Cross-breeding restores sight to blind cavefish

Blogging on Peer-Reviewed ResearchIn the caves of Mexico lives a fish which proves that a million years of evolution can be undone with a bit of clever breeding.

Blind cavefishThe blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. They now navigate through the pitch-blackness by using their lateral lines to sense changes in water pressure.

But there is a deceptively simple way of restoring both the eyes and sight that evolution has taken, and Richard Borowsky from New York University’s Cave Biology Research Group has found it. You merely cross-bred fish from different caves.

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Whales evolved from small aquatic hoofed ancestors

Blogging on Peer-Reviewed ResearchTravel back in time to about 50 million years ago and you might catch a glimpse of a small, unassuming animal walking on slender legs tipped with hooves, by the rivers of southern Asia. It feeds on land but when it picks up signs of danger, it readily takes to the water and wades to safety.


The animal is called Indohyus (literally “India’s pig”) and though it may not look like it, it is the earliest known relative of today’s whales and dolphins. Known mostly through a few fossil teeth, a more complete skeleton was described for the first time last week by Hans Thewissen and colleagues from the Northeastern Ohio Universities. It shows what the missing link between whales and their deer-like ancestors might have looked like and how it probably behaved.Whales look so unlike other mammals that it’s hard to imagine the type of creature that they evolved from. Once they took to the water, their evolutionary journey is fairly clear. A series of incredible fossils have documented their transformation into the masterful swimmers of today’s oceans from early four-legged forms like Pakicetus and Ambulocetus (also discovered by Thewissen). But what did their ancestors look like when they still lived on land?

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Fake cleaner fish dons multiple disguises

Guess which is which? (The top one is the real deal)

Blogging on Peer-Reviewed Research

Nature is rife with charlatans. Hundreds of animals have evolved to look like other species in order to fool predators into thinking they’re more of a threat, or to sneak up on unsuspecting prey. In the Indo-Pacific lives a fish that does both and has the rare ability to switch between different disguises – the bluestriped fangblenny.

Common though it is, mimicry is usually restrictive and most fakers are stuck with one disguise. Until a few years ago, the only known animal that could switch between different acts was the amazing mimic octopus, which contorts its flexible body to look like seasnakes, lionfish, flounders and other poisonous underwater denizens.

Cleaner and faker

In 2005, Isabelle Cote and Karen Cheney from the University of Queensland discovered that a small reef fish called the bluestriped fangblenny (Plagiotremus rhinorhynchos) is also a dynamic mimic.

Its model is the bluestreak cleaner wrasse Labroides dimidiatus, an industrious species that provides a cleaning service for other reef visitors by picking off parasites and mucus from hard-to-reach places. The fangblenny’s intentions are less welcome. Its resemblance to the helpful wrasse allows it to get close enough to mount quick attacks on larger fish, biting off scales and skin (see image below for why it got it’s name).

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Trout with salmon parents could help to revive endangered fish species

Japanese researchers have developed a way of using one species of fish as a surrogate parent for an endangered one by transplanting the sexual equivalent of stem cells. If enough of these cells can be preserved, an extinct species could be resurrected.

Getting excited when fish produce sperm would usually get you strange looks. But for Tomoyuki Okutsu and colleagues at the Tokyo University of Marine Science and Technology, it’s all part of a day’s work. They are trying to use one species of fish as surrogate parents for another, a technique that could help to preserve species that are headed for extinction.

Baby trout can be born from salmon parents using transplanted sex cells.Okutsu works on salmonids, a group of fish that includes salmon and trout. Many members of this tasty clan have suffered greatly from over-fishing in the last few decades, and their populations are dwindling their way to extinction.

If stocks fall below a critical level, they may need a jump-start. One strategy is to freeze some eggs to be fertilised artificially, in the way that many human eggs are in fertility clinics. But it’s much harder for fish eggs – they are large and have lots of fat, which makes them difficult to freeze effectively.

Okutsu’s group have hit on a more effective solution. They use transplanted sexual stem cells to turn another species of fish into surrogate parents for the endangered ones.

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Genetic study puts damper on gray whales’ comeback

The eastern Pacific gray whale has bounced back from the brink of extinction to a healthy population of 22,000 individuals. But by measuring the genetic diversity of these whales, scientists have estimated that the original population was up to five times larger. The whales aren’t out of the danger zone yet, and climate change may explain why.

