Many of us are familiar with colorful images of tropical, shallow water coral reefs, and
the abundant life they support.
But did you know that deep in the dark, cold depths of the ocean, another type of coral
has flourished for millions of years?
Deep-sea corals, also known as cold-water corals, are found across the globe in waters
deeper than 50 meters where little to no light penetrates.
Like their shallow water counterparts, these corals are considered to be foundation species,
providing habitat, shelter, and food for many other organisms.
Some deep-sea corals form huge mounds over thousands, or even hundreds of thousands of
years, growing upon the skeletons of their predecessors.
Other types of corals form extensive groves and can resemble fans, columns, spirals, or
branching trees.
But these aren't plants.
Corals are actually animals.
The coral structures we see here consist of many individual polyps, a soft-bodied animal
related to jellies and anemones.
These polyps can live alone, but more often hundreds or thousands live together as a colony.
Hi, I'm Liz with NOAA Ocean Exploration.
Deep-sea corals are ancient and long lived organisms.
Some of the oldest colonies date back to the time the last wooly mammoths roamed the earth.
Yet, there's still so much we don't know about them.
To take a deeper dive into the world of deep-sea corals let's talk to an expert.
Today we're joined by Dr. Randi Rotjan, a professor of biology at Boston University
and co-Chief Scientist of the Phoenix Islands Protected Area Conservation Trust.
Welcome Randi.
Hi, thanks so much for having me.
I'm so glad to be here.
I'm wondering if you can start us off by telling us a little bit about your work and how do
deep sea corals fit in?
You know, it's interesting, I've been working on corals for, you know, 20 years or so now
and most of that time has been thinking about shallow water corals, you know, corals that
you're probably more familiar with.
And I really, um very recently started to work in the deep sea.
You know, I really had to sort of think through and learn the language of the deep sea, um,
in order to sort of connect with these coral animals.
Why are deep sea corals so important, both in your line of research and in general?
Corals are important because they, they're the trees that make the forest.
Do you know what I mean?
It’s like, these are the organisms that are the biodiversity multipliers.
These are the creatures that so many other animals flock to and use and so they really
augment and enhance the diversity of life on the deep seafloor.
Deep-sea corals are also important because there may be implications for us, for humans.
You know, we're just beginning to understand some of the ways that they do what they do.
They defend themselves, or have really interesting or different immune systems may have implications,
uh, for medicines or for other therapeutics here on the surface.
It seems like there's a lot we still don't know about deep-sea corals.
Can you explain why that is?
What are some of the challenges of doing your work?
It's really hard to get down that deep.
You know, the deep sea starts at around 200 meters and can go all the way down to 5,000
or more meters deep.
So I would say that, you know, the past 20-50 years have really been a renaissance of technology
that have enabled deep-sea science and deep-sea technology and engineering to get us down
there and explore this part of our planet, which has always been there, right?
It completely precedes us, but we're just getting to learn about it now.
So I love that you mentioned that deep-sea corals pre-date us.
That's something that I find personally really fascinating, but also really hard to understand.
Can you explain just how old are deep-sea corals?
I am so glad you asked that.
It's such a cool timeframe to think about.
Scientists think that corals evolved in the deep sea, at least 400 million years ago.
But the modern, tropical, shallow-water corals that you still see alive on the reef today,
are a much more recent development.
So that's, that's sort of the evolutionary timescale, but then there's the timescale
of the corals themselves, I mean, some individual colonies can live for over a thousand years.
You know, the oldest ones that we know are about 4,000 years old, which is a pretty incredible
thing.
Wait a second, what exactly allows a deep-sea coral, in such an inhospitable environment,
to live for so long?
I think it's kind of truly a marvel that deep-sea organisms are so long lived when they just
don't have a whole lot of food.
Corals and sponges, they're indeterminate growth animals, meaning that they can just
grow and grow and grow.
They don't have a set body size, right?
Whereas humans, you know, we kind of have a range, but we have a set body size.
Because they're so food limited, and they have such a slow metabolism, and because there's
nothing really limiting their growth, and their colonial, right, they can just create
new polyps and new polyps and grow and grow and grow.
They grow slowly but steadily and they can just persist for really long periods of time.
Is there anything else that does eventually limit their growth, or what causes them to
die?
A lot of things can cause them to die, but I think the game of life, right, is to try
to eat and not be eaten.
And I think with deep-sea corals and sponges on the seafloor, that's true too.
You know, they, they can die from stress or change in conditions or, um, you know, a number
of things, but they can also be eaten.
They have to use some of the limited energy they have to defend themselves either physically
or chemically.
So that's not the first time that you've mentioned corals and sponges together.
What exactly is the association between deep-sea corals and deep-sea sponges?
Wouldn't we all like to know?
I think, you know, the interactions between those two organisms, there's something that,
you know, is an active area of study.
