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The Microbe That Invaded Caribbean Coral Reefs

Think of giant pythons from southeast Asia, ending up in the Florida everglades and suffocating any small mammal they could find. Think of cane toads from South America, relentlessly marching over Australia, swallowing bird eggs and native frogs. Think of rats from pretty much any mainland country, stowing away onto pristine islands and eating their way through the helpless local birds. These are all classic examples of invasive species.

Here is another, and it’s very different. It’s a microscopic alga called Symbiodinium trenchii. Unlike the python or the cane toad or the rat, this tiny brown bauble seems fairly benign—even beneficial. It lives in the cells of corals and provides them with food, by harnessing the sun’s energy to make sugars. It typically does this in its native waters in the Indo-Pacific Ocean. But somehow, it recently found its way to the Caribbean, on the other side of the world. And there, it displays all the characteristics of an invasive species.

Tye Pettay from Pennsylvania State University has shown that S.trenchii has spread through a large number of Caribbean corals. It provides its hosts with nutrients but is less generous than the native coral-associated algae that is has displaced. It is especially common in populations that have been ravaged by heat or pollution or disease. It looks for all the world like an opportunistic infection, of the kind that takes hold in people whose immune systems have been weakened. “It’s all over the Caribbean and it’s not going away,” says Todd LaJeunesse, who led the study.

Colony of the symmetrical brain coral, Pseudodiploria strigosa, is a common reef–building coral in the Caribbean. When under stress, these animals may harbor a species of mutualistic microbe, which was recently discovered to be introduced from the Indo-Pacific. This microscopic symbiont, a dinoflagellate, increases the stress tolerance of native corals, but may significantly reduce the animal’s rate of calcification, thus diminishing their role as reef builders (coral image by Robin T. Smith, Science Under Sail. Symbiodinium mastigote image was provided by Sung Yeon Lee and Professor Hae Jin Jeong, Seoul National University).
Brain coral, Pseudodiploria strigosa (Credit: Robin T. Smith, Science Under Sail) with Symbiodinium algae (inset; credit: Sung Yeon Lee and Hae Jin Jeong, Seoul National University).

Corals are stinging colonial animals that form partnerships with many species of Symbiodinium algae. These allies—these symbionts—provide them with the energy they need to construct their impressive reefs. But if oceans get too hot, the corals evict their symbionts, losing a source of both energy and colour. That’s why they are said to be “bleached”. If they stay too long in this condition, they die. Solitude is no life for a coral.

But there’s a way out. Some Symbiodinium species make their coral hosts more tolerant to heat and other stressful conditions. If a coral can swap its algal partner for a hardier one, it could survive.

This is what happened in 2005. That year, the Caribbean experienced exceptionally high temperatures; as a result, more than 80 percent of its corals bleached. It was a catastrophe, but for S.trenchii, it was an opportunity. In the months before the bleaching event, LaJeunesse found this species in less than 1 percent of the corals. During the event, he saw it in 20 percent of them. “We found it in the most severely stressed animals,” he says. “We have never seen it behave like this elsewhere.”

Through almost a decade of work, LaJeunesse’s team has confirmed that S.trenchii does indeed behave very differently in separate parts of the world. In the Indo-Pacific, it exists as a genetically diverse population, which has probably been around hundreds of thousands of years—if not millions. It is not alone, either. S.trenchii is part of a lineage of Symbiodinium called “Clade D”, which arose in the Indo-Pacific Ocean and diversified into many species, each of which associates with certain types of coral.

In the Caribbean, things are very different. S.trenchii is the only member of Clade D around, and it lives inside a wide variety of coral hosts. This population has absurdly little genetic diversity. Even across hundreds of kilometres of oceans, individual cells of S.trenchii are almost identical. It’s like looking at a sea of clones.

S.trenchii must have hitched a ride to the Caribbean on some kind of ship, perhaps as recently as a few decades ago. It then spread across the entire region, perhaps taking advantage of the rough times that the local corals were experiencing. “The poor Caribbean has been trashed with sea surface temperature anomalies and pollution and a huge human population,” says LaJeunesse. “It’s severely degraded. If it were pristine, if we went back 100 years, I’m not sure S.trenchii would be so successful.”

