In the deep ocean, the brownsnout spookfish can look up and down at the same time, with some of the animal kingdom’s strangest eyes. Each one is split into two connected parts, so it looks like the spookfish has four eyes. One half points up and the other points down, allowing the fish to look at both sky and abyss simultaneously. The downward eye is unique. Unlike the eyes of all other back-boned animals, which use a lens to focus light, this one uses mirrors. It uses hundreds of tiny crystals, arranged in a curved shape, to collect and focus light.
By reflecting light, rather than refracting it, these outer eyes could produce brighter images with higher contrasts that lens-carrying eyes normally would. That must give the fish a great advantage in the deep sea, where the ability to spot even the dimmest and briefest of lights can mean the difference between eating and being eaten.
Read more: Spookfish eye uses mirrors instead of a lens