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.
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.
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.
It’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.
The 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.
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.
In 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 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.
It’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.
Climate 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 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.
A 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.