Twenty-two thousand sounds like a huge number. It’s happens to be number of eastern Pacific gray whales currently swimming off the coast of North America. It’s certainly much larger than 140, the number of whales that aboriginal people of this area are allowed to hunt. And it’s far, far bigger than zero, the population size that the whales were rapidly approaching in the mid 20th century.

The gray whale hasn’t fully recovered from a century or more of huntingObviously, it’s all relative. Twenty-two thousand is still much less than ninety-six thousand. That’s the size of the original gray whale population and it’s three to five times the current count. Not exactly cause for conservational complacency, then.

Previously, conservationists and whalers alike could only speculate on the number of whales that lived before their flirtation with extinction. But now, Elizabeth Alter and Stephen Palumbi from Stanford University have managed to pin down a figure by looking at the genetic diversity of living whales. And their results suggest that despite a rebound that Hollywood would envy, the grays are still a pale shadow of their former strength.

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Moray eels attack ‘Alien-style’ with second pair of jaws

A skeletal model of a moray eel bite, showing the pharyngeal jaws at work.In the Alien movies, the eponymous monster killed shipmates and marines with a fearsome set of double jaws. That may have been science fiction but science fact isn’t too far off. In our planet’s tropical oceans, moray eels use a ballistic set of second jaws to catch their prey.

These ‘pharyngeal jaws’ are housed in the eel’s throat. When the main jaws close on an unlucky fish, the second set launches forward into the mouth, snags the prey with terrifying, backward-pointing teeth and drags it back into the throat. In fractions of a second, the prey is bitten twice and swallowed. (Have a look at this Quicktime video of the pharyngeal jaws in action)

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Bleached corals recover in the wake of hurricanes

Hurricanes can physically cool coral reefs but they can also save them, by cooling the surrounding ocean and reversing the effects of bleaching.

Hurricanes can reverse coral bleaching by cooling surrounding waterIn 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.

Rising temperatures, dying corals

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.

Together with scientists from the Universities of Miami and the US Virgin Islands, he measured the extent of bleaching in reefs off Florida and the US Virgin Islands over the course of 2005.

Corals bleach when temperatures riseBy September, both reefs were suffering from equal amounts of bleaching. But while the situation continued to worsen in the storm-free Virgin Islands, the advent of four hurricanes in Florida turned the tide in the reefs’ favour.

Hurricanes vs bleaching

The storms – Dennis, Rita, Wilma and the infamous Katrina – each left behind an imprint of cooler water and the seas within 400km of their paths cooled by up to 3.2°C and stayed that way for up to 40 days. Two weeks after the fourth hurricane, Wilma, had passed, the corals had almost completely recovered.

Manzello’s study shows that the benefits of hurricanes on coral reefs can sometimes outweigh the localised physical wear and tear they cause. The question now is whether this is an isolated incident or a common occurrence.

Manzello isn’t sure. Based on the numbers of bleaching events and hurricane landfalls in Florida since the 19th century, the odds of both happening at the same time (as in 2005) is about one in seven. But the actual probability is likely to be higher especially since the same factors that cause bleaching, such as warmer water, also encourage the growth of hurricanes.

Even so, it would be extremely foolish to expect hurricanes to bail corals out completely – only conservation projects and addressing rising temperatures can do that.

Reference: Manzello, Brandt, Smith, Lirma, Hendee & Nemeth. 2007. Hurricanes benefit bleached corals. PNAS doi.10.1073/pnas.0701194104.

Related posts on corals:
Hope for corals – swapping algae improves tolerance to global warming
Corals survive acid oceans by switching to soft-bodied mode

Related posts on the hidden effects of climate change: Icebergs are hotspots for life
Human nitrogen emissions indirectly capture carbon by fertilising forests
When the heat is on, male dragons become females
Climate change responsible for decline of Costa Rican amphibians and reptiles

Icebergs are hotspots for life

Icebergs are hotspots for Antarctic life. The water around them teems with nutrients, plankton and animals – a mobile community dragged along by the drifting ice. Together, they enrich over a third of Antarctic waters.

Icebergs are hotspots for Antarctic life.Say the word iceberg, and most people are likely to free-associate it with ‘Titanic’. Thanks to James Cameron (and, well, history too), the iceberg now has a reputation as an cold murderous force of nature, sinking both ships and Leonardo DiCaprio. But a new study shows that icebergs are not harbingers of death but hotspots of life.

In the late 1980s, about 200,000 icebergs roamed across the Southern Ocean. They range in size from puny ‘growlers’, less than a metre long, to massive blocks of ice, larger than some small countries.

