If you're a person with sight, your two eyes are your only window into the visual world.

And while it might be hard to imagine seeing with any other part of your body, it's not actually so rare for other animals to use different organs for sight.

Like slugs. Slugs have eyes, but even if they lose their eyesight, they can see using light receptors on their brains. And understanding how they do this could help us understand the evolution of sight. Now, the fact that slugs can see with their brains might not sound that special, since technically humans use their brains to see, too.

At least, our brains transform signals from the eyes into visual scenes. But human brains can't see on their own, and slug brains can. Scientists have known for a while that slugs have neurons in their brains that respond to light, but they didn't know exactly how they worked or what role they played.

So in a 2019 study, researchers decided to investigate. Now, normally, slugs make their way around the world guided by highly sensitive eyes on the tips of two tentacles. They try to avoid light as much as possible since it can dry them out.

And since slugs are mostly active at night or on cloudy days, they're especially sensitive to short-wavelength blue light, which is visible even when it's not bright out. But the researchers wanted to see how they'd respond to light without eyes, so they cut them off.

And they found that even blinded slugs would avoid short-wavelength light. Now that was only true if it was shining on their heads. The slugs didn't react to light shining on their tails, which helped the researchers narrow down where the response was coming from.

In the end, the slugs seemed to be detecting the light through the same light-sensitive proteins researchers had seen in past studies, known as opsins. Opsins become activated whenever they absorb light, and they're not unique to slugs--they play a role in vision throughout the animal kingdom, from jellyfish to humans, often as part of an eye. The opsins slugs have in their brains are not very helpful in telling them where light is coming from--so, in the experiment, the slugs sort of stumbled around until they found their way into a darker area.

But even so, it still served as a useful system to help slugs navigate into the shade. And slugs aren't the only creatures with brains that are sensitive to light. Many other animals have this trait, too, like fireflies, tadpoles, and baby fish.

In fact, in vertebrates, this form of light detection is shared so widely among distantly related species, scientists think it might represent a common evolutionary trait. One that seems to be older than eyes. So, in the end, as strange as it sounds, studying eyeless slugs might actually be shedding light on unknown steps in the evolution of vision.

Thank you for watching this episode of SciShow. If you like slugs and you want to find out more about the wonderful things that we can learn from the world's amazing slimy creatures, you should check out our episode on "Nature's Most Unusual Slime".