But we can’t go back, and it is successful. “It is what it is,” says LaJeunesse. “There’s nothing we can do about it.” And indeed, we might not want to do anything about it. There’s a tendency to view all invasive species as villains, but they’re not all like cane toads or Burmese pythons. There’s some evidence that these invaders can have positive effects on their new homes. Take S.trenchii. You could argue that its invasion benefited the corals, by allowing them to weather the warm spell of 2005.

This silver lining comes with a cloud. Pettay’s experiments showed that S.trenchii is a more selfish partner than the native algae of the Caribbean. It produces just as much sugar as its peers, but it hands over much less to its coral partners. As a result, the corals can only build their rocky reefs at half their usual rate. It might be better for them to have a stingier partner than no partner at all, but in the long-term, incompatibilities with S.trenchii might ultimately harm them and the reefs that they build. For now, no one knows where the balance of benefits and risks lies.

LaJeunesse wants to find out. He wants to know how S.trenchii affects the growth rates of its coral hosts. Also, what makes it such a good invader? And why is it hardier than other related species? “It’s the only member of Clade D whose genome has been duplicated,” he says. “It’s speculative, but maybe that’s something to do with it.”

He also wants other coral biologists to pay more attention to the microbial side of the coral-algal symbiosis. Many of them talk about corals “choosing” or “shuffling” their symbionts, as if the symbionts were passive halves of their own partnership. “It drives me crazy,” he says LaJeunesse. “I think people work on corals because they like corals, so they take the host’s point of view.”

But the symbionts are incredibly complex, too. They aren’t just bacteria; they are very complex organisms. They have as much DNA in their cells as you do in yours, and even more genes. Humans and corals have just over 20,000 genes in their genome; Symbiodinium has between 40,000 and 50,000, and we have no idea what around half of those do. “Who is the master of this house?” asks LaJeunesse.

Reference: Pettay, Wham, Smith, Iglesias-Prieto & LaJeunesse. 2015. Microbial invasion of the Caribbean by an Indo-Pacific coral “zooxanthella”. PNAS http://dx.doi.org/10.1073/pnas.1502283112

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Half a Billion Years of Suicide

For us to live, parts of us must die. Every day, billions of our cells shrink, break up into small parcels, and get tidied away by other janitorial cells. This gentle, organised cellular suicide is called apoptosis, and we depend upon it. Our hands start off as solid lumps; it’s apoptosis that sculpts our fingers by killing off the cells in between them. Now and then, our cells threaten to grow out of control; it’s apoptosis that stops them from becoming tumours.

There are many ways of triggering apoptosis, and one route involves two large groups of proteins: the tumour necrosis factors (TNFs), and the receptors that they stick to. When they meet, they set off a chain reaction inside the cell. A large network of proteins is recruited, united, and activated, until the cell eventually dies. Think of TNF as a key twisting in the lock of a door, triggering a Rube-Goldberg machine that ends with the entire room catching fire.

Now, Steven Quistad from San Diego State University has discovered that corals—small tentacle animals that build mighty reefs—have their own TNFs and TNF receptors. Compared to our versions, these coral proteins are made of slightly different building blocks, but they fold into very similar three-dimensional shapes.

In fact, these shapes are so similar that the coral proteins are interchangeable with ours. A coral TNF can persuade our cells to kill themselves by sticking to our receptors. Likewise, human TNFs can kill coral cells by sticking to their receptors. We last shared a common ancestor with corals around 550 million years ago. Our respective lineages have been diverging ever since but our keys fit in their locks, and vice versa.

“[That’s] amazing”, says Marymegan Daly from Ohio State University. “I think this highlights that at some level, animal cells are animal cells. The differences among animals are in the ways that the cells are organized rather than in how they work.”

Quistad made his discovery after analysing the recently sequenced genome of Acropora digitifera, a coral that looks like a mound of miniature Christmas trees. And to his surprise, he didn’t just find TNFs, he found lots of them. We have genes for 18 different TNFs and 29 corresponding receptors, and the coral has a similar number—13 TNFs and 40 receptors. “That’s more receptors than anyone had ever seen in any organism,” says Quistad.

And why is that surprising? Because “they were expected to have just one,” he says.

Two of the animals that biologists have studied most intensely—the fruit fly and the nematode worm—have just one TNF and one TNF receptor each. Based on this, biologists deduced that ancestral animals were similarly poorly stocked. In back-boned animals, this lone pair of proteins diversified into the big families that we humans possess.