They may be inert frozen lumps, but icebergs are secretly in the business of nutrient-trafficking. As the ice around Antarctica melts in the face of global warming, some parts break free from the parent continent and strike out on their own. As they melt, they release stored minerals into the water around them, and these turn them into mobile homes for a variety of life.

A tale of two icebergs

Kenneth L. Smith Jr, from the Monterey Bay Aquarium Research Institute, and other scientists from San Diego discovered the true extent of these icy ecosystems by studying two icebergs floating in the Antarctic Weddell Sea.

Even the smaller of the two, W-86, has a surface area larger than 17 football pitches. The larger one, A-52 was over a thousand times bigger, with a surface area of 300 km2 and extending 230 metres into the freezing waters.

Smith and crew identified the duo through satellite imaging, and tracked them down by boat. Their ship spiralled around the blocks of ice collecting water samples as it went, from a dangerously close distance of a few hundred feet to a safer five miles away.

Hotspots for life

Diatoms cover the underside of icebergs and form a ring of life around them.The skies above the two icebergs were patrolled by seabirds, including Cape petrels and Antarctic fulmars. Below the water, Smith explored the icebergs’ undersides with a remote-operated vehicle and found them teeming with life.

Below W-86, he saw a lattice-like surface, and the ridges of these were home to diatoms (right). These single-celled algae are part of the phytoplankton, microscopic creatures that make their energy from the sun and form the basis of the ocean’s food web. In between these diatom-covered ridges, baby icefish and segmented worms swam among the lattice’s nooks and crannies.

A-52 was even more varied, with large caves extending deep into the iceberg’s core. The team found diatoms here too, along with Antarctic krill (below), small shimp-like animals with a taste for diatoms. Among these were various invertebrates – comb jellies, colonial jellyfish-like animals called siphonophores, and predatory torpedo-shaped worms called chaetognaths.

Further out, the area immediately around the iceberg was void. But just beyond that, the ice was encircled by another halo of phytoplankton. These creatures, along with the diatoms on the ice itself, were thriving on the nutrients released by the melting ice, such as iron.

When Smith exposed diatoms to tiny mineral-rich particles filtered from his collected water samples, they grew slowly and steadily, while other diatoms cultured in normal water did not.

Diatoms fed by meltwater nutrients from icebergs, act as food for krill.These drifting islands of ice were dragging entire communities along with them. As they drift and melt, they release small amounts of important nutrients. That triggers the growth of creatures at the bottom of the food chain and provides the foundations for larger animals like krill and seabirds. A-52 alone enriched a massive ring of water about the size of the Isle of Man.

To estimate the effect of other icebergs, Smith used satellites to count the number of bergs in a sample area. Within this space, the satellites spotted almost a thousand individual icebergs that, together, covered less than 0.5% of the ocean’s surface. But even this small amount was enough to enrich over 39% of the Southern Ocean!

Icebergs and climate change

By providing support for phytoplankton, the icebergs were also inadvertently helping to mitigate the effects of climate change. Just like land plants, phytoplankton make their own energy through photosynthesis. And just like land plants, they absorb carbon dioxide to do so. By eating the phytoplankton and excreting the remains, krill cause carbon to fall down into the ocean depths in a rain of droppings.

So even as their parent continent melts and releases carbon into the atmosphere, icebergs serve to draw planet-warming carbon away from the air and transfer it to the deepest sea. Smith believes that climate modellers need to take this into account to better predict the effects of melting Antarctic ice.

The disappearing ice can reveal underlying rock which absorbs more heat, hastens melting and releases even more trapped carbon – this is known as ‘positive feedback’. But as the ice melts, icebergs break off and these help to suck in carbon from the atmosphere – this is ‘negative feedback’. The next task is to understand how these two processed balance out.

Reference: Smith Jr, Robison, Helly, Kaufmann, Ruhl, Shaw, Twining and Vernet. 2007. Free-drifting icebergs: hot spots of chemical and biological enrichment in the Weddell Sea. Science doi.10.1126/science.1142834.

Other posts on ecology:
Shark-hunting harms animals at bottom of the food chain
The fox and the island – an Aleutian fable
Parasites can change the balance of entire communities
Human nitrogen emissions indirectly capture carbon by fertilising forests

Restoring predator numbers by culling their prey

Helping out a threatened predator by culling their prey seems like a really stupid idea. But Scandinavian scientists have found that it might be the best strategy for helping some of our ailing fish stocks.

