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Single-Celled Creature Has Eye Made of Domesticated Microbes

The oceans are full of eyes. Giant squid scan the depths with the world’s largest ones, which are oddly similar to those of the sperm whales that hunt them. Mantis shrimps watch for prey using eyes that work like satellites. Starfish stare through the tips of their arms, chitons look up through lenses made of rock, and scallops peer at the water through dozens of eyes with mirrors inside them. But to see the strangest eyes of all—eyes so weird that we can’t even be sure that they are eyes—you have to squint.

These maybe-eyes belong to a group of rare, free-swimming algae called warnowiids. Each consists of a just one round cell, so small that a few hundred of which could fit in this full stop. Under the microscope, each warnowiid contains a conspicuous dark dot. This is the ocelloid. It consists of a clear sphere sitting in front of a dark red strip, and has components that resemble a lens, an iris, a cornea, and a retina.

Eyes are meant to be animal inventions. They’re supposed to comprise many cells. They are icons of biological complexity. And yet, here’s a non-animal that packs similar components into its single cell. Is the ocelloid actually an eye? Can it sense light? What does a warnowiid use it for? These questions are still mysteries, but in trying to answer them, Gregory Gavelis from the University of British Columbia has discovered something about the ocelloid that’s even weirder. At least two of its components—the “retina” and the “cornea”—seem to be made from domesticated bacteria.Warnowiid

The first scientist to notice the ocelloids was a German zoologist named Oscar Hertwig. In 1884, while working at a research station in Naples, he was distracted by a tiny speck in a Petri dish. It appeared to be jumping up and down in the water as if crying for attention. Hertwig sucked it up with a straw and stuck it in ethanol—which was a mistake. The creature started to disintegrate, and Hertwig speed-drew it as quickly as he could, until it had completely broken down. He then published his observations, describing what looks like an eye.

Karl Vogt, a more senior zoologist, wasn’t having any of it. He accused Hertwig’s of grossly misinterpreting a horribly distorted specimen. A single cell couldn’t possibly have an eye; instead, the creature must have just scavenged an eye from a dead jellyfish—and yes, some jellyfish have eyes. The debate raged back and forth until Hertwig, who never found a second warnowiid to study, moved on to other things (and great acclaim).

No one saw the creatures again until 1921, when Charles Atwood Kofoid and Olive Swezy showed that they live all over the Pacific coast of North America. They were rare, though, and many of the species that Kofoid and Swezy drew have never been seen since. This rarity makes warnowiids extremely hard to study. You can’t culture them. You can barely find them. “You’d be lucky if you ever saw more than five in a single Petri dish,” says Gavelis.

You can, however, study their genes. Sequencing technology has progressed to the point where scientists can parse the DNA of a single cell. Gavelis’s team, led by Brian Leander, used these techniques to study the “eyes” of two warnowiids—Erythropsidinium (the species that Hertwig drew) and Warnowia. In particular, he focused on a curved red structure called the retinal body, so named because it seems analogous to our light-detecting retinas.

Gavelis found evidence to support an old idea that the retinal body is a plastid—a type of compartment found inside the cells of plants and algae. The green chloroplasts that allow these organisms to make their own food, by harnessing the sun’s energy, are a type of plastid. They evolved from a free-living bacterium that was engulfed by an ancient cell and forced into servitude. Over time, this bacterium became an inextricable part of its host, and turned into the plastids we see today.

Cells can acquire plastids by engulfing and taming their own bacteria. Alternatively, they can steal someone else’s. The ancestor of the dinoflagellates—the group of algae that warnowiids belong to—did exactly this. It swallowed another red alga and claimed its plastids for its own.

In warnowiids, Gavelis thinks those pilfered plastids make up the retinal body. He dissected out these structures from the main ocelloids and amplified the DNA within them. Among these sequences, he found several active genes that are involved in photosynthesis and are only used in algal plastids. And when he repeated the same technique on entire cells, including the huge amount of DNA in the warnowiids’ main genomes, he found a far smaller proportion of photosynthesis genes.


Down the microscope, the team saw that the retinal body has physical features that are characteristic of plastids. Weirder still, it seems to sit within a network of interconnected plastids that look different, but are enveloped by a single membranous web. There could be just one plastid, or dozens of them. “They’re like drops of oil in a lava lamp,” says Gavelis. “The degree of specialisation in this one structure just boggled my mind.”

Gavelis also showed that the “cornea” of the ocelloid consists of little bean-shaped structures called mitochondria. Mitochondria also descend from free-living bacteria that were domesticated by ancient cells, in an extremely unlikely event that may have given rise to all complex life. For a few billion years, they have provided complex cells with power. In the warnowiids, they also… well, it’s not clear what they do. A continuous layer of them surrounds the “lens”, and seem to send small protrusions into it. They could be helping to collect light in the style of a true cornea, or they could be supplying the lens with energy.

That’s the biggest mystery about the ocelloid: what does it do? It certainly looks like an eye. It has components that would seem to focus light onto the retinal body. But for the retinal body to then respond to that light, it needs some kind of light-sensitive pigment. Chlorophyll is a possibility; the thing’s a plastid, after all. Gavelis’ team are also looking for traces of opsins—the proteins that are universally found in all animal eyes, from starfish to giant squid.

Even if the ocelloid is an eye, what could it possibly see? Fernando Gómez of the University of São Paulo recently told New Scientist that they help warnowiids to aim harpoon-like stings at their prey (he compared them to “snipers”). But Gavelis is sceptical. With just one ocelloid, each warnowiid has at most a one-pixel view of the world. “There were only so many things that it could do with such limiting processing power,” he says. “Even resolving an outline or a shadow is way beyond what anyone has demonstrated that a cell can do.”

Alternatively, the warnowiids could just be using their ocelloids to sense absolute light levels, so they can swim towards bright areas or keep in shade. But Gavelis isn’t happy with that idea either. Other dinoflagellates can sense light levels using much simpler eyespots, which have no lenses. The warnowiids must surely be putting their more complex structures to more complex uses.

Gavelis’ favoured idea is that they are looking for their prey: other dinoflagellates. These creatures reflect a particular kind of light called circularly polarised light, which could betray their presence. Perhaps the warnowiids use the ocelloids to detect this tell-tale signal, and swim towards the creatures that emit it.

Tom Cronin, a vision scientist at the University of Maryland, Baltimore County, is not convinced. “It’s a monstrous stretch,” he says. For a start, many other dinoflagellates eat their own kind and do so without complex ocelloids. Also, complex eyes don’t necessarily imply complex behaviour. The simple box jellyfish has 24 eyes, eight of which are surprisingly similar to our own camera-type peepers. “They have outstanding optics,” says Cronin, “but they’re primarily useful for orienting the animal, or for detecting edges of shadowed areas.”

Finally, he wonders if the ocelloid isn’t really an eye, but more of a “glorified chloroplast”. Maybe its function is to provide its owner with energy, and the “lens” is just a way of focusing more light? Maybe the eye-like elements help the creature to orient itself in the brightest direction?

“That’s inconsistent with the evolutionary history of these critters,” counters Gavelis. The earliest members of the group almost certainly made all of their own food through photosynthesis, but later members are almost totally predatory—and they’re the ones with the most complex ocelloids. Given that, the hunting hypothesis is looking good.

Solving this mystery will be hard, given how hard the warnowiids are to find and study. Cronin says, “In the end, we know more about the structure of the strange eye-like ocelloids, but their function is just as obscure as ever!”

