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. They suggest that the world’s biggest eyes evolved to spot one of the world’s biggest predators – the sperm whale.
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
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.
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”.
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.
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.
As a fish swims over the ocean floor, it’s being watched by hundreds of rocks. The rocks are actually the eyes of a chiton, an armoured relative of snails and other molluscs. Perhaps uniquely among living animals, it sees the world through lenses of limestone, and its eyes literally erode as it gets older.
Chitons are protected by a shell consisting of eight plates. The plates are dotted with hundreds of small eyes called ocelli. Each one contains a layer of pigment, a retina and a lens. People have known about the ocelli for years, but no one knew what they were made from or how much the chitons could actually see with them.
Daniel Speiser from the University of California, Santa Barbara has solved the mystery by studying the charmingly named West Indian fuzzy chiton. It all started with a surprising bath. Speiser had removed the lenses from a chiton and dipped them in a mildly acidic liquid, which was meant to clean them. Instead, it quickly dissolved them!
This is an old article, reposted from the original WordPress incarnation of Not Exactly Rocket Science. I’m travelling around at the moment so the next few weeks will have some classic pieces and a few new ones I prepared earlier.
In the caves of Mexico lives a fish which proves that a million years of evolution can be undone with a bit of clever breeding.
The 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.
Bifocal glasses allow wearers to focus on both far and near objects by looking through different parts of the lens. It’s commonly said that Benjamin Franklin invented these lenses, but they have actually been around for millions of years. In the streams of North America, the nightmarish larva of the sunburst diving beetle hunts with a pair of natural bifocal lenses.
The beetle relies on its keen eyesight to stalk other insect larvae amid often murky streams. It sees the world through no less than six pairs of eyes and in 2006, Elke Buschbeck discovered that each of these has at least two retinas. One of her students Annette Stowasser has focused on the front pair, and shown that they are unlike any other in the animal kingdom.