Revealing The Largest Deep-Sea Coral Reef In The World

12:14 minutes

A 3-D graphic elevation model of an undersea surface.
A map showing the topography of the Stetson Mesa north region of the Million Mounds, an important habitat supporting a diversity of marine life. Credit: NOAA

Scientists recently discovered the largest known deep-sea coral reef in the world. It’s called Million Mounds, and it stretches from Miami, Florida, to Charleston, South Carolina, covering around 6.4 million acres of the seafloor.

A large patch of white coral, looking like a pile of tumbleweeds, sit at the bottom of a dark ocean.
Dense fields of Lophelia pertusa, a common reef-building coral, found on the Blake Plateau knolls. The white coloring is healthy—deep-sea corals don’t rely on symbiotic algae so they can’t bleach! Credit: NOAA

Unlike the colorful reefs found in sunlit tropical waters, this one is mostly made up of a stony coral that’s usually found from about 650 to 3,300 feet underwater—depths where it’s very cold and pitch black.

Ira Flatow talks with Dr. Erik Cordes, marine biologist and professor at Temple University in Philadelphia, Pennsylvania, who collaborated on the study. They discuss what makes deep-sea corals different from those found in shallower waters, why it’s important to map them, and what it’s like to visit one in a submarine.

Further Reading

  • See some underwater photos of NOAA’s expeditions through the Million Mounds ecosystem via their website.

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Segment Guests

Erik Cordes

Dr Erik Cordes is a marine biologist and professor at Temple University in Philadelphia, Pennsylvania.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, sickle cell treatments, rethinking fingerprints, and a honey computer.

But first, scientists recently discovered the largest known deep sea coral reef in the world. It’s called Million Mounds, and it stretches all the way from Miami to Charleston, South Carolina. It’s mostly made up of a stony coral that’s usually found deep, deep down from about 650 feet to 3,300 feet deep. And when you’re that far underwater, it is very cold and very dark and very unlike the colorful tropical coral reefs we’re used to seeing shining in sunlit waters.

So how different are deep sea corals? Why map them? What can we learn from them? Let’s find out. Dr. Erik Cordes is a marine biologist and professor at Temple University in Philadelphia. Welcome to Science Friday.

ERIK CORDES: Thanks, Ira. I’m glad to be here.

IRA FLATOW: I’ve been around corals my whole life as a scuba diver. Don’t they need sunlight for the photosynthetic zooxanthellae that live in their tissues and feed them?

ERIK CORDES: Yeah, most people think of those when you’re talking about corals, but the truth is that there are actually more species of corals in deep water without those photosynthetic symbionts than there are in shallow water.


ERIK CORDES: Yeah. That’s true. And based on some of this recent work, we’re now learning that the coral reefs that exist in the deep ocean are actually more widespread and more common than shallow water coral reefs.

IRA FLATOW: That’s amazing. I had no idea. So how do you get down there and describe for me what you see when you get down there.

ERIK CORDES: Well, there’s a few different ways to get down there. My favorite way, of course, is in a crewed submersible where you would get inside and take a ride down to these depths. We got to survey a few of these sites that were discussed in this paper for the first time using the Alvin submersible from the Woods Hole Oceanographic. It’s probably the most famous submersible in the world. It was the one that dove on the Titanic, the one that discovered the hydrothermal vents for the first time–

IRA FLATOW: And you got to ride on it. Wow.

ERIK CORDES: Yeah, I’ve been lucky enough to ride in it a few times, and actually visiting this site was one of my favorite and most memorable dives of all time.

IRA FLATOW: What does it look like?

ERIK CORDES: So we started about 750 meters, and we climbed up almost 100 meters. That’s 300 feet or about the height of the Statue of Liberty during this dive. And we were on a coral mound the entire time.

So these corals form this sort of loose framework. They lay down their skeleton just like the shallow water corals do. It’s calcium carbonate, and it builds up over thousands, tens of thousands, in some cases, hundreds of thousands of years to this massive mound that is just topped with this beautiful thriving white coral. The different coral species that live down there can be red or purple or all sorts of different colors. The one that creates this habitat, the main builder of these reefs, is called lophelia, and it’s naturally white.

IRA FLATOW: That is cool. What’s the job of these deep sea corals in this ecosystem?

ERIK CORDES: They are reliant on the food that rains down from the surface. It’s actually snowing down is more accurate. We call it marine snow because it looks like this gently falling snow when you’re down there. And that’s bringing all the food from the surface down to feed the base of the food web in these systems.

So they’re feeding on this material and the little crustaceans that are feeding on that little shrimp and these things called amphipods that are swimming around, and then that goes up to small fish and then even larger fish. And we have this whole big food web down there. They’re taking this food that’s snowing down from the surface, and they’re processing it. They’re sequestering a lot of the carbon that comes from surface waters down in the deep sea to build these massive reefs, and they’re recycling a lot of nutrients that go back into the water. And this is right below the Gulf stream, so those nutrients go up into the Gulf stream and are carried away and fuel surface productivity downstream and produce the nutrients that we need to feed the fish in shallow waters that we go out and catch.

IRA FLATOW: What kinds of animals do you see around there? What kinds of fish?

ERIK CORDES: Well, there’s lots of little crabs and worms and everything that are hiding in the choral framework just like you would see when you’re scuba diving on a shallow reef. But every once in a while, you get one of these larger fish coming through. During my dive and Alvin, the sub was actually attacked by a swordfish.