Reference: Gavelis, Hayakawa, White, Gojobori, Suttle, Keeling & Leander. 2015. Eye-like ocelloids are built from different endosymbiotically acquired components. Nature http://dx.doi.org/10.1038/nature14593



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Octopuses, and Maybe Squid, Can Sense Light With Their Skin

Octopuses, squid, and cuttlefish, the animals collectively known as cephalopods, are capable of the most incredible feats of camouflage. At a whim, they can change the colour, pattern, and texture of their skins to blend into the background, baffle their prey, or communicate with each other.

As if that wasn’t amazing enough, Lydia Mäthger and Roger Hanlon recently discovered that the common cuttlefish has light-sensitive proteins called opsins all over its skin. Opsins are the engines of sight. Even though animal eyes come in a wondrous variety of shapes and structures, all of them use opsins of one kind or another. The discovery of these proteins in cuttlefish skin suggested that these creatures might be able to sense light over their entire surface, giving them a kind of distributed “sight”.

It was a tantalising suggestion, but far from a definitive one. Opsins are used in many other contexts, such as sensing the time of day, which still involve detecting light but have nothing to do with seeing images. To work out what exactly opsins are doing in cephalopod skin, the team needed more evidence.

For example, when opsins are struck by light, they change shape. This triggers a Rube Goldberg-esque chain of further changes in other proteins, which culminates in an electrical signal travelling through a nerve towards the brain. That’s the essence of vision. It’s what happens in a cephalopod’s eye. Does it also happen in their skin?

That’s exactly what Alexandra Kingston from the University of Maryland, Baltimore County decided to find out. Working with Hanlon and vision expert Tom Cronin, Kingston studied the skins of the longfin inshore squid, the common cuttlefish, and the broadclub cuttlefish, looking for proteins that act downstream of opsin.

She found them. Several of them are present in the animals’ skin, and only in the chromatophores—the cells that are primarily responsible for their shifting patterns. Each chromatophore is an elastic sac of pigment, surrounded by a starburst of muscles. If the muscles relax, the sac contracts into a small dot that’s hard to see. When the muscles contract, they yank the sac into a wide disc, revealing the colour it contains. Kingston showed that these living pixels contain the same Rube Goldberg set-up that exists in their owners’ eyes.

Her team couldn’t, however, show that the chromatophores actually respond to light. “All the machinery is there for them to be light-sensitive but we can’t prove that. It’s been very frustrating,” says Cronin. The chromatophores might be detecting local light levels to prime them for either expansion or contraction. They could communicate with each other so that small clumps of chromatophores react to light as a unit. Or they could send signals directly to the brain to provide their owners with more information about light levels in their environment. These possibilities could all be right or wrong; no one knows.

“We don’t know if they contribute to camouflage or are just general light sensors for circadian cycling or are driving hormonal changes. They have a job to do but we don’t know what it is,” says Cronin. “That’s biology!” he adds, resignedly.

Cuttlefish. Credit:  Peter Hellberg
Cuttlefish. Credit: Peter Hellberg

Meanwhile, Desmond Ramirez and Todd Oakley from the University of California, Santa Barbara had better luck with a different cephalopod—the California two-spot octopus. When the duo shone bright light onto isolated patches of skin, they found that the chromatophores would dramatically expand. They called this light-activated chromatophore expansion, or LACE.

Ramirez and Oakley showed that the octopus’s skin also contains opsin, but not in the chromatophores. Instead, its opsins reside in small hair-like structures called cilia. People used to think that the octopus used these cilia as organs of touch; they still could be, but they might also detect light too. And echoing Cronin, Oakley says, “We don’t know yet how this is used, or indeed if it is used, in the living animal.”

ColorLACE(1)Neither study is definitive, but they certainly complement each other. They strengthen the case that these animals really are detecting light with their skins, independently of their brains and eyes.

They also serve as useful reminders that cephalopods are a diverse group of very different animals, with different branches separated by over 280 million years of evolution. It shouldn’t be surprising that octopus skin readily responds to light, but squid and cuttlefish skin doesn’t seem to. Or that, in octopus skin, opsins are found in cilia, while in squid and cuttlefish, they live in chromatophores.

They behave differently, too. “Cuttlefish and squid do seem to display to each other more than octopuses,” says Cronin. “Octopuses do pattern dramatically in response to environmental changes, but we don’t know of displays in octopuses designed for other octopuses.” Perhaps each species uses its skin opsins for different tasks.

Reference: Ramirez & Oakley. 2015. Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.110908

Kingston, Kuzirian, Hanlon & Cronin. 2015. Visual phototransduction components in cephalopod

chromatophores suggest dermal photoreception. Journal of Experimental Biology. http://dx.doi.org/10.1242/jeb.117945

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Sloths and Armadillos See The World In Black-and-White

Armadillos have terrible vision. In 1913, American zoologists Horatio H. Newman and J. Thomas Patterson wrote, “The eyes [of the nine-banded armadillo] are rudimentary and practically useless. If disturbed an armadillo will charge off in a straight line and is as apt to run into a tree trunk as to avoid it.”

The three-toed sloth isn’t much better. “If an infant sloth is placed five feet away from its mother on a horizontal branch at the same level, at once the young sloth begins to cry, the mother shows that she heard it calling and turns her head in all directions. Many times she looks straight in the direction of her offspring but neither sight, hearing nor smell apparently avail anything,” wrote Michel Goffart in 1971. And more comically: “Infuriated male [sloths] try to hit each other when they are still distant by more than a metre and a half.”

Now, decades after these descriptions were written, Christopher Emerling and Mark Springer from the University of California Riverside think they know why armadillos and sloths are so poor of sight. These animals have broken copies of the genes that build colour-detecting cone cells in their eyes. That leaves them with only rod cells, which have poorer resolution and work best in dim light. They see the world in coarse black-and-white, and they struggle to cope with bright light.

This discovery supports the idea the armadillos, sloths, and anteaters—a group collectively known as the xenarthrans—evolved from a burrowing ancestor that spent much (if not all) of its time underground. With light in short supply, these ancestral animals may have prioritised the sensitive rod cells over the sharp and colour-tuned cones.

They eventually re-surfaced and, in the case of sloths, even took to the trees. But they still retain traces of their burrowing past, including sturdy front legs, curved claws, and skeletal features that gave them a powerful digging stroke (the word “xenarthran” means “strange joints”). Anteaters use these traits to rip through ant nests, while sloths use them to hang from branches.

But they also carried their ancestors’ cone-less retinas. These, according to Emerling and Springer, might have constrained their evolution in important ways. With poor vision, they couldn’t take up many of the lifestyles that other mammals developed, like fast-running, active-hunting, or gliding. And armadillos “have minimal ability to see approaching cars when crossing roads, a fact all too familiar to residents of Texas,” says Emerling.

This discovery is part of a much larger narrative for mammals—one that highlights evolution’s fickle nature. A wide range of animals, including many birds, fish, reptiles, and amphibians, have eyes with four types of cones, allowing them to discriminate between a huge range of colours. Mammals, however, evolved from a nocturnal ancestor that had already lost two of its cones, and many have stuck with this impoverished set-up. Dogs, for example, still only have two cones: one tuned to violet-ish colours and another tuned to greenish-blue. (Contrary to popular belief, a dog’s world isn’t black-and-white; they see colours, albeit a limited palette.)

Humans and other primates partly reversed the ancient loss by reinventing a third red-sensitive cone, which may have helped us to discern unripe green fruits from ripe red/orange ones. Ocean-going mammals, meanwhile, took the loss of cones even further and disposed of their blue/violet-sensitive ones. And the great whales have lost all their cones entirely. They only have rods. The ocean is blue, but a blue whale would never know.

