Malawi cichlids – how aggressive males create diversity

Blogging on Peer-Reviewed ResearchCertain groups of animals show a remarkable capacity for quickly evolving into new species to seize control of unexploited niches in the environment. And among these ecological opportunists, there are few better examples than the cichlids, a group of freshwater fishes that are one of the most varied group of back-boned animals on the planet.

Malawi cichlidsIn the words of Edward O. Wilson, the entire lineage seems “poised to expand.” The Great Lakes of Africa – Tanganyika, Malawi and Victoria – swarm with a multitude of different species; Lake Malawi alone houses over 500 that live nowhere else in the world.

All of these forms arose from a common ancestor in a remarkably short span of time. Now, a new study suggests that this explosive burst of diversity has been partly fuelled by rivalry between hostile males.

Michael Pauers of the Medical College of Wisconsin found that male cichlids have no time for other males that look like them and will bite, butt and threaten those who bear the same colour scheme. In doing so, they encourage diversity in the lake since mutant males with different tints are less likely to be set upon by territorial defenders.

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Blind cavefish not so blind, Beetlemania and other tidbits…

Stories about cavefish are like buses – you get a seeming infinity of nothing and then loads turn up at once. Just 10 posts ago, I wrote about a study which found that you can restore sight to blind cavefish by cross-breeding individuals from different caves.

The different populations lost their eyes through changes to different sets of genes and in the hybrids, each faulty version was paired with a working one. As a result, the hybrids had fully formed and functional eyes despite having lived in darkness for a million years.

Now, a new study shows that the larvae of blind cavefish can detect light (or more accurately, shadows) too, even without working eyes. They can detect shadows and seek shelter in them, just like the sighted surface-dwelling versions of the same species. The key to the behaviour is their pineal gland, a small organ that regulates the body clock and, in some species, is sensitive to light.

I wrote up the research for Nature News; mosey on over for the full story and some possible explanations for why the fish’s pineal has retained the ability to detect light, even though its eyes have been lost.

Some other things to mention:

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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Solving the San Francisco plankton mystery

Blogging on Peer-Reviewed ResearchLook into the oceans past the sharks, seals and fish and you will find the tiny phytoplankton. These small organisms form the basis of life in the seas but if their populations get to big, they can also choke the life from it by forming large and suffocating algal blooms.

Solving the San Francisco plankton mysteryThe waters of San Francisco Bay have never had big problems with these blooms and if anything, scientists worried that the waters didn’t have enough phytoplankton. All that changed in 1999, when the phytoplankton population started growing. It has doubled in size since.

Now, scientists from the United States Geological Survey (USGS) have found that the blooms are the result of a long chain of ecological changes in the area. The plankton are just players in a large ensemble drama involves clams, mussels, fish, crabs and a cold snap.

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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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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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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.

Practicalities

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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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

Asymmetrical brains help us (and fish) to multi-task

The asymmetry of the human brain may allow us to cope with multiple demands that compete for our attention. The link between brain asymmetry and multi-tasking has now been confirmed in experiments with artificially-bred fish.

As you read this article on the internet, your computer is busy. You may be running multiple programs in the background, with email clients, anti-virus software or file sharing software all competing for valuable memory. The ability of computers to multi-task has grown substantially in recent years, as processors have become increasingly powerful.

Evolution has chartered a similar course, and humans have a particularly strong talent for dividing our attention among multiple priorities. Now scientists are showing that the asymmetrical differences between the two sides of our brain are essential for this ability to multi-task.

In the animal world, the ability to multi-task is a matter of life and death. Many species must be ever-watchful for food, while simultaneously looking out for predators who would view them in the same way.

Like too many open applications that slow down a computer, these multiple tasks compete for the brain’s finite resources. Those who survive life’s challenges are those with an edge at efficiently dealing with multiple demands.

The human brain - not symmetricalOne way of doing this is to use parallel processing – to delegate different parts of a problem to different pieces of hardware.

This is exactly the situation found in the human brain, with two asymmetric hemispheres associated with different mental abilities. And this ‘lateralisation’ is not unique to us, but seems to be present in all back-boned animals, from fish to apes.

An explanation for this asymmetry now becomes obvious – it may allow animals to multi-task, acting as a sort of cerebral division of labour.

(Un)evenly-brained fish

Marco Dadda and Angelo Bisazza at the University of Padova decided to test this idea, by looking at small freshwater fish called killifish. They bred different strains of this species with either symmetrical brains or asymmetric ‘lateralised’ ones.

The fish were kept in a special tank consisting of two parts separated by a trap door. In one half – the ‘feeding zone’ – the researchers placed a live brine shrimp for the fish to eat.

