they provide some food to the coral through photosynthesis,

  • as well as manage any waste produced by their coral hosts.

  • In fact, they're largely responsible for the corals' vibrant colors.

  • If they leave or are ejected from the coral's mostly clear tissues,

  • we see the hard, white calcium carbonate skeleton below.

  • This is known as coral bleaching.

  • And it occurs when a coral gets stressed;

  • like when the waters they live in become too warm.

  • Shallow corals are so dependent on these algae

  • that if one goes too long without them, it will die.

  • But sunlight quickly disappears the deeper you go.

  • So things are a bit different with deep corals.

  • The shallower deep corals, the ones living between 50 and 150 meters down,

  • do see a bit of sunlight.

  • And they do have zooxanthellae in their tissues.

  • Trouble is, the amount of sunlight that reaches these algae

  • isn't ideal for photosynthesis, which makes it hard for them to hold up

  • their sugar-making end of the bargain.

  • The corals have evolved a nifty workaround though.

  • They have fluorescent proteins in their tissues

  • that convert the available light into more algae-friendly wavelengths!.

  • Still, once you descend past about 200 meters, there's basically zero sunlight.

  • At those depths, not even fluorescent proteins can drive enough photosynthesis.

  • But most deep corals are found below this point!

  • So, the majority of species have ditched their algal buddies altogether.

  • Since they don't have any colorful symbionts,

  • they can't bleach like their shallower cousins.

  • But, that's not to say they're all a boring white!

  • Deep sea corals actually come in a stunning array of brilliant colors.

  • For example, the so-called black corals aren't black in life;

  • they're pink, yellow, orange and red.

  • They get their namesake from the color of their skeletons,

  • which are different from the skeletons of most shallow water corals.

  • Instead of using the white mineral calcium carbonate, they build their skeletons

  • with protein and a fibrous substance called chitin

  • which, when dried out, is black.

  • These chitin and protein skeletons are more flexible than carbonate ones,

  • yet still tough enough to support the animals.

  • So they allow black corals to live where they prefer:

  • in strong water currents that provide them with more food.

  • Other deep corals called octocorals, named for the 8 tentacles on each polyp,

  • are also intensely colored.

  • Like, there's one species that is a stunning shade of purple.

  • These, too, are a little different than your average shallow coral.

  • Most are considered soft corals,

  • which means they don't have a hard internal skeleton.

  • Instead, the polyps are supported by little needle-like pieces

  • of calcium carbonate known as sclerites.

  • No one is fully certain why deep corals are so colorful,

  • but researchers have a theory.

  • While pigments aren't visually helpful in the deep sea,

  • they may be acting as antioxidants and antibacterial compounds.

  • You see, the pigments found in deep corals are carotenoids

  • the same class of compounds often responsible for the brilliant colors

  • in the veggies and fruits we eat!

  • In fact, much like us, corals have to get carotenoids from their diet.

  • So it's likely part of the reason deep corals vary so much in color

  • is that the local snacks contain different pigments!.

  • Now, you might be imagining all of these colorful deep corals growing

  • on top of one another, like shallow corals do.

  • But in the deep, that kind of reef building is the exception.

  • In fact, out of the over 3300 deep sea coral species described so far,

  • there are only six that build that kind of reef.

  • Instead, most are found standing separate from each other, like trees in a forest.

  • They can pepper the seafloor or cling to the sides

  • of underwater structures like seamounts.

  • We still call the habitats they create reefs, though.

  • And just like shallow reefs, they provide essential habitat

  • for ecologically and commercially important species.

  • And some are massive.

  • Like, in 2018, researchers were exploring the Atlantic Ocean off the coast of

  • South Carolina and discovered a deep reef that stretched for 85 miles!

  • Others are much smaller.

  • But even solitary deep corals create oases of life

  • in the desert of the deep seafloor.

  • They're found covered in all sorts of critters

  • like brittle stars, squat lobsters, and anemones.

  • Some sharks lay their eggs on them!

  • Plus, grouper, snapper, sea bass, and rockfish

  • all use deep sea corals as spawning grounds.

  • Even marine mammals like the endangered Hawaiian monk seal get in on the

  • deep reef action, since these habitats are full of their preferred prey.

  • Deeper reefs don't just pop up,though.

  • It can take hundreds of years to establish these vital ecosystems,

  • because deep corals live life in the ultra slow lane.

  • The oldest coral that we currently know of

  • is estimated to be over 4,000 years old.

  • It was just a little polyp when the Sumerians were inventing writing!

  • And that 85 mile long coral reef I mentioned?

  • On reefs like that one, the corals at the top are still young

  • a mere 700 years old!

  • But they built on the skeletons of dead corals.

  • And that structure is likely to be at least 20,000 years old.

  • One of the main reasons these corals grow slowly

  • and live a long time is that they inhabit much colder waters.

  • Below 200 meters, the average water temperature is around 4 degrees Celsius

  • similar to the inside of your refrigerator.

  • And cold slows all sorts of biological processes,

  • including growth, because it literally slows down molecules.

  • And there's also the matter of foodor, I should say, the lack thereof.

  • Because deeper corals don't have algae to help with food,

  • they must subsist on whatever floats by or rains down from above.

  • And that's not always a lot.

  • Many position themselves in strong water currents

  • to get more food more consistently.

  • Still, they tend to get less than shallower corals,

  • which means less energy for growth.

  • And, unfortunately, food isn't the only thing floating down from above.

  • Researchers have found that microplastics are making their way into the deep

  • sea food chain, which seem to be slowing the corals' growth even more.

  • I wish I could say this is the only way our species

  • is hurting deep reefs, but it's not.

  • Because of their beautiful skeletal colors,

  • we've harvested deep sea species to wear as jewelry for hundreds of years.

  • That's led to lower diversity and fewer coral babies in the species we covet most.

  • Also, there are several fishing practices which cause tons of damage

  • to these fragile communities.

  • For instance, during bottom trawling, a massive weighted net

  • is dragged along the seafloor, demolishing entire ecosystems.

  • And since the corals take decades to recover because of their slow growth,

  • the diversity of life they support is also slow to rebound

  • including the productive fishing grounds the trawling

  • was targeting in the first place.

  • Deep corals are largely spared from one of the major threats

  • their shallow cousins face: warming-induced coral bleaching.

  • Like I mentioned before, since they generally don't rely on algae,

  • most deep sea corals cannot bleach.

  • However, the majority are still susceptible to another climate-related threat:

  • ocean acidification.

  • When we put more carbon dioxide into the air, more also ends up in the water.

  • There, it reacts with seawater to form carbonic acid

  • which makes the ocean more acidic.

  • And researchers have recently found that as our ocean becomes more acidic,

  • the skeletons of corals are becoming porous and fragile.

  • This is similar to the weakening

  • observed in human bones undergoing osteoporosis.

  • So, they have coined this process coralporosis,

  • and it's happening in both shallow and deep corals.

  • And it's not just that coral skeletons are becoming weaker