Springer’s team discovered the rod-only whale retinas a few years ago. It seemed to make sense, since these are deep-diving animals that spend a lot of time at depths where little light penetrates. The same team also showed that golden moles also lack cones—they are ‘rod monochromats’. Again, this made sense: these animals spend most of their time underground.

But the nine-banded armadillo was more puzzling. Emerling looked at its genome and saw that several of its cone-building genes had picked up debilitating mutations, which should saddle it with a rod-only retina. “I was convinced this was a sequencing error,” he says,” since armadillos are often active in the daytime. They don’t dive deep in the ocean and don’t live underground.”

Emerling checked the results more carefully and showed that the armadillo still has working copies of all its rod-specific genes, but has broken versions of seven cone-specific ones. He found similar examples of broken cone genes in the genomes of other xenarthrans, including five other armadillo species, three anteaters, a living sloth, and even an extinct ground sloth.

By comparing these genes, he concluded that the group’s last common ancestor had already lost one of its cones thanks to a disabling mutation. Two different branches—the armadillos, and the sloths/anteaters—then independently disabled different genes involved in building the last remaining cone.

“It was even more of a shock to discover the same thing for sloths,” says Emerling. “Sloths live in trees! Of all the things to get rid of as an arboreal mammal, daylight vision does not seem like it should be one of them!”

But Tom Cronin from the University of Maryland, Baltimore County cautions that “all-rod vision is not incompatible with daylight activity”. The great whales spend a lot of time at the ocean’s surface in full daylight, he says. And there are even people who have rod-only vision—they do well in all but brilliant sunlight, and have sharp enough vision to read in normal light. (Then again, Emerling says that this condition is sometimes called “day blindness”, and that “it’s frequently painful for these individuals to keep their eyes open during the day.”)

Cronin also notes that it’s hard to draw firm conclusions about an animal’s vision through genetics alone. Xenarthrans could have compensated for their broken cone genes, so to prove that they really have rod-only sight, you’d have to examine their retinas and actually test their vision.

Emerling acknowledges this, but points to other lines of evidence. For example, xenarthran eyes lack a muscle that helps other mammals to focus their lenses, which is suggestive of poor eyesight. There are also many anecdotal accounts of sloths, armadillos, and anteaters having terrible daytime vision. And in one experiment, sloths behaved no differently when wearing eye masks. Put all of this together, and “it seems extremely probable that armadillos and sloths lack cones entirely,” he says.

But if that’s the case, it raises another mystery: why have so many burrowing mammals, including African mole-rats, European moles, and many species of rodents, kept their cones? Despite spending their lives underground, they all have retinas that are far closer to those of day-living mammals than nocturnal ones. Subterranean living doesn’t necessarily equate to cone loss.

“There is no obvious reason why either xenarthrans or [whales] should have given up their cones,” says Cronin. So why did they?

Reference: Emerling & Springer. 2014. Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2014.2192

Correction: This post originally stated that marine mammals lost their green-sensitive cones. They actually lost their blue/violet-sensitive ones.

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Nature’s Most Amazing Eyes Just Got A Bit Weirder

Eyes are testaments to evolution’s creativity. They all do the same basic things—detect light, and convert it into electrical signals—but in such a wondrous variety of ways. There are single and compound eyes, bifocal lenses and rocky ones, mirrors and optic fibres. And there are eyes that are so alien, so constantly surprising, that after decades of research, scientists have only just about figured out how they work, let alone why they evolved that way. To find them, you need to go for a swim.

Eyes of the peacock mantis shrimp. The black bands show where it's looking. Credit: Mike Bok
Eyes of the peacock mantis shrimp. The black bands show where it’s looking. Credit: Mike Bok

This is the eye of a mantis shrimp—an marine animal that’s neither a mantis nor a shrimp, but a close relative of crabs and lobsters. It’s a compound eye, made of thousands of small units that each detects light independently. Those in the midband—the central stripe you can see in the photo—are special. They’re the ones that let the animal see colour.

Most people have three types of light-detecting cells, or photoreceptors, which are sensitive to red, green and blue light. But the mantis shrimp has anywhere from 12 to 16 different photoreceptors in its midband. Most people assume that they must therefore be really good at seeing a wide range of colours—a “thermonuclear bomb of light and beauty”, as the Oatmeal put it. But last year, Hanna Thoen from the University of Queensland found that they’re much worse at discriminating between colours than most other animals! They seem to use their dozen-plus receptors to recognise colours in a unique way that’s very different to other animals but oddly similar to some satellites.

Thoen focused on the receptors that detect colours from red to violet—the same rainbow we can see. But these ultra-violent animals can also see ultraviolet (UV). The rock mantis shrimp, for example, has six photoreceptors dedicated to this part of the spectrum, each one tuned to a different wavelength. That’s the most complex UV-detecting system found in nature. Michael Bok from the University of Maryland wanted to know how it works.

Like us, mantis shrimps see colour with the help of light-sensitive proteins called opsins. These form the basis of visual pigments that react to different wavelengths of light, allowing us to see different colours. If a mantis shrimp has six UV receptors, it should have at least six opsins that are sensitive to different flavours of UV.

Except it doesn’t. Bok could only find two.

To which: huh?

How could there possibly be six types of photoreceptors with only two opsins? There was one possibility. Something could be filtering the light hitting the different receptors.

The rock mantis shrimp. Credit: Mike Bok.
The rock mantis shrimp. Credit: Mike Bok.

Here’s an analogy: say you’ve got a big crowd lining up in front of six security guards, each of whom must shout out when they spot someone with a specific name. One recognises Adams, another targets Bobs, and so on. But the guards aren’t too bright; they wouldn’t know Adam if he introduced himself. So you make their job easier. You rig the queuing system so that only Adams line up in front of Adam-blocking guard, only Bobs reaching the Bob-blocker, and so on. The guards shout pretty much indiscriminately, but they still do their jobs correctly. They’re not specific; you impose specificity onto them.

That’s exactly what happens in the mantis shrimp’s eye. When light enters the units in its eye, it must first pass through a crystalline cone, which lies over the receptors. Bok found that these cones contain UV-blocking substances called MAAs (or mycosporine-like amino acids, in full). There are four, possibly five, of these, which block slightly different wavelengths of UV. Combine these filters with the two underlying opsins, and you get six different classes of UV receptor.

Many marine animals have one or two MAAs. They use these as sunscreens to block UV from reaching their skin and eyes, and causing damage that could eventually lead to cancer. The mantis shrimps also use MAAs to block UV but for a unique purpose: to turn their eyes into incredibly sophisticated UV detectors.

Where do the MAAs come from? It’s not clear. No animal can make these chemicals themselves, so they must get them from their environment, possibly from their diet or from microbes. But two of the MAAs that Bok discovered have never been seen before, so it’s possible that the mantis shrimps can somehow change any incoming MAAs into five different types.

“We presented these results last summer at a big vision conference and one of my colleagues said: Now, you’ve solved all the problems. What are you going to do next?” says Tom Cronin, who led the study. “We sort of feel that way. The big problem now is: What does this all have to do with vision?” Why do mantis shrimps have such ridiculously complicated eyes? That’s the big question, and no one really knows.

The team are now trying to study how mantis shrimps react to different UV signals. For example, they find some short wavelengths of UV so repulsive that they’ll avoid food that’s paired with those wavelengths. Maybe this has something to do with aggressive signals? Mantis shrimps have rich social lives and they might communicate with ultraviolet patterns reflecting off their bodies.