But the corals refute this story of simple beginnings. They belong to one of the earliest animal groups—the cnidarians. They’re even more ancient than the last common ancestor of humans, flies and worms. If they have a large number of TNFs and TNF receptors, that must have been the initial status quo. Flies and worms then lost the vast majority of this original diversity. Corals and humans kept and perhaps even expanded upon that old repertoire, but all the while keeping the same lock-and-key interactions. After all, apoptosis is so important for so many aspects of our lives that it is not easily tweaked.

Could corals and humans have evolved our TNF families independently? It’s unlikely, given how compatible the two proteins are. The fact that coral TNFs can kill human cells points to a shared ancestry—and a very deep one to boot.

“This is part of a really cool shift that’s happening in evolutionary biology,” says Quistad. “We’ve learned a lot from flies and worms, but they have led us to these erroneous conclusions about the evolution of all animal life. The assumption has been that older things should be simpler. If we saw something in flies and worms, it should be even simpler in a more ancient organism like a coral. But corals are actually more similar to humans in multiple ways, and flies and worms turn out to be very strange animals.”

Quistad’s work adds to the evidence that the machinery for apoptosis is at least half a billion years old, dating back to the origin of many of the animal groups we know today. “These main players in the immune system were already there, and everything has been tinkered with since that time,” he says.

Thomas Bosch from the University of Kiel notes that other scientists have already found proteins involved in apoptosis in cnidarians. For example, in 1999, Charles David and Angelika Böttger found caspases in a green, tentacled cnidarian called Hydra. If TNFs help to set cells down the road to death, caspases are the executioners that actually carry out the sentence. “Programmed cell death was one of the key inventions in the evolution of animal multicellularity,” says Bosch.

Coral reefs around the world are in decline, and a third of reef-building corals are in danger of extinction. If they vanish, countless species would disappear too, and coastal communities would suffer a huge economic blow. But we’d also lose valuable clues about the origins of the animal kingdom. These ancient and supposedly simple creatures can give us insights about the origins of the animal kingdom, in ways that more familiar workhorses like flies and worms cannot. “Corals have a lot more to teach us about how our own immune systems work and where they came from,” says Quistad.

Reference: Quistad, Stotland, Barott, Smurthwaite, Hilton, Grasis, Wolkowicz & Rohwer. 2014. Evolution of TNF-induced apoptosis reveals 550 My of functional conservation. PNAS http://dx.doi.org/10.1073/pnas.1405912111

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Will we ever… lose all our corals?

Here’s the 14th piece from my BBC column

John Bruno remembers swimming through Florida’s coral reefs as a child in the 1970s. At the time, the reefs were dominated by elkhorn and staghorn corals, whose vivid, branching structures provided shelter for a smorgasbord of marine life. “It was like snorkelling over a wheatfield – vast, golden coral coverage as far as you could see,” says Bruno, now a marine biologist at the University of North Carolina.

Such days are gone. In just a few decades, the Caribbean’s reefs have collapsed. Golden beds of elkhorns and staghorns have disappeared and been replaced by thick mats of green algae. The proportion of the reef covered by live coral has plummeted from 50% in the 1970s to just 8% now, changing the fish communities dramatically. “Florida was a scary place to snorkel then, with hammerhead sharks, groupers and sailfish,” says Bruno. “Now, it’s like snorkelling in an aquarium.”

It’s not just the Caribbean. A third of reef-building corals are in danger of extinction, and reefs the world over are in serious decline. Even Australia’s Great Barrier Reef, long held as a shining testament to careful management, has lost the majority of its coral. “Ten years ago, we thought, ‘At least we have the [Great Barrier Reef]’, but even that’s starting to look pretty grim,” says Bruno. The question now is not whether things will get worse – they assuredly will – but whether we will lose our reefs entirely.

This is a chilling prospect. To lose the reefs would be to lose the planet’s most diverse ecosystems – habitats that make even tropical jungles seem impoverished. “I’m not dissing rainforests, but you could walk for kilometres and see a thousand beetles and a hundred birds, all variations on a theme,” says Rick MacPherson from the Coral Reef Alliance. “But in one square metre of a reef, you could get every animal phylum known.”