A killifishBoth strains were equally adept at catching the shrimp, but the lateralised strain gained a massive advantage when a predator was brought into play.

Dadda and Bisazza occasionally placed a tank containing the larger predator pumpkinseed sunfish in front of the feeding area, so that any killifish swimming across found itself face-to-face with a threat. When this happened, the symmetrically-minded ones took twice as long to catch their meals, but their lateralised peers took barely a second longer.

When the fish had to divide their attention between watching the predator and catching the shrimp, the lateralised brains acted as parallel processors, allowing them to cope with both tasks simultaneously.

Dadda and Bisazza confirmed this interpretation by carefully watching the fish as they swam after their meals. Killifish catch their food by approaching from the side, using one eye to monitor the prey.

In the absence of the sunfish, they were equally to strike from either side. But with the predator around, in 7 out of 10 times they snapped at the shrimp on the same side, watching it with one eye, and the predator with the other.

The pros and cons of asymmetry

In the case of the killifish in this experiment, partitioning roles to different parts of the brain seems to have brought clear benefits. But Dadda and Bisazza are quick to point out that the situation in the wild is less obvious.

In many cases, species with lateralised brains show lateralised behaviours, preferring to turn, keep watch or catch prey on one side of their body. Their other side is less often guarded, and natural selection penalises them for it by producing predators that prefer to approach from the unguarded side.

In these cases, regardless of parallel processing power, an asymmetric brain is clearly a disadvantage. The two scientists believe that the tipping point between these pros and cons comes when an animal has to perform difficult mental tasks.

Other studies have shown that asymmetrical brains endow wild chimpanzees with superior termite-fishing skills, and (equally wild) human children with better mathematical and verbal abilities than their classmates.

It may be that over the course of evolution, our brain’s halves started to work together more effectively as they became more different and specialised. It is ironic and sad then, that the opposite seems to hold true for the divergence of human cultures.

Dadda & Bisazza. 2006. Animal Behaviour 72: 523-529.

Farmed salmon decimate wild populations by exposing them to parasites

Salmon migrations serve to protect newly hatched youngsters from the parasites that afflict their parents. But salmon farms undermine this protection and jeopardise wild stocks by exposing young salmon to large numbers of parasitic sea lice.

The next time you buy salmon from your local supermarket, think about the hidden costs in each succulent fillet. Compared to wild fish, farmed salmon is far less likely to burden your wallet. But by buying it, you may be placing a much larger burden on the environment. Fish stocks around the world are declining due to over-fishing and ‘aquaculture’ – the farming of fish – was originally thought to help. But farming brings with it a host of ecological problems.

Wild salmon suffer fewer parasites than their farmed cousins. If the farmed fish are meat-eaters, as salmon are, they must be fed on the proteins and oils of wild fish, which does nothing to alleviate the stress on wild populations. Domesticated farmed fish are also genetically different to their free counterparts, and escapees risk spreading their genes and replacing local genetic diversity.

But the biggest and most immediate problem may be to do with the spread of parasites. Large, crowded and trapped animal populations are an easy target for parasites, and salmon farms are no exception. Farmed salmon are often infested with sea lice, parasitic relatives of prawns and shrimp, that cause direct damage, starve their host and increase vulnerability to disease. But the real problem comes when infected farmed salmon pass their parasites onto wild fish.

Martin Krkošek and colleagues from the University of Alberta believe that salmon farming may be disrupting behaviour that evolved in salmon to protect their young from parasites.

Newborn salmon are especially vulnerable to parasitesSalmon are known for their massive and demanding migrations in order to mate and lay eggs. Because of these treks, the young salmon enter the ocean several months before the adults and their parasite passengers return. In this way, the youngsters gain precious months’ respite from infections during which they can develop unhindered. It’s the same strategy that human parents use when they move to the country to raise their children in safer surroundings.

But this safe period is shattered if the ocean is crammed with lice-ridden farmed populations. Across the North Atlantic from Canada to Norway, wild juveniles are being infested with sea lice. The farms are providing the lice with new routes for infecting even more hosts, and the young salmon are not ready for them.

To adults, sea lice are irritating but tolerable, but to the much smaller young, carrying more than two lice is always lethal. Not only do they take up valuable nutrients the juveniles need to grow but they make them more vulnerable to predators and weaken their immune systems.

Krkošek analysed data on salmon and lice populations off the western coast of Canada and revealed disturbing trends. The lice were decimating many local populations of salmon, killing up to 95% of juveniles in some regions.

As aquaculture continues to spread, this study provides us with a harsh reality check and consumers around the world have the power to reverse the trend by protesting with their money. Choosing wild fish over farmed varieties sends a message to the fishing industry that the benefits of buying cheaper fish are outweighed by the costs to wild populations.