“That’s the leading hypothesis but it has its own problems,” says Cronin. “Signals don’t evolve unless you have the visual system to see them. So you generally don’t have a system in place to see signals unless it’s there to see something else.”  So the team are also looking at the patterns of UV light in the places where mantis shrimps live. But even if that line of research pans out, many animals share the same waters, and none of them have such a complex eye. So why does the mantis shrimp?

When I spoke to Marshall last year, he said that the mantis shrimp’s style of vision might help it to process images very quickly without much contribution from its brain. That might be useful to a predator that uses some of the fastest strikes in the animal kingdom. But of course, that’s still a hypothesis.

And there’s another baffling layer of complexity: the receptors that detect red to violet colours are connected to different nerves than the ones that detect UV, and both streams lead to different parts of the brain. The mantis shrimp didn’t just evolve an absurdly over-engineered way of seeing, it did it twice.

Reference: Bok, Porter, Place & Cronin. 2014. Biological Sunscreens Tune Polychromatic Ultraviolet Vision in Mantis Shrimp. Current Biology http://dx.doi.org/10.1016/j.cub.2014.05.071

More on mantis shrimps:

Mantis shrimps have a unique way of seeing

The Mantis Shrimp Sees Like A Satellite

Mantis shrimp eyes outclass DVD players, inspire new technology

The mantis shrimp has the world’s fastest punch

Why are stabby mantis shrimps much slower than punchy ones?

How mantis shrimps deliver armour-shattering punches without breaking their fists

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The Mantis Shrimp Sees Like A Satellite

The most extraordinary eyes in the animal kingdom belong to the mantis shrimps, or stomatopods—pugilistic relatives of crabs and prawns, which are known for delivering extremely fast and powerful punches. Their eyes sit on stalks and move independently of one another. Each eye has “trinocular vision”—it can gauge depth and distance on its own by focusing on objects with three separate regions. They can see a special spiralling type of light called circularly polarised light that no other animal can. And they have a structure in their eyes that’s similar to technology found in CD and DVD players, only much more effective.

And now, Hanne Thoen from the University of Queensland has found that mantis shrimps see colour in a very different way to all other animals.

Most people have three types of light-detecting cells, or photoreceptors, in their retinas. These are sensitive to red, green and blue light, respectively. Birds, reptiles and many fish have a fourth photoreceptor that detects ultraviolet light. Four is plenty. Mathematical models tell us that you only need four receptors, maybe five, to effectively encode the colours within that range.

The mantis shrimp has twelve different photoreceptors.

Eight of these cover the parts of the spectrum that we can see, while four cover the ultraviolet region. That seems like a ludicrous excess. If four or five receptors are all an animal needs, “why on earth do stomatopods need 12 channels?” says Justin Marshall, who led the new study.

The obvious answer is that they’re very good at discriminating between different colours. That would be a handy skill: mantis shrimps live in coral reefs, which are bursting with colours. Many of them are brightly coloured themselves, and use their lurid body parts to communicate with one another. “With 12 receptors, you’d think that they can detect colours much better than any other animal,” says Marshall.

“Actually, they’re much worse!”

Thoen discovered their surprising ineptitude by studying a small species called Haptosquilla trispinosa. She presented the animals with two optic fibres, each displaying a different colour. If they attacked the right one, they earned a tasty snack. Thoen then changed the colour of the off-target fibre to the point when the mantis shrimp could no longer tell the difference between the two.

If a human did this test, we’d be able to tell the difference between colours whose wavelengths are 5 nanometres apart—compare the left and middle columns in the image below. A mantis shrimp would struggle with that. The can only discriminate between colours with a 15-25 nanometre difference—compare the middle and right columns.

A human could tell the difference between the colours in the left and middle columns with a 50% accuracy. A mantis shrimp could only do the same for the colours in the middle and right columns.

Despite their 12 photoreceptors, mantis shrimps are worse at telling apart different colours than humans, honeybees and butterflies.

“Thoen is a very careful scientist, so the data are completely convincing, if quite surprising,” says Tom Cronin from the University of Maryland, Baltimore County, who studies mantis shrimp vision. “We certainly would have predicted a much more competent sense of color discrimination than this!  However, behaviour is the ultimate test of what an animal can do, so this is what the animals say that they are capable of.“

They must be using the information from those receptors in a very strange way.

We see colours by making comparisons between our three receptors. By comparing the outputs of the red and green receptors, we can tell the difference between reds and greens. And by comparing their combined output against that of the blue receptors, we can discriminate between blues and yellows. This is called the “colour opponent process” and it’s what every colour-sighted animal does.

Every animal… except the mantis shrimps. Given their poor performance in Thoen’s tests, they cannot possibly be making these comparisons. What are they doing instead?

“The simple answer is: Dunno,” says Marshall. “I’ll admit right up front we don’t fully understand.”

Their working hypothesis is that the mantis shrimps analyse the outputs from all of their 12 receptors at once. Rather than making comparisons between those receptors, they pass the entire pattern of outputs onto the brain, without any processing. “One could imagine that they have a look-up table in their brain,” says Marshall. So rather than discriminating between colours like we do, their eyes are adapted for recognising colours.

“Oddly enough, the closest device to stomatopods would be a satellite,” says Marshall. “Remote sensing algorithms have look-up tables of colour to fill in the image that the satellite forms.”

Marshall suggests that this way of dealing with colour should be much faster than ours, since there is no need to send the photoreceptors’ signals through any intermediary neurons. And speed matters for mantis shrimps. These ambush hunters attack their prey with rapidly unfurling arms, which end in either stabbing spears or pounding clubs. The clubbed species, known as smashers, can hit their targets with the force of a rifle bullet and deliver the fastest punches in the animal kingdom. They need fast eyes to complement their fast arms.

And they only have a small brain. “A mantis shrimp only has a fraction of our cortical processing power, yet it handles 4 times more input,” says Nicholas Roberts from the University of Bristol. “The non-comparative processing system they have evolved represents a novel solution for increasing data acquisition while minimising any downstream processing overhead.”

Of course, this is still conjecture. Thoen and Marshall have shown that mantis shrimps definitely don’t see colours in the same way as us, but what they actually do is a mystery. Now, they’re trying to work out what happens to signals when they leave the photoreceptors, and how these cells are connected to the brain.

Cronin also wants to know “whether these animals combine their colour receptor signals in different ways for different tasks. Perhaps analysis of mate displays or colour signals demands a more thorough discrimination than food recognition.”

Marshall adds that the mystery is relevant to one of the most important questions in neuroscience: How does a nervous system make sense of information from the outside world. “This is clearly a very different way of computing that information,” he says. “It’s not just about weird shrimp biology. It touches on a number of neuroscience questions.”

Reference: Thoen, How, Choiu & Marshall. 2013. A Different Form of Color Vision in Mantis Shrimp. Science http://dx.doi.org/10.1126/science.1245824

More on mantis shrimps:

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Starfish Spot The Way Home With Eyes On Their Arms

Most starfish have eyes on the tips of their arms. They’re hard to see and even if you spot them, you might not recognise them as eyes. But they can see you (as long as you’re not moving too fast).

The starfish in the top image is an Indo-Pacific species called the blue star (Linckia laevigata). Here’s a close-up of one of its arms. That groove runs up the entire underside of the arm and contains thousands of tube feet, which the animal uses to crawl about. The eye sits at the end of the groove, where the white arrow is pointing.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

It’s a tiny red nub, barely half a millimetre wide. It’s usually exposed, but the starfish can retract it into the arm if danger threatens.