People would also suffer. More than 450 million people live close to coral reefs and rely on them as sources of tourism revenue and protein, and as buffers that dampen the energy of incoming storms. “There are humans that depend on them for a daily basis not just as a nice place to visit for a holiday,” says MacPherson.

One-two punch

Coral reefs are besieged on many fronts. “It’s not like trying to protect rhinos, where we know that the cause of the decline is poaching,” says Bruno. “There are so many things happening and at such big scales that local managers are largely powerless to do anything.”

Rampant overfishing removes herbivorous fish that keep competitors like seaweed and algae at bay. Cyclones and hurricanes physically batter the reefs, as do sticks of dynamite thrown by fishermen. Diseases, some of which are exacerbated by bacteria carried in human sewage, kill them off. The voracious crown-of-thorns starfish – an evil-looking sunburst of spikes – liquefies them with its extruded stomach. Agricultural run-offs flood the oceans with nutrients, spurring the growth of algae and plankton that choke the waters and block out sunlight. Coastal construction projects cut down trees that hold topsoil together, allowing rain to wash sediment into the reefs, smothering the corals.

But climate change is the “big bad”, according to McPherson. The greenhouse gases that we pump into the atmosphere create an insulating blanket that warms the seas along with the rest of the planet. In warmer water, corals expel the algae that live inside their tissues and produce nutrients by harnessing sunlight. Without these lodgers, the corals lose their energy supply and their bright colours, becoming bleached and weak. Meanwhile, carbon dioxide also dissolves in the oceans, making them more acidic and depleting the carbonate ions that the corals need to build their limestone fortresses. They dissolve faster than they can be rebuilt. Hit by the one-two punch of hotter and more acidic waters, the corals, homeless and starving, become more vulnerable to the other threats they face.

Last year, ecologist Roger Bradbury provided a bleak outcome in an opinion piece for the New York Times, saying: “There is no hope of saving the global coral reef ecosystem.” It was a controversial claim, and many other coral scientists see less doom and gloom. “The trajectory is one of decline across the board, but I see areas of incredible resilience in even the most severely hit ecosystems,” says McPherson. “There are so many places you can go where reefs, if not bouncing back, are at least holding their own in a suppressed state.”

Not all corals, or all reefs, are the same. Some, like those in American Samoa, have genetic advantages that allow them to thrive in shallower warmer waters, and some can recruit strains of algae that tolerate higher temperatures. Others grow in waters that are naturally acidic, where carbon dioxide seeps from the ocean floor. Corals can acclimatise to changing conditions, and there’s some evidence that reefs which bleach extensively for one year are better able to handle warmer waters a decade later. And reefs can change at the community level, shifting from sensitive species like the elkhorns to sturdy, robust ones like big boulder corals.

Buying time

Humans can help, particularly by setting up marine protected areas – underwater national parks – where fishing is verboten. Not only do they allow local reefs a chance to recover, but they can seed nearby areas with coral larvae. “That’s our best weapon in our arsenal right now for coral conservation,” says MacPherson, “but they need to be managed.” While a quarter of the world’s reefs already lie in marine protected areas, many are protected on paper only. Their restrictions have to be actively enforced, and they need strong support from local communities. All of this takes money, education, and expertise.

But in the end, the fate of coral reefs comes down to global warming. “We stop climate change, or that’s that,” says Bruno. “That’s a precondition for conserving almost everything.” Setting up marine reserves, fighting coral diseases, reducing pollution… all of these measures are about buying time.

And with enough time, corals prove to be remarkably resilient. They have lived through several climatic fluctuations, including waters that were far warmer and more acidic than those today. Over geological timescales, corals have endured at least five severe crises, but have never been entirely wiped out. Small populations survived in refuges to restart their rocky kingdoms during more hospitable climates.

The difference, of course, is that these ancient changes played out across millennia, whereas we are causing similar upheavals within the space of decades. Based on the latest report from the Intergovernmental Panel on Climate Change (IPCC), it is entirely plausible that the oceans might be three or four degrees Celsius warmer by the end of the century. “Honestly, if we got to that, most reefs would be toast,” says Bruno. “Usually, one degree of warming is enough to cook and kill most corals. There are very few species that could survive three or four.”

However, those figures are a global average. Not every patch of water will warm equally, and some cold areas may even become more conducive to corals. “We’d lose a lot of what we have,” says Bruno, “but we probably wouldn’t lose everything and we’d gain reefs in some places. We could get close to a world without corals… but reefs aren’t going to go extinct, probably under any global scenario.”

