Unlike most jellies, box jellies are not passive drifters, but actively pursue prey. The box jellies Chiropsella bronzie is able to navigate through obstacle ridden mangrove swamps, in order to find and capture small copepods, which concentrate in shafts of sunlight poking through the mangrove canopy overhead. If the jelly gets too far from the mangroves, it risks getting caught in the open lagoon and starving. To keep close to the water’s edge, they keep a constant eye on the sky.Box jellies have many eye types, including an “upper lensed eye” that maintains a constant upward gaze. To do this, the jelly exploits gravity, by weighing down the base of an eye cluster, called a rhopalium. With one rhopalium at each corner of the jelly, there are a total of four rhopalia. The upper lens eye is one of four eye types found on the rhopalia, there are also slit and pit eyes, capable of detecting changes in light and dark, and a “lower lensed eye”. While the pit and slit eyes are quite simple, the lensed eyes are similar in structure to human eyes. The whole rhopalium structure sits on a long, thin and very flexible stalk. Opposite each upper lensed eye, on the underside of the rhopalium is a weight, formed by a small crystal. This weight, along with the flexible stalk, ensure that the upper lensed eye is indeed always facing up, even when the jelly is upside down or sideways.
The upper lensed eye has a perfect view of the world above. Like HAL’s many watching eyes, the upper eye is able to take in the full 180° view. This is achieved through a special trick of optics, called “Snell’s window”. Snell’s window is a 97° underwater circular window through which the full 180° view of the above-water world is compressed. In other words, light bends and distorts when it hits the water’s surface, so that the whole sky can be seen in a circular view:
But is the jelly really using its ever up-gazing upper lensed eye to navigate, keeping a constant watch on Snell’s window? To test this, Anders Garm and colleagues went jellyfishing. They placed wild jellies in a circular tank, and tracked their movement while the tank was inched farther and farther away from the mangroves. Sure enough, when the jellies were in their mangrove home with canopy overhead, they moved randomly. But as they got nervously far from the canopy, they did a b-line back to shore, watching the trees fade into the distance, and determining that indeed this was quite far enough.
The upper lensed eye enables the jelly to move through space, like a tiny ship navigating the stars. But what about the lower lensed eye? No one is quite yet sure. I wonder if it perhaps helps the jelly catch prey, or track its distance from the bottom. Much like Dave and Frank in 2001: A Space Odyssey, it’s hard to know just where and what this strange creature can see. But one thing is for certain, if you find yourself looking down at a Chiropsella bronzie, it will be looking back up at you.
 Box Jellyfish Use Terrestrial Visual Cues for Navigation