Ed Yong in National Geographic:
“If you ask people what animal eyes are used for, they’ll say: same thing as human eyes. But that’s not true. It’s not true at all.”
In his lab at Lund University in Sweden, Dan-Eric Nilsson is contemplating the eyes of a box jellyfish. Nilsson’s eyes, of which he has two, are ice blue and forward facing. In contrast, the box jelly boasts 24 eyes, which are dark brown and grouped into four clusters called rhopalia. Nilsson shows me a model of one in his office: It looks like a golf ball that has sprouted tumors. A flexible stalk anchors it to the jellyfish.
“When I first saw them, I didn’t believe my own eyes,” says Nilsson. “They just look weird.”
Four of the six eyes in each rhopalium are simple light-detecting slits and pits. But the other two are surprisingly sophisticated; like Nilsson’s eyes, they have light-focusing lenses and can see images, albeit at lower resolution.
Nilsson uses his eyes to, among other things, gather information about the diversity of animal vision. But what about the box jelly? It is among the simplest of animals, just a gelatinous, pulsating blob with four trailing bundles of stinging tentacles. It doesn’t even have a proper brain—merely a ring of neurons running around its bell. What information could it possibly need?