Caribbean corals under threat

One of the most endangered marine ecosystems are Caribbean coral reefs that have seen a massive die-off of the main reef-building species
10 December 2013

Interview with 

John Bruno, University of North Carolina


John BrunoHelen -  So, there are many threats facing the oceans around the world from pollution and climate change to over fishing and coastal development.  We need to understand how those threats are having particular effects on marine habitats and species if we're going to find ways of successfully protecting them.

Chris -   Helen, obviously coral pays a very big part in that, but what actually is coral?  I've never really understood these.

Helen -   Corals and reefs, yes.  A lot of people look at a piece of coral and perhaps think it's a dead bit of rock.  But in fact, it's a living animal.  It's a tiny microscopic animal.  A relative of anemones that you might have seen in a rocky pool on the beaches around the British Isles, but very, very much smaller and they secrete calcium carbonate, chalky skeletons essentially and they build reefs.  So, the reefs are built by these colonies of animals and they're very important for various reasons for the biodiversity hotspot that coral reefs are.  But unfortunately, one of the most endangered marine ecosystems we have are the coral reefs in the Caribbean where there's been a massive die off in one of the main species or two of the main species of coral that build the reefs.  One of the main suspects for that die off is the spread of coral disease.  Well, awhile ago, I paid a visit to Abaco Island in the Bahamas and had the rather wonderful job of going diving with Professor John Bruno from the University of North Carolina.

Chris -   Do you ever get all the best jobs?  I don't know how you do it.

Helen -   You know, that's marine biology for you.  You could've done it.  Anyway, I was there basically to find out more about how and why the reefs of the Caribbean are in so much trouble.

John -   So, Caribbean reefs have changed dramatically since the 1980s.  I grew up in South Florida and in the '70s and early '80s, you could snorkel over a coral reef and it was just like a golden wheatfield of coral that's incredibly complex community that the corals had built up.  It was inhabited by fish and all kinds of invertebrates.  So, a really biodiverse ecosystem.  That changed enormously and you don't have to be a scientist to recognise that.  So, the corals have all disappeared.  There are still some left, but they're far less abundant than they were just 2 or 3 decades ago.  There's a variety of causes for that.  The sea is warming, leading to a phenomena called 'coral bleaching' that kills the corals.  There's sediment pollution, in some cases, possibly nutrient pollution.  But here in the Caribbean, the really big driver of coral loss is coral diseases.  One disease in particular, White-band disease, wiped out what were then the dominant species, the staghorn coral and the elkhorn coral.  Their populations have plummeted by 99%.  So just envision kind of like a Carolina or Southeastern US pine forest where the whole forest is made up by a couple of dominant species and then within 12 months, boom, those species are just gone.  It just completely transforms the landscape.  There's certainly other things going on, but when you lose those key stone species, those what we call "foundation species" that built up the whole system, it's fairly easy in this case to point to one primary cause.

Helen -   If we're looking out for diseased corals, what are we looking for?  What are the key signs?

John -   Yellow blotches on some of the corals are tell-tale sign of coral infection, dead portion of the coral crowning where the coral is either white because the tissue has died off or they've lost their symbiotic
zooxanthellae.  Some of the coral diseases are these very clear black bands.  So usually, some sign of infection, either a white or a black band on the coral.  So, the signs of infection aren't subtle.  It's quite obvious that something is different and there's only really half a dozen common diseases in the Caribbean.  So, it's actually quite easy to kind of identify them.

Helen -   Alright.  Well, I think it's time we went in and see what we could find.

John -   So, there's really no question that disease was one of the primary drivers of coral loss in the Caribbean and it's still really a big mystery.  We don't know what many of the pathogens are that cause the disease.  So, we don't know what the bug is, what the infection agent is in many cases.  Even when we do, we don't necessarily know why it's become so common.  And so, it's really kind of hard to nail down whether there's a new pathogen, whether the host is stressed out.  So, you know, when we get stressed out, we become more susceptible to common colds and all kinds of diseases, or if something about the environment changes and either made the host more susceptible or one of those environmental conditions might make the pathogen more - what we call, "virulent".  So, it's more able to attack the host and cause problems for it.  My lab used to focus a lot on nutrient pollution and now we're focused a lot on temperature, how increases in temperature can lead to outbreaks of coral diseases.  I'm also collaborating with a coral immunologist to try to understand how corals essentially fight against disease.  So, corals have very simple immune systems.  Essentially, how they fight off disease and so, some corals were able to fight it off and others aren't.  Just like with people.  Some people are just naturally resistant to colds and others are really susceptible to them.  It's really still a big mystery why we're seeing these increases in diseases.  We really have no idea how to manage them.  So, we're kind of at the mercy of this ongoing phenomena right now.

