He calculated that the jellyfish can see the mangrove canopy up to 8 metres away from its edge, and they can swim towards it. 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. With its upwards pointing eyes, the box jellyfish gets a complete 180 degree view of the world above the 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. This is Snell’s window, and it’s caused by refraction, where light bends as it moves from air to water. You’ll see a 180 degree view of everything above the water, compressed into a bright circular window.
Dive to the bottom of a pool and look upwards. This is a phenomenon that underwater photographers will be familiar with, and you can see it yourself the next time you go swimming. With a visual field of 95 degrees, the jellyfish is perfectly poised for looking up through “Snell’s window”. And it just so happens that at a certain depth, the entire world above water is funnelled through that cone. Imagine a cone sitting on top of the eye, with an angle of 95 degrees – the eye can see everything within the cone. And, as Garm found, the jellyfish is beautifully adapted to keep an eye (or four) on the canopy.īy 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). It needs to stay within the canopy to find its food if it drifts into the open lagoon, it will starve. These light patches attract small animals called copepods, which the jellyfish eats. The water is broken up by shafts of light, which pierce through holes in the canopy above. This adaptation helps the jellyfish navigate through the murky water of its mangrove home. This animal is perpetually looking straight up, even if it’s swimming upside-down. 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. The whole structure is a weighted ball, dangling from a string. The upper lens eye sits at the top of the cluster, and there is a heavy crystal called a statolith on the bottom. Each rhopalia sits at the end of a flexible stalk. Now, Anders Garm from the University of Copenhagen has found that the jellyfish always keeps its upper lens eyes pointing towards the sky. They can actually see images, with the aid of light-focusing lenses. But the other two – the “upper lens eye” and “lower lens eye” – are far more advanced. Four of these are simple pits or slit that can do little more than detect the presence of light. The eyes are grouped into four clusters called rhopalia, each containing six eyes. Scientists have known about these for over a century, but people are still trying to work out what they do.
It’s an active, manoeuvrable predator, and it finds its way around with no fewer than 24 eyes. The box jellyfish ( Tripedalia cystophora) is far from a simple blob with tentacles. But their owner couldn’t be more different – it’s a box jellyfish, and it’s looking for some shade. They’re assembled using the same genetic building blocks, and they have lenses, retinas and corneas. These eyes are surprisingly similar to yours. In the mangrove swamps of Puerto Rico, four eyes are permanently fixed on the sky.
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