But, one of the shared things they have in common, besides both living in this incredibly
different, deep, dark habitat, is the fact that they are some of the earliest animals
on our planet - sponges, even older than corals.
So I know these deep-sea coral and sponge communities are found on seamounts or canyon
walls.
How did they get there in the first place?
You know, corals and sponges move through their next generation.
They move through their gametes.
They produce these larvae, that then are carried by the currents and what's amazing is that
they can get pretty far away from the parent colony.
And they are really at the whim of the current.
And in fact, a lot of these coral gametes get lost in the process.
You know, they just get swept out and never find an appropriate place to settle.
But some of these coral larvae and sponge larvae are transported to a place that's got
the optimal conditions, it's got the right flow.
You know, it has enough food, it's good enough protection and they can settle and start to
grow and that's how they move from place to place, it’s, they send their next generation
off into the world.
That is a brutal way to start your life.
It is a brutal way to start your life, but once you're there, once you're settled, you
can live for a really long time.
So you bring up the idea of food, right?
So they live in these deep, dark locations, they stay in one spot for life.
So what exactly are they eating?
You know, they are eating food from the surface, believe it or not.
We have all of this surface productivity fueled by nutrients and sunlight that fuels the growth
of phytoplankton and it, it you know, there's a whole food chain that cascades from this
growth at the surface.
And as these organisms either excrete, or die and decompose and decay, they start to
fall through the water column.
But as it falls slowly down, it's trapped and caught by sponges, which are filtering
the water or by corals, which have their polyps extended and can capture those tiny particles
and, um, eat them.
So even though they live so far away from the surface, they are surface dependent on
the food that falls gently down.
What we see is this wide diversity of corals and I was wondering if you could describe
that a little more?
You know, even though they're not plants, I often think about flowers and different
botanical strategies, right?
How plants try to capture sunlight because they're trying to maximize their surface area
to get as much sun as they can.
Of course, there's no sun in the deep sea, but corals and sponges are doing the same
thing.
They're trying to maximize their surface area to capture as many food particles as they
can in the position of flow.
So sometimes that's straight up and down as things just filter down and sometimes it's
actually, you know, turned sideways as they're trying to capture particles that are moving
past them.
So I think a lot of the shapes that we're seeing have a lot to do with where these corals
like to position themselves and it's all about trying to capture as much food as they can.
What gives corals their shape?
What helps them stand upright?
There are different kinds of corals.
There are hard corals, scleractinians, that actually produce an aragonite skeleton.
And then there are soft corals, um, octocorals, hexacorals, gorgonians actually have calcite
skeleton, which is not quite the same as, you know, when some of these hard corals,
but it's still strong enough and rigid enough to give them structure.
You know, you want to be strong enough to stand up, um, in the current, but you also
need to be flexible enough to kind of go with the flow so that you don't just break.
I would love if you could elaborate - what are some current areas of research in the
field or unanswered questions about deep-sea corals?
I think the great thing about this field is that there is no shortage of questions.
We are finally at the point where we understand a little bit about these ecosystems, and who
the species are, and sort of how they fit together evolutionarily.
And now I think it's time for some behavior, for us to understand the interactions between
organisms and how they do what they do on the seafloor.
So for me, I think there's many, many, many more lifetimes of study in trying to understand
not just who these corals are, or what they do, but how they do what they do.
One of the most popular questions we often get is - what are the implications of climate
change on the deep-sea corals?
Deep-sea corals and deep-sea ecosystems are definitely going to be impacted by climate
change.
Um, the easiest way to think about this is from the surface productivity that feeds them,
right?
Changes in food production at the surface are going to change the food availability
for corals that live deeper down.
When food falls down, that's also how we sequester carbon.
So our carbon sequestration equation on a planetary scale might change and, um, the
deep sea has a lot to do with that.
You know, in addition, the deep sea is not immune to changes in temperature or ocean
pH.
We might see up to a one degree in the next 80-100 years, and to have an increase of a
degree is a really big deal.
Well, Randi, you've told us a lot about deep-sea corals today, but what I want to know now,
is what do you think is the most interesting thing about them?
That they exist.
I mean, come on!
These are like the most incredibly cool creatures and they just defy imagination in so many
different ways.
And to me, it just feels really important to know that they're here.
They are on the one hand, incredibly resilient, and on the other hand, incredibly dependent,
fragile and reliant on the decisions that we make here on the surface.
I think that's a really powerful message too, to leave us on.
Thank you so much for joining us Randi.
Thank you so much for having me and I hope to learn lessons from these students in the
future, right?
It is all about the next generation.
The world of deep-sea corals is full of contradictions.
They're remote yet reliant on the waters above; important, yet poorly understood; ancient,
but far less studied than most organisms living closer to the inquisitive minds of humans.
What questions do you still have about deep-sea corals and how would you go about answering
them?