Helen -   So, we've just got back off our dive.  We're just drying off and what we saw was - to me, it just seem like a landscape that showed what it used to be.  You could see these huge structures that clearly used to be coral.  There were big boulders that are now dead, but maybe with few patches of coral growing on them.  Great big spires and turrets what I think must've been these acropora corals, the staghorn and the elkhorn.  Possibly the elkhorn particularly at that site.

John -   Yeah, it was incredible.  I mean, there's all kind of caves and tunnels.  There was so much structure.  I feel like I was flying through a city from which like humanity had been annihilated.  Didn't it feel like that?

Helen -   It did, yeah.

John -   It was just incredible.

Helen -   There were some fish, but I guess not as much as we were expecting.  Just no corals to speak of.

John -   Yeah, very little coral, but the skeletons are all there.  So, the structure they built up over the last 3, 000 or 4,000 years is all still intact.  It won't be forever.  So, you can see exactly what it was like just a couple of decades ago.

Helen -   We saw one instance of disease.  There was the white patch growing across that piece of montastrea coral I think.

John -   Yeah, that's right.  So, it was called "White Plague" disease.  So, there was a montastrea coral with about 6 or 7 different lobes and all of them were infected with white plague and they're kind of just itching its way across the coral crown and chewing up the tissue.

Helen -   But as a whole, it was this ghostly landscape.  Is there a chance that the corals will come back?

John -   Absolutely.  I mean, there's been no coral extinctions in the Caribbean.  I mean, we're threatened with seeing coral extinctions, possibly even in my lifetime, but all the species are still here.  So, if we manage to turn things around, I honestly think we could be back to where we were.  It may take a century or so.  Yeah, we could see near complete recovery.

Helen -   With Caribbean coral doing so badly in the wild, I paid a visit to the Smithsonian Marine Ecosystems Exhibit at Fort Pierce in Florida where Bill Hoffman took me behind the scenes of the aquarium to see how he's growing some of the most endangered coral species.

Bill -   So, this is cervicornis, that is staghorn coral - very distinct smell which are the terpenoid compounds that they used to kind of protect themselves and make them maybe taste bad to most things.

Helen -   That's the first time I've ever smelled a coral.  That's incredible.  So, they smell.  Amazing!

Bill -   This is an elkhorn coral fragment.  We've started to provide them to researchers and it's pretty hard otherwise to get threatened corals to bring them back into the lab and usually to kill them or to see what kills them.  But I think we're very fortunate to be able to help research as well as show our public some of these threatened species.  This is the way that most elkhorns and staghorn corals in nature are propagated.  In fact, it's known that within a say, 20-meter or so area of a reef, chances are, they're clones.  They're produced this way.  A branch breaks off.  In fact, the branches are even a little bit thinner at the base, maybe to make them a little predisposed to snapping off.  And then if they land in the right spot, they'll just lay on the bottom.  You can see this one is dying on the bottom where it was laying this way, but what you can also see are branches growing up.  If it wasn't being picked up on a regular basis, it would start to attach on the bottom side.  You can see that we use epoxy.  Sometimes we even use super glue to attach them to the rocks.

Helen -   We've come down to the aquarium to have a look at of what's on display for the public and having just got back from the Bahamas where I was diving on wild reefs, this one is a completely different scene.

Bill -   I can't speak of the Bahamas but in Florida, I sadly tell people that you'll see more corals in this aquarium than you would during an entire dive, or at least for sure, more diversity of corals than you would see on a dive.  And that was truly a sad state.  I think if people had better appreciation for the diversity and complexity of marine ecosystems and why they are important.


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