Credit: Garm & Nilsson, 2103. Royal Society.

Scientists have known about starfish eyes for ages, and they’ve assumed (reasonably) that the animals use these organs to see. But no one has properly tested this, or probed the limits of their vision. Anders Garm from the University of Copenhagen and Dan-Eric Nilsson from Lund University are the first.

They started with a detailed study of the blue star’s eyes. Each one sits on the end of a modified tube foot, and contains 150 to 200 separate light-collecting units called ommatidia. You can see them below, stained with a red dye. The set-up is similar to an insect’s compound eye, but with one big difference: insect ommatidia have lenses to focus light onto the underlying cells, while starfish lack lenses of any sort.

Credit: Garm & Nilsson, 2013. Royal Society.
Credit: Garm & Nilsson, 2013. Royal Society.

Each eye has a fairly large visual field that extends over 210 degrees horizontally and 170 degrees vertically—a slightly wider range than what your eyes can cover.  And since the starfish has five of these eyes at the end of its flexible arms, it can probably see in every direction at once. Alternatively, it might be able to narrow its view to certain directions by moving the flexible black tube feet that surround each eye, and using them as blinders.

In the wild, blue stars live on coral reefs. When Garm and Nilsson placed them in front of a real reef, the starfish walked around randomly at first. But when they got within 2 metres of coral, they made a bee-line for it.

They needed their eyes for that. If Garm and Nilsson dissected out the eyes (don’t worry—they grow back after a few weeks), the blinded starfish couldn’t navigate home. They walked at the same speed, but they couldn’t find the coral.  This is clear evidence that the blue star sees with its eyes, and uses them to navigate.

However, its vision is rather poor. It’s colour-blind, and sees the world only in shades of light and dark. Its light-detecting cells work very slowly, so fast-moving objects are invisible to it. And it has poor spatial resolution, so it can’t see fine detail.

But none of this matters. Garm and Nilsson suspect that the starfish doesn’t need to see colours, details, or speedy objects, because it mostly uses its eyes to detect coral reefs. Which are large environmental shapes that don’t move. The reefs would also appear as dark splotches against a bright ocean, in the starfish’s monochrome view of the world.

“When vision first evolved, it must have supported some tasks,” says Garm. “What behaviours could you control with a very crude image? Detecting large predators? They move fast and you’d need fast vision.” And to process images quickly, you’d need a decent amount of brainpower—something that the owners of the first eyes probably lacked.

An alternative, and one that Nilsson has championed, is that the first eyes evolved to detect large, stationary objects, so that animals could recognise their habitats and find their way home. “To do that, you only need poor eyesight,” says Garm. A starfish, guided back to a reef by its five arm-tip eyes, could be giving us an example of what the first visual systems were used for.

Reference: Garm & Nilsson. 2013. Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.3011

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How A Fish Unleashed Its Evolutionary Potential And Went Blind

I’ve been to fancy restaurants where dozens of cooks toil away in the kitchen. They have their own quirks and varying degrees of skill but they all produce the same plates of food—those dictated by the head chef. He or she ensures that their recipes are followed to exacting specifications. I’ve sometimes wondered what would happen if you deposed the head chef, and let the others loose to play around with the recipes. Some would produce culinary disasters, but a precious few would probably create gastronomic triumphs.

That’s roughly what a scientist named Conrad Waddington did in the 1950s, but rather than deposing chefs, he stressed some fruit fly pupae with bursts of heat or chemicals. When the adult flies emerged, they had an array of weird features, like broken veins in their wings or extra body segments.

The genes for these odd traits hadn’t arisen out of the blue; they were already there in the flies, but their effects were somehow being masked. They were like the underling cooks in the fancy kitchen, committed to enacting the same recipes regardless of their own variation. When Waddington heated or chemically treated the flies, he lifted this repression and allowed their hidden genetic variation to manifest as physical variations.

But how does genetic variation get first concealed and then later released? What’s the molecular equivalent of the draconian head chef?

Susan Lindquist at MIT found a candidate four decades after Waddington’s experiments. It’s a protein called Hsp90. When she depleted it in flies, they grew up with weird features from extra hairs to severe deformities, just like in Waddington’s studies. These changes weren’t caused by fresh mutations, but pre-existing ones that had been unmasked in Hsp90’s absence.

As I wrote for Scientific American earlier this year, Hsp90 helps other proteins to fold in the right way, and stops them from unfolding at high temperatures. This allows the proteins to tolerate and accumulate mutations that might otherwise catastrophically distort their shapes. If the temperature goes up, Hsp90 can’t meet the demand for its services. Suddenly, proteins start to fold in a variety of different ways.

For this reason, Lindquist describes Hsp90 as an evolutionary capacitor, after the devices that store and release electrical charge. It does the same, but for genetic variation.

Lindquist’s team have gone on to show that Hsp90 can store cryptic variation in many other species, including microbes, worms, mustard cress plants, and yeast. Her former postdoc Daniel Jarosz found that this capacitor conceals a whopping fifth of all the variation it the yeast genome.

It looked like Hsp90 could be a major driving force in evolution, allowing genetic variation to build up behind the scenes and unleashing it in one burst when times are tough. Suddenly, different versions of the same gene (alleles) that were once corralled down the same path can produce a smorgasbord of new traits. Natural selection gets a bonanza of physical variation to act upon.

But there were three niggling problems.

First: virtually all of the traits that Hsp90 unleashed were defective rather than adaptive—broken-veined flies are hardly evolution’s next breakout stars. Second: most of these experiments involved lab-bred creatures, so no one knew how important Hsp90  is in the wild. Finally: what is the wild equivalent of the experimental heat shocks? “It’s hard to imagine a small browsing dawn horse encountering a particularly hot summer and instantly giving birth to the modern large grazing horse,” says Clifford Tabin from Harvard Medical School.

Lindquist recognised these problems, and she started searching for a genuine case where Hsp90 unshackled some cryptic variation, which led to the evolution of adaptive traits. She asked Tabin, who had studied the development of many unusual animals, from Darwin’s finches to hopping rodents. He thought they should look at species that suddenly find themselves in a completely new environment. He thought they should look at blind cavefish.

Millennia ago, several groups of Mexican tetra—a popular aquarium fish—swam into dark caves, and eventually evolved to be blind. Today, their embryos are born with eyes that gradually waste away, leaving hollow orbits. Tabin and Lindquist’s teams, headed by postdoc Nick Rohner, have now compiled a strong case that Hsp90 was involved in this change.

They exposed the larvae of sighted, surface-dwelling tetras to a chemical that blocks Hsp90. Some larvae ended up with much larger eyes than are ever found in nature, while others had much smaller ones. Overall, the range of sizes shot up by 83 percent. So, the surface fish have a lot of cryptic variation in eye size. The team deliberately exposed this variation by with their Hsp90-blocking drug, but they think that entering dark caves would have achieved the same effect.

They examined the water in one of the caves where the blind fish live, and showed that it’s much less electrically conductive than nearby surface waters, thanks to the calcium carbonate that leaches into it from the surrounding rocks. This low conductivity can mess with a fish’s ability to control the ions in its body, creating physical stress on a par with what Waddington’s heat-shocked flies experienced. Indeed, when Rohner raised surface tetras in water with low conductivity, they developed a much larger variety of eye sizes.

So, when these fish swam into Mexican caves, they would have experienced conditions that unleashed the cryptic variation stored by Hsp90. Some would have grown up with unusually large eyes, and others with unusually small ones.