Corals, after all, live in a fine mosaic of salinity, temperature and light, where adjoining areas experience different conditions to their neighbours. It’s that patchiness that creates abundant niches for life, and has turned reefs into engines for evolution. And it’s that patchiness that might help to save them, even if we cannot. “That is my rationale for hope,” says MacPherson.

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Corals Summon Gardening Gobies to Clean up Toxic Seaweed

For corals, gardening’s a matter of life and death. Corals compete with algal seaweeds for space, and many types of seaweed release chemicals that are toxic to corals, act as carriers for coral diseases and boost the growth of dangerous microbes. These dangers require close contact—the seaweed poisons won’t diffuse through the water, so they need to be applied to the corals directly. And that gives the corals an opportunity to save themselves. When they sense encroaching seaweed, they call for help.

Danielle Dixson and Mark Hay from the Georgia Institute of Technology have found that when Acropora corals detect the chemical signatures of seaweed, they release an odour that summons two gardeners – the broad-barred goby and redhead goby. These small fish save the corals by eating the toxic competitors. In return, one of them stores the seaweed poisons in its own flesh, becoming better defended against its own enemies.


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Disease from human sewage is killing Caribbean corals

Forty years ago, the elkhorn coral was one of the most common species in the Caribbean. Five years ago, it was listed as critically endangered. The coral’s woes are many but, aside from the warming temperatures, predators and storms that affect all corals, the elkhorn is also plagued by a highly contagious malady called white pox disease. White lesions erupt all over the coral’s branches, representing areas where its animal tissue has wasted away to leave the white skeleton.

Now, Kathryn Patterson Sutherland from Rollins College in Florida has discovered the cause of white pox disease, and it’s an unexpected one – us. We have literally landed the elkhorn in s**t.


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World’s 2nd deadliest poison, in an aquarium store near you

In 2007, a man from Woodbridge, Virginia was rushed into hospital after inhaling an aerosolised version of one of the deadliest poisons on the planet. He was not the victim of a terrorist attack. He wasn’t working in a biohazard laboratory. He was trying to clean out his fish tank.

The man, who posts on the Reef Central Forums as Steveoutlaw, was trying to get rid of a colony of zoanthids – a relative of corals and sea anemones – that was infesting his aquarium rocks. He had heard that boiling water would do the trick. When he tried it, he accidentally inhaled some of the steam.

Twenty minutes later, his nose was running and he had a cough. Four hours later, his breathing was laboured and he was headed to the emergency room. By the time he arrived, he was suffering from severe coughing fits and chest pains. He was stabilised, but he developed asthma and a persistent cough, and had to use steroids and an inhaler for at least two months.

The reason for his sudden illness was palytoxin, a speciality of zoanthids, and the second deadliest poison in the natural world. One gram of the stuff will kill more than a hundred million mice. This poison, liberated by the boiling water, had risen into Steveoutlaw’s airways in a cloud of steam.


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Baby corals swim home by following the sounds of reefs

Coral_larvaeIt’s the open ocean, and a small animal is swimming home. Listening out for the hustle and bustle of a coral reef, the creature changes direction and heads straight towards the sound. If it eventually arrives at its destination, it will settle down and add to the reef’s mighty structures. This intrepid traveller is a baby coral.


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Overfishing gives toxic seaweeds an edge in their competition with corals

Coral_reefThe world’s coral reefs are disappearing. At least a third of the world’s reef-building species face extinction and in the Caribbean, the average cover of hard corals has fallen by around 80% in the last thirty years. The rich habitats they create are giving way to simpler, less vibrant communities, dominated by seaweeds. But seaweeds aren’t just opportunistic colonisers of waters abandoned by corals – they are coral-killers themselves.

Douglas Rasher and Mark Hay from the Georgia Institute of Technology have found that grazing fish typically keep seaweeds in check. If those fish start disappearing, as they often do because of human hooks, the seaweeds run rampant and corals suffer. Anywhere between 40-70% of the most common seaweed species release compounds that drive away the algae that allow corals to derive energy from the sun. Bereft of energy, the corals ‘bleach’ and die. The message is clear – through overfishing, we are accomplices in seaweed-mediated coralcide.