IRA FLATOW: Really? What do you mean? A swordfish tried to attack the sub?

ERIK CORDES: Yeah, that happens from time to time with all this light and noise that you’re creating in the submarine. It startles them, and they are defending their turf. And they can actually hurt themselves attacking the submarine. So the pilot, my friend Bruce Strickrott, was actually quick enough to give the swordfish a little tap on the tail with the manipulator to scare him off, and he went swimming away very, very rapidly after that.

IRA FLATOW: I’ll bet. You didn’t expect to see a big thing like you there.


IRA FLATOW: But your work was involved mostly in revealing just how large Million Mounds is, and was it surprising to see just how big it was?

ERIK CORDES: Oh, it was totally amazing to see how big this site is. We’ve known that there were corals in this area that we refer to as the Blake Plateau between Florida and South Carolina. We’ve known that there were coral sites there for a few decades going back even about 100 years, but we had no idea until we got to use some of the modern mapping tools that we now have that we were able to see just how extensive it was. And we thought of these as individual separate coral sites in this patchwork and now we understand that they’re all completely connected and it’s just this one continuous coral habitat for 100 miles.

IRA FLATOW: I can picture how you map the land. How do you map a coral reef?

ERIK CORDES: Yeah. So you can map the land from satellite, and you can map the surface of the moon or Mars from satellites. Those maps are actually higher resolution than our map of the ocean floor.


ERIK CORDES: Yeah. So the only way to get a good high-resolution map of the ocean floor is to actually take a ship out there and run it back and forth like you’re mowing the lawn and collect sonar information. So now we have what’s called a multibeam echosounder, so instead of just one burst of sound that you’re sending to the seafloor, we can send dozens of them at the same time and then tell how long it takes for that sound to get from the ship down to the sea floor and bounce back. And then we can tell the distance, and that can give you a good three dimensional view of the seafloor.

IRA FLATOW: Why is the reef called Million Mounds? Are there a million mounds?

ERIK CORDES: Not quite a million but there are over 10,000 mounds. One of the cool things that was done in this study was Derek Sowers– is the lead author. As part of the mapping effort, he compiled all of those data into one big map, and then he wrote an algorithm that could count those peaks because you could imagine counting 10,000 peaks by hand would take quite a while. So this algorithm that he put together can tell the difference between a mound and a longer ridge feature and the slopes of a mound versus the peak. And we went out with the submersibles to try and test some of those ideas.

IRA FLATOW: Is this part of a bigger effort to map the seafloor? We really don’t know very much about what’s really down there deep, do we?

ERIK CORDES: No, that’s true. This is a part of a much bigger effort to map the seafloor. It’s called Seabed 2030, and it’s an international effort to get a complete map of the ocean floor by 2030. It’s a massive undertaking. You’re trying to map the entire planet, and we’re at about 25% of the oceans mapped right now.

IRA FLATOW: Just– just 25%? Yes.

ERIK CORDES: Just 25%. Yeah, that’s right. But that’s pretty good because only a few years ago, we only knew what about 5% of it looked like. So these maps are getting better and better, and we’re actually on pace to achieve this mapping effort of the entire globe by 2030.

IRA FLATOW: Now the coral reefs near the surface where we live are being affected by climate change. Is that true of these deep sea corals?

ERIK CORDES: They are definitely vulnerable to climate change. We’ve watched the temperature rise in the deep ocean, and it’s actually absorbed a lot of the heat from the shallow ocean and done us a favor by building a big reservoir of that heat and keeping it out of the atmosphere. But we’re starting to document what are referred to now as marine heat waves that are going by, and we saw some of these heat waves come by where the temperature would go from 40 to 55 degrees Fahrenheit over a couple of hours.

And that doesn’t sound too extreme to us, but these corals live in such a constant environment in the deep ocean that that kind of a temperature change is a big shock to them. And they might be able to deal with one or two of those heat waves, but as they increase in frequency and start affecting them more often, then they get more and more stressed out.

IRA FLATOW: For most people, these corals are out of sight and maybe out of mind because we don’t see them. Tell us why we should care so much about these reefs.

ERIK CORDES: People really are not aware that there are deep sea corals. That’s something even when I was in college that I didn’t know. I learned about that much later, and it is really amazing how widespread they are. They do a lot of the same things that shallow water coral reefs do for us, and they host this really high biodiversity. And there are all sorts of natural products that come out of the marine environment that we’re looking at for anti-cancer drugs and antibiotics and all sorts of other pharmaceutical purposes, but they’re also regenerating all of the nutrients that fuel the entire Gulf Stream ecosystem. A lot of the nitrates and phosphates, the things that are in fertilizer, that’s what these coral reefs are giving off, and we measured higher concentrations of those nutrients over the reef than we’ve measured anywhere else in the North Atlantic.

IRA FLATOW: Interesting.

ERIK CORDES: They are really fueling that system, and because we now are starting to understand that these deep water coral reefs are even more widespread than shallow water coral reefs, it just shows you how important they really are to the global ecosystem and these global cycles that we’re really just beginning to learn about.

IRA FLATOW: Well, Dr. Cordes, you have certainly taught us a lot. I’ve been scuba diving for years and had no idea about these deep water corals. So thank you.

ERIK CORDES: Yeah, you’re welcome.

IRA FLATOW: Dr. Erik Cordes is a marine biologist and professor at Temple University. That’s in Philadelphia, Pennsylvania.

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