Lennart Persson and colleagues from Umeå University came up with this counterintuitive idea by running a 26-year natural experiment with the fish of Lake Takvatn, Norway.

The brown trout increases the numbers of its prey by eating it!At the turn of the 20th century, the top predator in Lake Takvatn was the brown trout. Over-fishing sent its numbers crashing, and it was virtually gone by 1980.

In its place, a smaller fish – the Arctic char ­– was introduced in 1930. Char should make a good meal for trout, so it was surprising that when the trout were reintroduced they failed to flourish despite an abundance of food.

It was only in the 1980s, when the researchers removed over 666,000 char from the lake that the trout started bouncing back. While their prey population fell by 80%, the trout have increased in number by 30 times. The lake’s temperature and nutrient levels were mostly constant during this time, so why did the trout do better when they prey was culled?

Persson believes that it’s not the numbers of the char, but their size that matters, and that changed irrevocably when the trout first vanished.

More predation means more prey?

Paradoxically, predators like trout, can actually increase the numbers of small prey by eating them. It seems like a strange idea, but it happens because the remaining prey face less competition for food. As a result, they grow more rapidly, mature faster and give rise to more young.

By growing too large, the Arctic char muscled the trout out of Lake Takvatn.This means the population becomes, on average, smaller, since individuals spend less time growing and fill the water with baby fish. And that’s good news for predators. But take the predator out and the whole system grinds to a halt.

Conservationists often like to believe that an over-hunted predator will just bounce back into its original niche once hunting is stopped. But things are rarely that simple. In the predator’s absence, other species will rush in to fill the gap and the entire system can settle down into a new balance, which the predator can find very hard to slot back into.


In Lake Tyvatn, the absence of the trout meant that the char population faced no threats and competed heavily for resources. They grew and reproduced slowly, reaching sizes too large for the trout to tackle. The proportions of small char fell to a level which could not support reintroduced trout. Essentially, while the predator was away, the prey took it easy and locked the door so it couldn’t get back.

When the char were culled, this mimicked the effects of trout predation by removing the largest individuals. As a result, the numbers of smaller, trout-friendly char doubled and began to dominate the lake. And that shift finally allowed the trout to regain a foothold (or finhold) in the lake. The two fish have now established a balance in numbers for over 15 years.

Persson’s study clearly shows that removing a predator from its habitat (an all-too common occurrence) doesn’t create a predator-shaped hole in the ecosystem, ready to be filled again. Instead, it causes drastic changes to local food webs, that can only be reversed with counter-intuitive and ingenious strategies.

Stocks of predatory fish, including sharks, salmon, cod and trout, are threatened by over-fishing all over the world, and Persson believes that his strategy could help them to recover.

For example, the falling cod population in the Baltic sea could potentially be restored by fishing for its prey, like herring or sprat. It’s so crazy, it just might work.


Reference: Persson, Amundsen, de Roos, Klemetsen, Knudsen & Primicerio. 2007. Culling prey promotes predator recovery – alternative states in a whole-lake experiment. Science 316: 1743- 1746.

Related post on over-fishing:
Shark-hunting harms animals at bottom of the food chain
and others on changing ecosystems:
Attack of the killer mice – introduced rodents eat seabird chicks alive
The fox and the island: an Aleutian fable
Farmed salmon decimate wild populations by exposing them to parasites

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Simple sponges provide clues to origin of nervous system

The possible origins of the nervous system have been found in the simple sponge, an animal with no nervous system of its own. Sponges carry the genetic components of synapses, which may have been co-opted by evolution as a starting point for proper nerve cells

Sponges are the most primitive of all animals. They are immobile, and live by filtering detritus from the water. They have no brains or, for that matter, any organs, tissues or nervous system of any sort. If you were looking for the evolutionary origins of animal intelligence, you couldn’t really pick a less likely subject to study.

Over time, evolution co-opted the early PSD of the sponge and used it to craft true nervous systems.So it was with great surprise that Onur Sakarya from the University of California, Santa Barbara found that sponges carry the beginnings of a nervous system.

With no neurons to speak of, these animals still have the genetic components of synapses, one of the most crucial parts of the nervous system. And their versions share startling similarities with those of humans.

