Why box jellyfish always have four eyes on the sky

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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.

This adaptation helps the jellyfish navigate through the murky water of its mangrove home. The water is broken up by shafts of light, which pierce through holes in the canopy above. These light patches attract small animals called copepods, which the jellyfish eats. It needs to stay within the canopy to find its food; if it drifts into the open lagoon, it will starve. And, as Garm found, the jellyfish is beautifully adapted to keep an eye (or four) on the canopy.

By analysing the upper lens eye, Garm found that it has a visual field of around 95 to 100 degrees (which is roughly what ours is). Imagine a cone sitting on top of the eye, with an angle of 95 degrees – the eye can see everything within the cone. And it just so happens that at a certain depth, the entire world above water is funnelled through that cone.

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With a visual field of 95 degrees, the jellyfish is perfectly poised for looking up through “Snell’s window”. This is a phenomenon that underwater photographers will be familiar with, and you can see it yourself the next time you go swimming. Dive to the bottom of a pool and look upwards. You’ll see a 180 degree view of everything above the water, compressed into a bright circular window. This is Snell’s window, and it’s caused by refraction, where light bends as it moves from air to water. It looks as if the entire world had been warped into a cone with an angle of 97 degrees, which is almost exactly what the box jellyfish can see with its upper lens eyes.

With its upwards pointing eyes, the box jellyfish gets a complete 180 degree view of the world above the water. To see what it sees, Garm went diving in jellyfish-filled lagoons, and used a wide-angle lens to snap pictures through Snell’s window. He calculated that the jellyfish can see the mangrove canopy up to 8 metres away from its edge, and they can swim towards it. Any further than that and they can’t see anything but pure blue sky. Without knowing where to go, they run the risk of being stranded in open waters and dying.

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Garm proved that his calculations were correct by capturing real jellyfish. He put them in a tank to block out any smells or pressure clues, and placed the tank back among the mangroves. If the tank sat within 8 metres of the canopy edge, the jellyfish would swim towards it. When Garm drove the tank further away, or covered with a sheet, the jellyfish swam randomly. They must have been looking at the forest overhead.

So far, this is the only known role for the upper lens eye. The lower lens eye seems to help the jellyfish to avoid obstacles, and to move away from dark objects, but the others are a mystery. “The pit eyes are not forming images, but seem to measure the light intensity at the edges of Snell’s window,” says Garm. He suspects that they calibrate the entire rhopalia. They connect to muscles on the stalk, and they probably allow the jellyfish to adjust the rest of its eyes into the right place.

The slit eyes are even stranger. “They form images, but only in the vertical plane,” says Garm. “Our working hypothesis is that they are used to estimate the depth the medusae are at and thereby help them stay at the surface.”

Garm thinks that this might be what eyes were like when they first evolved. Early animals would have different sets of eyes that were specialised for different tasks.  Only later did all these functions come under the auspices of a single set of eyes.

Many other animals have multiple eyes with different functions. Garm says, “The many eyes of spiders also have different functions. Normally, a pair of their middle eyes is larger and provide the highest resolution used, for example, in hunting and navigation. Others with less resolution are used for light intensity measurements and motion detection.”

And how does the box jellyfish process the information from its eyes? It has a nervous system, but a very simple one. Each rhopalia has around 1,000 nerves and a ring nerve connects them – that’s around 4 per cent of what a fly has, and 0.000004 per cent of what a human has. “The box jellyfish solution may thus be linked to the absence of a central brain, but it defeats the idea that a central brain is a prerequisite for advanced behaviour”, writes Garm.

Reference: Garm, Oskarsson & Nilsson. 2011. Box Jellyfish Use Terrestrial Visual Cues for Navigation. Current Biology.http://dx.doi.org/10.1016/j.cub.2011.03.054