For over a century, the mismatched eyes of cockeyed squid have bewitched the minds of scientists. These denizens of the deep have one bulging eyeball and one tiny peeper – a complete underwater oddity.

The squid’s right eye is small, blue, and deep set into their head. The left eye is bulbous, yellow, and more than twice the size of the right. How did such a difference come to be?

"You can’t look at one and not wonder what’s going on with them," said Kate Thomas, a biologist at Duke University and lead author of the study published in Philosophical Transactions B, in a statement. 

The first place to start when determining why such extreme eye evolution has taken place is to look at the creature’s environment. The mesopelagic region of the sea is a dim, dark habitat in which to dwell. At between 200 and 1,000 meters (650 and 3,280 feet) in depth, the eerie darkness has triggered an explosion of ocular marvels. In fact, this domain has the highest diversity of visual adaptations in the sea.

"The deep sea is an amazing natural laboratory for eye design, because the kinds of eyes you need to see bioluminescence are different from the kinds of eyes you need to see the basic ambient light," said senior author Sönke Johnsen, a professor of biology at Duke University. "In the case of the Histioteuthis, this cockeyed squid, they chose one eye for each."

The bulbous eye is used to search for shadows against the ambient light from the water’s surface, while the smaller eye roves the deep sea for flashes of bioluminescent prey and predators.

While such an idea has been proposed before, Thomas gathered the first behavioral evidence. She sifted through more than 150 underwater videos collected by the Monterey Bay Aquarium Research Institute (MBARI) via remotely operated vehicles. 

Thomas found 152 sightings of the cockeyed squid, more often than not drifting upside down, if such a thing exists in the deep sea. The creatures' big eye routinely peered up towards the surface, while the small peeper pointed down towards the depths. 

Over time, it’s likely that the eye pointing up benefits from being larger, improving its sensitivity to discern shapes in the dim light. On the other hand, an eye searching for flashes of bright light against a dark backdrop has little need to be big.

"The eye looking down really only can look for bioluminescence," Johnsen said. "There is no way it is able to pick out shapes against the ambient light. And once it is looking for bioluminescence, it doesn’t really need to be particularly big, so it can actually shrivel up a little bit over generations. But the eye looking up actually does benefit from getting a bit bigger."

Not only that, but two bulbous eyes are more costly to maintain than one. If you don’t need an optic organ that large, it’s a waste of resources. Evolution is all about efficiency, even at the sacrifice of symmetry.

So there you have it, two eyes for two different purposes – an ingenious, if odd, solution.