Seaweed” is a loose colloquial term for a wide variety of algae, which hail from a few different kingdoms of life. For a while, it’s been clear that they can compete with corals for the same patches of ocean, but the exact nature of that competition has been controversial. To settle the debate, Rasher and Hay set up field experiments in two different reefs, one in Fiji and one in Panama. In both cases, they pit the common coral Porites porites against seven common species of local seaweed. The competitors were lashed against one another using a grid and some rope and left in place for 20 days.

At such prolonged close quarters, the corals became heavily bleached compared to others that were seaweed-free. Their ability to photosynthesise was shot by anywhere from 52 to 90%. Only the parts that actually touched the seaweeds were harmed; the areas on the sides stayed healthy.


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Clock gene and moonlight help corals to co-ordinate a mass annual orgy

This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.

Every month, at the full moon, tourists and students gather on the beach at Koh Phangan, Thailand for a night of booze, dancing, and debauchery. But the moon-themed antics of these party-goers look positively tepid when compared to those of the Great Barrier Reef‘s corals. With the help of two genes and a spot of moonlight, the corals synchronise one of the greatest spectacles of the natural world – a mass annual orgy.

When it comes to sex, corals play a numbers game. Encased in their rocky shells, direct contact is out of the question so they reproduce by releasing millions of eggs and sperm directly into the surrounding water.

This strategy only makes sense if all the corals release their sex cells en masse and sure enough, every individual within a third of a million square kilometres of reef does so during the days after the October full moon.

The corals’ co-ordination would put even the most organised flash-mobs to shame and until now, scientists had no idea how they did it, especially with neither eyes nor brains. Aside from the obvious contribution of moonlight, the only other available clue was that corals seem to be especially sensitive to blue light.


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Bleached corals recover in the wake of hurricanes


Blogging on Peer-Reviewed ResearchIn 2005, corals in the large reef off the coast of Florida were saved by four hurricanes. Tropical storms seem to be unlikely heroes for any living thing. Indeed, coral reefs directly in the way of a hurricane, or even up to 90km from its centre, suffer serious physical damage. But Derek Manzello from the National Oceanic and Atmospheric Administation has found that corals just outside the storm’s path reap an unexpected benefit.

Hurricanes can reverse coral bleaching by cooling surrounding waterHurricanes can significantly cool large stretches of ocean as they pass overhead, by drawing up cooler water from the sea floor. And this cooling effect, sometimes as much as 5°C, provides corals with valuable respite from the effects of climate change.

As the globe warms, the temperature of its oceans rises and that causes serious problems for corals. Their wellbeing depends on a group of algae called zooxanthellae that live among their limestone homes and provide them with energy from photosynthesis. At high temperatures, the corals eject the majority of these algae, leaving them colourless and starving.

These ‘bleached’ corals are living on borrowed time. If conditions don’t improve, they fail to recover their algae and eventually die. But if the water starts to cool again, they bounce back, and Manzello found that hurricanes can help them to do this.


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Worrying slowdown of coral growth in the Great Barrier Reef

Blogging on Peer-Reviewed ResearchIt’s not a good time for corals. Last year, a third of coral species went straight into the endangered lists after being assessed for the first time, and it looks like 2009 isn’t going to bring any reprieves to the doom and gloom. In particular, a new study provides hard evidence that the mightiest of coral super-colonies – the Great Barrier Reef – is in trouble.

Like reefs across the world, the Great Barrier Reef faces many threats, including pollution, physical destruction, predatory starfish and perhaps most importantly, the many effects of climate change. Glenn De’ath and colleagues from the Australian Institute of Marine Science have found that the corals among this greatest of reefs are starting to yield under these multiple assaults, adding new material to their limestone skeletons at ever-declining rates. The Reef’s growth is slowing to a worrying degree, the likes of which are unprecedented in at least the last 400 years.

De’ath’s group focused on one group of corals called Porites. They are a widespread and important group, and like most of their kin, they build reefs by laying down external skeletons of aragonite, a version of calcium carbonate or limestone. Like trees, they have annual growth rings that reveal how quickly they expand. And because coral growth depends on a variety of environmental conditions, the skeletons of the Porites provided a potted history of environmental changes, recorded in unchanging limestone.