Synapses (and proto-synapses)

Synapses are junctions between two nerve cells that are allow the cells to pass on signals to each other. Signals are carried by molecules that cross the synaptic gap called neurotransmitters. When they reach the receiving cell, they come across an elaborate tangle of proteins called the post-synaptic density (PSD; labelled in red below). The PSD processes the neurotransmitters, among many other important roles, and allows the receiving cell to respond appropriately to the nervous signal

Sakarya searched for equivalents of the human PSD proteins in the genomes of other animals. For a start, he found an almost complete set in the starlet sea anemone (Nematostella vectensis). The anemone (like its cousins, the jellyfish) is one of the Cnidarians, a group of animals that have the most rudimentary of nervous systems. Finding PSD genes in them is surprising but reasonable.

The synapse relays signals from one cell to another.But Sakarya was really surprised when he found the vast majority of the PSD assemblage in the sponge Amphimedon queenslandica, an animal that doesn’t even have a nervous system! The sponge’s PSD proteins bore remarkable resemblances to those of humans and other animals, and were built of similar arrangements of domains.

One in particular, the PDZ domain, allows PSD proteins to recognise one another and assemble correctly. When Sakarya compared the structures of the human and sponge PDZ domains, he found that at the atomic level, the parts they used to interact with other proteins were almost 90% identical. So not only does the sponge have the full set of PSD parts, it can assemble them into a fully-functioning whole.


So what is the PSD, part of the nervous system, doing in an animal without one? Sakarya believes that the PSD is an example of exaptation, a process where evolution co-opts an existing structure for another purpose. Bird feathers are a good example of this – they evolved in small dinosaurs to help them regulate their body temperature, and were only later used for flight.

Exaptation can explain how complex, integrated structures like the nervous system can evolve. Rather than building the whole thing from scratch, evolution took ‘off-the-shelf’ components, like the PSD, and put them together in exciting new ways.

Sponges are the simplest of animals but even they have genetic components of synapsesIn the same way, the PSD of sponges is switched on in a type of cell called the ‘flask cell’. Flask cells are only found in sponge larvae, which, unlike the adults, are free-swimming. These cells could help the larvae to sense their environment, and could well have been a starting point for the evolution of neurons.

Sakarya cautions that there could be another explanation. Sponges could be degenerate relics of a more advanced branch of animals, that stripped away their complexity in favour of life in the (very) slow lane. In this scenario, the flask cells are evolutionary remnants of neurons proper.

Nonetheless, under both scenarios, these findings strongly suggest that the common ancestor of all living animals already has an early working version of the PSD. This practically pre-adapted it for the evolution of nervous systems. With minimal additional evolutionary steps, this early scaffold could have been transformed into the functional synapses that drive our thoughts today. The ancestor was pre-adapted to a future with neurons.

It’s worth noting that this discovery was only made possible because the genome of Amphimedon has been fully sequenced. In an age where genome sequencing could start to be taken for granted, this drives home the importance of sequencing a wide variety of living things that represent crucial junctures in evolution.

Reference: Sakarya, Armstrong, Adamska, Adamski, Wang, Tidor, Degnan, Oakley & Kosik. 2007. A post-synaptic scaffold at the origin of the animal kingdom. PLOS One 6, e506: 1-7.

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Related stories about evolution:
Of flowers and pollinators – a case study in punctuated evolution
Natural selection does a handbrake turn – quick evolution at work
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Human cone cell lets mice see in new colours
Living optic fibres bypass the retina’s back-to-front structure

Related stories about nervous systems:
Maternal hormone shuts down babies’ brain cells during birth
No new brain cells for you – settling the neurogenesis debate
Bats create spatial memories without making new brain cells

Corals survive acid oceans by switching to soft-bodied mode

Biologists fear that the world’s beautiful coral reefs may be early victims of climate change, succumbing to the increasing acidity of the planet’s oceans. But new research provides a small glimmer of hope, by showing that corals may be able to weather the upcoming storm by shifting to a temporary soft-bodied lifestyle.

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, 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.

In acid water, corals lose their shells and live as soft-bodied polyps.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.

Even without their protective skeletons, they survived for over a year and seemed to be going about business as usual. They thrived, they reproduced normally and they still kept the symbiotic algae that allow them to produce energy through photosynthesis.

And when they were put back in normal conditions, they readily gave up their independence and re-formed both colonies and hard shells.

Fine and Tchernov’s findings suggest that corals may be able to survive upcoming climate changes by adopting soft-bodied, free-living lifestyles. And there is evidence that they have used this trick before.