It takes energy to maintain functioning eyes—energy that’s wasted in pitch-black caves, where eyes are of little use. In these dark worlds, the small-eyed fish would probably have done better than their big-eyed peers. Rohner simulated this evolutionary pressure by taking the Hsp90-blocked fish with the smallest eyes and breeding them with normal peers. He found that their offspring, despite having fully functioning Hsp90, still had smaller-than-average eyes.

Here’s why. The initial generation had different alleles for the genes that control eye size, and blocking Hsp90 unmasked this variation. Now, Rohner could select for the small-eye alleles by picking the small-eyed fish for his breeding experiment. These alleles became more common in the next generation of fish, which developed smaller eyes on average.

If selection acts consistently enough, you’d expect the once-cryptic variation to slowly dwindle away. Only alleles that produce advantageous traits—in this case, small eyes—would remain in the population.

That’s exactly what Rohner found when he repeated his experiment in the blind cavefish. The adults lack eyes, but they still have hollow orbits where their eyes should be. And when Rohner blocked their Hsp90, the size of these orbits stayed the same. The cryptic variation in their eye size had already been exposed earlier in their evolutionary history, and they were only left with the alleles for small eyes.

Waddington did the same thing with his lab-bred flies, almost 60 years ago. At first, he needed heat or chemicals to produce the weird individuals. As he bred these mutants with each other, he eventually ended up with generations that developed their odd features under normal conditions. He had uncovered cryptic variation, and then selected for a specific set of exposed alleles. Now, Rohner has finally found a natural example of the same process.

It’s an amazing example that proves the importance of cryptic variation in a fascinating natural context,” says Joanna Masel from the University of Arizona. “It shows that cryptic genetic variation is important, and that Hsp90 depletion can uncover that variation, but it doesn’t decisively prove that Hsp90 was the historical means by which the natural adaptation occurred.”

Joshua Plotkin from the University of Pennsylvania is more enthused. “It’s a terrific study that, at long last, demonstrates an important role for cryptic variation in the evolution of natural populations of the ‘everyday’ organisms that we can easily see in front of us,” he says. “I cannot imagine a more beautiful system in which to study this question than Mexican cavefish.”

Of course, this is just one example. We still don’t know whether Hsp90, by concealing and exposing genetic variation, steers the evolution of many species, or just a few. The process seems to rely on sudden environmental changes, and you’d certainly get those in places like deep-sea vents and glacial lakes, or at microscopic scales. But for bigger organisms, “most evolution occurs in gradually changing environments,” says Tabin. Are the blind cavefish the exceptions that prove the rule, or merely the first good example of a common phenomenon?

Finally, remember that this style of evolution is no different to what Darwin envisaged centuries ago. You still have variation that get passed from one generation to the next, and produces traits that affect the success of their owners. The only difference here is that the variation already exists in a hidden form, and gets exposed to natural selection all at once. As Plotkin once said to me: “It’s standard vanilla evolution.” It’s just faster.

Reference: Rohner, Jarosz, Kowalko, Yoshizawa, Jeffery, Borowsky, Lindquist & Tabin. 2013. Cryptic Variation in Morphological Evolution: HSP90 as a Capacitor for Loss of Eyes in Cavefish. Science http://dx.doi.org/10.1126/science.1240276.

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Why Are Reindeer Eyes Golden In Summer But Blue In Winter?

When Glen Jeffery first took possession of a huge bag full of reindeer eyes, he didn’t really want them.

Jeffery is a neuroscientist from University College London who studies animal vision, and his Norwegian colleagues had been urging him to study the eyes of reindeer. They wanted to know how these animals cope with three months of constant summer sunlight and three months of perpetual winter darkness. “I thought it was a dumb idea,” says Jeffery. The animals would probably adapt to the changing light through some neurological trick. The eyes weren’t the right place to look.

But the Norwegians persisted, and they eventually sent him a bag full of eyes, taken from animals killed by local Sami herders. The eyes were divided into two sets—one from animals killed in the summer, and another from those killed in the winter. Jeffery started dissecting them. “I opened them up and went: Jesus Christ!” says Jeffery. “Hang on. They’re a different colour”

In the summer, reindeer eyes are golden. In the winter, they become a deep, rich blue. “That was completely unexpected,” says Jeffery.

That was 13 years ago. Since then, he has been working to understand the secrets behind the chameleon-like eyes, along with Karl-Arne Stokkan from the University of Tromsø and others.

The bit that actually changes colour is the tapetum lucidum or “cat’s eye”—a mirrored layer that sits behind the retina. It helps animals to see in dim conditions by reflecting any light that passes through the retina back onto it, allowing its light-detecting cells a second chance to intercept the stray photons. The tapetum is the reason why mammal eyes often glow yellow if you photograph them at night—you’re seeing the camera’s flash reflecting back at you.

Most mammals have a golden tapetum, and so do the reindeer in summer. So why does this layer become blue in winter? Through years of dissections and measurements, Jeffrey’s team think that they have the answer. And it begins in darkness.

Credit: Alexandre Buisse

In dark conditions, muscles in your irises contract to dilate your pupils and allow more light into your eyes. When it’s bright again, the irises widen and the pupils shrink. The same thing happens in reindeer, but the interminable Arctic winter forces their pupils dilate for months rather than hours. Over time, this constant effort blocks the small vessels that drain fluid out of the eyes. Pressure builds up inside the eyeballs, and they start to swell. “The animal’s moving towards glaucoma,” says Jeffery.

These events also change the tapetum. This layer is mostly made up a collagen, a protein whose long fibres are arranged in orderly rows. As the pressure inside the eye builds up, the fluid between the collagen fibres gets squeezed out, and they become more tightly packed. The spacing of these fibres affects the type of light they reflect. With the usual gaps between them, they reflect yellow wavelengths. When squeezed together, they reflect… blue wavelengths.

So: as reindeer spend months of darkness, their permanently dilated pupils lead to swollen eyes, compressing the fibres in their tapetum and changing the colour of light they reflect.

The team also think that this makes the eyes more sensitive. They tested the retinas of reindeer eyes, both isolated ones and those still in the heads of live, anaesthetised animals, and found that the blue winter ones are at least a thousand times more sensitive to light than the golden summer ones.

Jeffery explains that when yellow light reflects off the tapetum, most of it bounces straight out again. The retina gets just one more chance to intercept it. But blue light gets scattered. “Instead of the photons bouncing back out of the eye, they bounce around and gets captured, which increases the sensitivity” says Jeffery.

But other scientists aren’t convinced by this explanation. Dan-Eric Nilsson, a vision expert from Lund University, is excited that the sensitivity of the reindeer eyes and the colour of their tapetum change with the seasons. Both are interesting, but the latter does not explain the former.

Here’s his argument: Let’s say that the retina captures around 50 percent of the light that enters the eye, and that the tapetum reflects all of the rest. The retina captures half of these reflections, ending up with 75 percent of the original total. Even if you assume that the retina was infinitely inefficient, the most the tapetum could do is to double its sensitivity. And Jeffery’s team found that the retina becomes around a thousand times more sensitive in winter. “They’ve found an interesting phenomenon, but failed in explaining it,” says Nilsson. He suspects that, instead, the reindeer is changing the levels of light-sensitive pigments in its retina.

Trevor Lamb, another eye expert at the Australian National University, agrees. “I wouldn’t be at all surprised if the retina managed to increase its sensitivity during winter through some kind of intra-retinal changes, quite separate from the tapetal ones,” he says, “but that is pure speculation on my part.”