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Corals survive acid oceans by switching to soft-bodied mode


Blogging on Peer-Reviewed ResearchClimate change is not just about surface warming and glacial melting. The carbon dioxide that human activity is pumping into the atmosphere also dissolves in the world’s oceans, slowly increasing their acidity over time. And that spells trouble for corals.

Corals, like this brain coral, find it harder to build their shells in acid water

Corals may seem like immobile rock, but these hard fortresses are home to soft-bodied animals. These creatures – the coral polyps – build their mighty reefs of calcium carbonate using carbonate ions drawn from the surrounding water. But as the water’s pH levels fall, these ions become depleted and the corals start to run out of their chemical mortar. The upshot is that in acid water, corals find it hard to build their homes.

Scientists have predicted that if carbon dioxide levels double, the reef-building powers of the world’s corals could fall by up to 80%. If they can’t rebuild quickly enough to match natural processes of decay and erosion, the reefs will start to vanish.

Now, Maoz Fine and Dan Tchernov from the Interuniversity Institute of Marine Science, Israel, have found that they have a way of coping with homelessness. They grew some fragments form two European coral species under normal Mediterrenean conditions, and others in water slightly more acidic, by a mere 0.7 pH units.

Those that spent a month in the acidic tank were quickly transformed. The skeleton dissolved and the colony split apart. The exposed and solitary polyps, looking like little sea anemones, still remained attached to rocky surfaces. When the going gets tough, the tough clearly go soft.


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Fishing bans protect coral reefs from devastating predatory starfish

CrownofThornsStarfish.jpgBlogging on Peer-Reviewed ResearchA complete ban on fishing can save coral reef communities in more ways than one. A few weeks ago, I blogged about a study which found that the coral trout, a victim of severe overfishing, was bouncing back in the small regions of the Great Barrier Reef where fishing has been totally forbidden. It certainly makes sense that fish will rebound when fishing ceases, but a new study reveals that the bans have had more indirect benefits – they have protected the corals from a predatory starfish.

The crown-of-thorns starfish (Acanthaster planci) is a voracious hunter of corals and a massive problem for reef conservationists. It’s bad practice for any science writer to anthropomorphise an animal, but the crown-of-thorns really does look incredibly, well, evil. Its arms (and it can have as many as 20) are covered in sharp, venomous spines. As it crawls over the reef, it digests the underlying coral by extruding its stomach out through its underside.

From time to time, their numbers swell into plagues of thousands that leave behind the dead, white skeletons of corals in their wake. These outbreaks eventually die off as the starfish eat themselves out of food supplies, but not before seeding downstream reefs with their tiny larvae that drift along the southern currents. During their peak, they destroy far more coral than other disturbances such as bleaching events or hurricanes.

Now, Hugh Sweatman at the Australian Institute of Marine Science has found that these outbreaks are much less frequent in the “no-take marine reserves”, where fishing is absolutely forbidden. Every year between 1994 and 2004, Sweatman carried out a census of starfish numbers in up to 137 areas across the Great Barrier Reef’s massive length.


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One in three species of reef-building corals face extinction

Blogging on Peer-Reviewed Research
If you’ve never had the pleasure of swimming among a coral reef, you might want to get your chance sooner rather than later. Yesterday, the journal Science published the first comprehensive global assessment of the status of the world’s reef-building corals, and it’s results don’t make for comforting reading. Almost a third of the 700-plus species surveyed face extinction; no group of land-living species, except possibly for the amphibians, are this threatened.

A team of 39 scientists led by Ken Carpenter, director of the Global Marine Species Assessment gauged the extinction risk faced by the world’s corals by using the International Union for Conservation of Nature’s (IUCN) famous Red List Criteria. These criteria measure extinction risk by looking at how quickly a population’s size falls over time. That sort of rigorous census data simply isn’t available for most corals, so Carpenter’s team settled for the next-best alternative – the rate at which a species’ habitat is lost within its known range. The results were adjusted for traits, such as the life cycle of each species and how resilient they are to habitat loss.

The results showed that the outlook for corals has worsened considerably in just the last 10 yeras. The team looked at the fates of 704 species and deemed that 176 were Near-Threatened, 201 were Vulnerable, 25 were Endangered and 5 poor species were Critically Endangered. Using earlier data, the team found that had the analysis been done in 1998 (before a mass “bleaching” event killed off large swathes of coral), only 20 species would have been classified as Near-Threatened and only 13 would have made it into the more severe categories.