The species supported by coral reefs may die off if the corals switch to a soft-bodied life.The hard shells of coral reefs fossilise easily, but the fossil record still has large gaps where no reefs are found. These may represent periods of time when corals were biding their time in their soft-bodied phase instead.

But while this new discovery is cause for hope, it should not be cause for complacency. Even though the corals themselves may persist in another guise, the vast diversity of species that depend on them may go for good if their reefs disappear.

More about corals: 
Bleached corals recover in the wake of hurricanes
Hope for corals – swapping algae improves tolerance to global warming

More about the effects of climate change: 
Icebergs are hotspots for life
When the heat is on, male dragons become female
Climate change responsible for decline of Costa Rican amphibians and reptiles

Reference:  Fine and Tchenov. 2007. Scleractinian coral species survive and recover from decalcification. Science 315: 1811.

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Shark-hunting harms animals at bottom of the food chain

Overfishing has disproportionately reduces the numbers of the ocean’s ‘apex predators’ and large sharks are disappearing particularly fast. Their absence allows their prey to flourish and the consequences of that can be disastrous for animals at the bottom of the food chain, and the humans that depend on them.

On the surface, plummeting populations of sharks do not seem like much cause for concern for humans or, for that matter, other sea life. But this simple viewpoint relies on splitting animals into two groups – predators and prey.

The sand tiger shark - one of the victims of overfishing.In practice, this distinction is far too crude. Too put it bluntly, there are predators and there are predators. Those at the top kill those in the middle, and stop them in turn, from killing those at the bottom. As the old saying goes, the enemy of my enemy is my friend.

The rise in shark fishing is mainly driven by a growing market for their fins. Sharks’ fins soup is a delicacy in China, which is utterly ludicrous given that the fins themselves are tasteless and merely add texture.

China’s strong economy has put this expensive treat in the range of the expanding middle classes and the world’s sharks are paying the price for it.

Ransom Myers from Dalhousie University, Halifax, decided to study the effects of declining shark numbers by analysing a uniquely comprehensive shark census taken over the last 30 years on the American eastern seaboard.

In these waters, several shark species have all but vanished since 1972, including 99% of the bull, dusky and smooth hammerhead populations.

And because fishing expeditions tend to catch larger individuals, the average size of the survivors has plummeted. Mighty animals like the tiger and black-tip sharks are now up to half as long as they used to be.

The hammerhead shark helps to control numbers of bottom-dwelling predators like rays and skates.Unsuprisingly, as the sharks declined, their prey benefited. Great sharks mainly hunt smaller predators, including their close relatives skates, rays, and indeed, smaller sharks, whose numbers surged in their absence.

The cownose ray, for example, is now ten times more common than it was in the mid-70s.

And here’s where the domino effects begin.

It turns out that large sharks inadvertently carry out a sort of protection racket for animals at the bottom of the food chain.By taking out the mid-level predators, they prevented these lesser hunters from decimating stocks of small fish and invertebrates.

The cownose ray feeds mainly on shellfish like scallops, clams and oysters. In the 80s, their small numbers made little dent on the local scallop population which sustained the economies and stomachs of local seaside towns.

In 1996, the ray explosion started to spell the end for the scallops. By 2004, the local North Carolina scallop fisheries which had thrived for centuries were forced to close and remain closed to this day. Little did the locals imagine that the disappearance of dangerous sharks from their waters could have such strongly felt economic consequences.

The ludicrous demand for shark’s fin soup will wreak irrevocable damage on oceanic ecosystems.This is far from an isolated incident. In Ariake Sound off Japan, shark fishing is particularly intense, and a booming population of long-headed eagle rays has decimated shellfish populations just like their cownose cousins in the Atlantic.

This is one of the first times that the removal of apex predators has been so thoroughly studied in the ocean. On land, the consequences are well-known.

Just last year, Australian scientists found that in some areas, persecution of the local top dog – the dingo – has allowed introduced predators like cats and rats to kill off up to two thirds of ground-dwelling mammal species.

As for the great sharks, the responsibility for preserving these great animals lies with the only predators they themselves face – the fishermen who kill them, and the Asian restaurant-goers who create the demand for their fins.

Surely, the price of irrevocably altering an ecosystem is too high to pay for the right to eat textured soup?

Reference: Myers, Baum, Shepherd, Powers & Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.

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Related posts on conservation and introduced animals:

The fox and the island: an Aleutian fable
Farmed salmon decimate wild populations by exposing them to parasites
Attack of the killer mice – introduced rodents eat seabird chicks alive