But Jeffery’s team has another piece of evidence for their hypothesis—one which they mention briefly in their new paper but will outline more fully in a future one. “We got halfway through this project and everything’s cruising brilliantly, and we suddenly hit a brick wall,” he says. “We suddenly found animals with a green tapetum.”

It turned out that these reindeer had been bought from Sami herders and kept in large pens, where they could just about see the sodium street-lights of nearby towns. Their pupils partly dilated during the winter, the pressure in their eyes increased a little, their collagen fibres became slightly squeezed together, and their tapetums stopped halfway along their yellow-to-blue transformation. Et voila. Green tapetum.

“And we measured the sensitivity in their eyes,” says Jeffery. “Way down.”

It could still be that the changes in the eyes are independently changing the colour of the tapetum and the sensitivity of the retina. It’ll require more evidence to link the two, but both observations alone are still pretty cool. As Nilsson say, “I am not aware of any other seasonal changes in the visual optics. In that respect, this is a novel and exciting discovery.”

Reference: Stokkan, Folkow, Dukes, Nevue, Hogg, Siefken, Dakin & Jeffery. 2013. Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.2451

PS: If, like me, you do a Google image search for tapetum lucidum pictures, you’ll find several images where the eyes look topaz, rather than yellow. This is very different from the deep, rich blue of the reindeer’s winter eyes. Partly, it happens because digital cameras automatically adjust the pictures they take. But it’s also because golden tapetums do have a topaz fringe, which takes over the reflections if you photograph the animal from an angle.

Update: This article has been corrected from an earlier version, which suggested that the colour difference was obvious when Jeffery opened up the bag, not the actual eyes. Thanks to Hester van Santen for pointing out the error.

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Giant Squids Have Huge Eyes to See Shiny Charging Whales

Discovery Channel viewers have been able to look into the eye of the giant squid, in the first ever footage of this elusive predator in its natural environment. In honour of this achievement, I’m republishing this story from last year, about why the squid’s eyes are so ridiculously big, even for its already huge body.


The giant squid sees the world with eyes the size of soccer balls. They’re at least 25 centimetres (10 inches) across, making them the largest eyes on the planet.

For comparison, the largest fish eye is the 9-centimetre orb of the swordfish. It would fit inside the giant squid’s pupil! Even the blue whale – the largest animal that has ever existed – has measly 11-centimetre-wide eyes.

So why the huge leap in size? Why does the giant squid have a champion eye that’s at least twice the size of the runner-up?

Dan-Eric Nilsson and Eric Warrant from Lund University, Sweden, think that the squid must have evolved its eye to cope with some unique challenge that other animals don’t face – to spot one of the world’s biggest predators, the sperm whale.

It’s generally true that bigger eyes can see more light. You’d expect that a big-eyed squid should be able to see further than a small-eyed one, which would be useful for finding mates or prey. But Nilsson and Warrant showed that this intuitive explanation can’t account for the squid’s extreme eye.

Using a mathematical model, they found that in the deep ocean, eyes suffer from a law of diminishing returns. Small eyes can see dramatically further if they grow a bit bigger. But once the pupil passes 2.5 centimetres, these improvements become tinier and tinier. Once the pupil reaches 3.5 centimetres, and the eye itself reaches 9 centimetres, there’s very little point in making it any bigger. And that’s exactly where fish have stopped. Even though the swordfish’s head is capable of holding a much larger eye, it doesn’t.

The giant squid weighs about the same as a swordfish, but its eye is around three times bigger. Why? Using their model, Nilsson and Warrant found that enormous eyes have advantages over eyes that are merely large. Specifically, they’re much better at spotting other large objects that give off their own light, in water deeper than 500 metres. There’s one animal that fits those criteria, and it’s one that giant squids really need to see: the sperm whale.

We know that sperm whales eat the giant squid and its even larger cousin, the colossal squid. The sharp beaks of both species have been found in sperm whale stomachs, and the whales often bear the marks of their battles in their skin – ring-shaped scars caused by the serrated ‘teeth’ on the squids’ suckers.

Hold on – whales don’t glow. In the dark oceans, how could the squids see them? Nilsson and Warrant note that while sperm whales don’t produce their own light, they frequently disturb animals that do. When they dive, they knock tiny animals like jellyfish and crustaceans that flash in response.

These shimmering outlines would be too faint for most animals to see, but not the giant squid. Nilsson and Warrant showed that its huge eyes could pick up this light from 120 metres away, and they can scan a huge sphere of water for those tell-tale flashes.

Sperm whales have a long-range detector too – sonar. They produce extremely loud clicks, and time the rebounding echoes to map the water around them. Their sonar has a range of a few hundred metres, so it should always spot a giant squid before the squid sees it coming.

Still, the squid’s eye would give it enough warning to allow it to flee. Nilsson says, “Squid are generally good at fast bursts, and the few observations that have been done on live giant squid show that they are powerful animals.“ They also have large bodies, which could help them to escape at speed. Perhaps the threat of sperm whales pushed the evolution of both the giant squid’s eyes and its body.

This arms race between giant predators and giant-eyed prey may have played out once before. During the reign of the dinosaurs, reptiles called ichthyosaurs swam in the seas. They looked a bit like dolphins, but they also had massive eyes – similar in size to those of the giant squid. There were no sperm whales around in those days, but there were other massive predators like Kronosaurus and Rhomaleosaurus. Maybe the ichthyosaurs used their large eyes to avoid these giant hunters, just as giant squids use theirs to avoid sperm whales.

For now, Nilsson and Warrant bill their idea as a hypothesis, albeit one with a solid foundation. “To confirm or disprove our theory it would be necessary to make direct observations on how giant and colossal squid evade hunting sperm whales,” says Nilsson. “But because this takes place in the darkness at depths of 600-1000 metres in the sea, such observations are extremely difficult.”

A note on sizes: Very few people have successfully measured the eye of a giant squid. In the few successful attempts, the eyes belonged to dead animals and were distorted. Thankfully, Nilsson and Warrant managed to get a photo of a freshly caught giant squid, captured by a Hawaiian fisherman in 1981. There’s a standard fuel hose running over the eye, and the duo used this to gauge its size – it was at least 27 centimetres wide. They also got access to a colossal squid – the largest one ever caught – which had been frozen in New Zealand’s Te Papa Museum. Its eye was the same size – 27 to 28 centimetres. There are some larger estimates of 40 centimetres or so, but the duo think that these are overblown.

Reference: Nilsson, Warrant, Johnsen, Hanlon & Shashar. 2012. A Unique Advantage for Giant Eyes in Giant Squid. Current Biology http://dx.doi.org/10.1016/j.cub.2012.02.031

Photo: squid by National Geographic, eye by Ernie Choy

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Giant squids’ huge eyes see the light of charging whales

The giant squid sees the world with eyes the size of soccer balls. They’re at least 25 centimetres (10 inches) across, making them the largest eyes on the planet.

For comparison, the largest fish eye is the 9-centimetre orb of the swordfish. It would fit inside the giant squid’s pupil! Even the blue whale – the largest animal that has ever existed – has measly 11-centimetre-wide eyes.

So why the huge leap in size? Why does the giant squid have a champion eye that’s at least twice the size of the runner-up?

Dan-Eric Nilsson and Eric Warrant from Lund University, Sweden, think that the squid must have evolved its eye to cope with some unique challenge that other animals don’t face – to spot one of the world’s biggest predators, the sperm whale.


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Jumping spiders use blurry vision to judge distance

We don’t like blurry vision, and we go out of our way to correct it with glasses and contact lenses. But some animals aren’t so fussy. The jumping spider not only tolerates blurry images, it deliberately produces them.

Jumping spiders, as their name suggests, leap onto their prey from afar. They judge their jumps using the two huge (and rather beautiful) eyes on the front of their faces. And to gauge how far away their targets are, they use special retinas that produce sharp images and out-of-focus ones at the same time.

Other animals have many different ways of judging depth, but none of them apply to jumping spiders. Humans mostly rely on our two eyes. Each gets a slightly different view of the world and our brain uses these differences to triangulate the distance to objects in front of us. But this ‘binocular vision’ only works if the two eyes see overlapping parts of the world. Those of jumping spiders do not.

Chameleons can judge distance by sensing how much they have to focus their eyes to bring an object into sharp relief.  But jumping spiders have no way of actively focusing their eyes. Finally, some insects judge distance by shaking their heads from side to side, which makes nearby objects move further across their field of view than far ones. But jumping spiders can accurately pounce onto their prey without moving their heads.

Without any of these three methods, how could they possibly gauge their precise killing pounces with any sort of accuracy? Takashi Nagata from Osaka City University has the answer.

Each of the front eyes has a unique staircase-shaped retina, with four layers of light-sensitive cells lying one over the other. By contast, our retinas only have one such layer. Scientists have known about the staircase retinas since the 1980s, but Nagata has finally shown exactly what they do.  He found that the top two layers are most sensitive to ultraviolet light. The two on the bottom have a penchant for green.

And that’s a bit odd. The way the layers are stacked means that green light only ever focuses sharply on the bottom one (layer 1). Blue light focuses on the one above it (layer 2), but those cells aren’t sensitive to blue. Instead, they see the world in fuzzy out-of-focus green.

Nagata thinks that this fuzzy vision isn’t a bug; it’s a feature. The amount of blur depends on an object’s distance from the spider’s eye. The closer it is, the more out of focus it is on the second retina. Meanwhile the first retina always gets a sharp image. By comparing the images on both layers, the spider can gauge depth with a single unmoving eye.

To test this idea, Nagata placed Adanson’s house jumpers in a special arena where they had to leap at prey. If the arena was flooded with green light, the spiders made accurate jumps. If Nagata used red light of equal brightness, they fell short of the mark. Nagata even created a mathematical model for the spider’s eye to predict how far it would miss its jump under different wavelengths of light. The model’s predictions matched the animal’s actual behaviour.

Humans actually do something similar. We can use the blurry nature of background images to get a sense of distance, even if all other cues are removed. Indeed, photographers often use blurry backgrounds to create a greater sense of depth. But this is just one of the tricks we use to judge depth, and perhaps a minor one. For the jumping spider, it seems to be the only trick in the playbook.

Reference: Nagata, Koyanagi, Tsukamoto, Saeki, Isono, Shichida, Tokunaga, Kinoshita, Arikawa & Terakita. 2011. Depth Perception from Image Defocus in a Jumping Spider. Science http://dx.doi.org/10.1126/science.1211667

Photo by Alex Wild

The eyes have it – a tour through the stunning world of animal eyes

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The sharp eyes of Anomalocaris, a top predator that lived half a billion years ago

Before killer whales and polar bears, before sharks and tyrannosaurs, the world’s top predator was probably a bizarre animal called Anomalocaris. It lived in the Cambrian period, over half a billion years ago, when life was confined to the seas and animals took on bizarre shapes that haven’t been seen since.

Many scientists believe that Anomalocaris ruled this primordial world as a top predator. At up to a metre in length, it was the largest hunter of its time. It chased after prey with undulating flaps on its sides and a large fan-shaped tail. It grabbed at them with large spiked arms. It bit into them with a square, tooth-lined mouth. And it tracked them with large stalked eyes. (See the Prezi below for a tour of Anomalocaris’ anatomy, or load a single image with all the info.)

Now, John Paterson from the University of New England, Armidale, has uncovered new fossilised eyes that he thinks belonged to Anomalocaris. If he is right, this hunter had extraordinarily acute vision for its day, rivalling that of almost all modern insects.


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Fossil eyes show wraparound three-dimensional vision, half a billion years ago

Each of our eyes sees a slightly different view of the world, and our brain combines these signals into a single three-dimensional image. But this only works in one direction, because our eyes face straight ahead and their respective fields of vision only overlap in a narrow zone. But there was once a creature that had binocular vision in a massive arc around its body, not just in front but to the sides as well. It’s called Henningsmoenicaris scutula and it lived around half a billion years ago.

H.scutula lived in the Cambrian period, the part of Earth’s history when most of today’s major animal groups exploded into existence. It was a crustacean, one of the earliest members of the group that includes crabs, prawns and lobsters. It was just a millimetre long and almost totally encased within a bowl-shaped shield. From beneath the shield, weird spike-tipped legs propelled it along, while two stalked eyes, each just half a millimetre across, peered out at the Cambrian oceans.

These eyes are compound ones, made up of several units or ‘ommatidia’. They’ve also withstood the test of time. Their organic tissues have since been converted into the mineral apatite, and the resulting fossils perfectly retain the shape and angle of each ommatidium. The eyes are so well-preserved that Brigitte Schoenemann from the University of Bonn could use them to reconstruct how H.scotula saw the world to a “quite impressive degree”.


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Sea urchins use their entire body as an eye

Purple sea urchins look like beautiful pincushions. They have no obvious eyes among their purple spines, but they can still respond to light. If you shine a spotlight on one, it will sidle off to somewhere darker. Clearly, the purple sea urchin can see, and over the past few years, scientists have worked out how: its entire body is an eye.

For decades, scientists knew that sea urchins can respond to light, even though they don’t have anything that looks remotely like an eye. The mystery deepened in 2006, when the full genome of the purple sea urchin was published. To everyone’s surprise, its 23,000 genes included several that are associated with eyes. The urchin has its own version of the master gene Pax6, which governs the development of animal eyes from humans to flies. It also has six genes for light-sensitive proteins called opsins.

While these genes are usually switched on in the developing eye, Maria Arnone found that the sea urchin’s versions are strongly activated in its feet. Sea urchins have hundreds of “tube feet”, small cylinders that sway around amid the spines. They can use the feet to move around, to manipulate food, and apparently to see.


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Why box jellyfish always have four eyes on the sky

In the mangrove swamps of Puerto Rico, four eyes are permanently fixed on the sky. These eyes are surprisingly similar to yours. They’re assembled using the same genetic building blocks, and they have lenses, retinas and corneas. But their owner couldn’t be more different – it’s a box jellyfish, and it’s looking for some shade.

The box jellyfish (Tripedalia cystophora) is far from a simple blob with tentacles. It’s an active, manoeuvrable predator, and it finds its way around with no fewer than 24 eyes. Scientists have known about these for over a century, but people are still trying to work out what they do.

The eyes are grouped into four clusters called rhopalia, each containing six eyes. Four of these are simple pits or slit that can do little more than detect the presence of light. But the other two – the “upper lens eye” and “lower lens eye” – are far more advanced. They can actually see images, with the aid of light-focusing lenses.

Now, Anders Garm from the University of Copenhagen has found that the jellyfish always keeps its upper lens eyes pointing towards the sky. Each rhopalia sits at the end of a flexible stalk. The upper lens eye sits at the top of the cluster, and there is a heavy crystal called a statolith on the bottom. The whole structure is a weighted ball, dangling from a string. As a result, it’s always vertical and the upper lens eyes are always pointing upwards, no matter how the jellyfish’s body is angled. This animal is perpetually looking straight up, even if it’s swimming upside-down.