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If the lights go out or you find yourself somewhere truly dark, your eyes are pretty useless. Unless we're wearing night vision goggles, even the most eagle-eyed humans just can't see in the dark. But some animals don't need goggles or fancy tech to see in the dark, like salmon.

Salmon can just turn on night vision in their eyes, and even though humans will never be capable of this trick, understanding how salmon do it reveals the amazing way biology and physics combine to create the experience of vision.

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Now it's not totally surprising that another animal can see things that we can't, because no seeing creatures see the world exactly the same way. Dogs can't see all the amazing colors we do, and we can't see all the incredible ultraviolet patterns in nature that it's believed bees and butterflies can perceive. We all see the world through different eyes, but in most of us - whether we're humans, dogs, butterflies, or bees - what we can see is hardwired into us.

Our eyes are tuned to see specific wavelengths of light down to our genes; we can't just change what's visible to us on the fly. But salmon can, and that's weird, because - broadly speaking - salmon do see a lot like us: They have eyes with these special cells called photoreceptors, just like we do. And inside these photoreceptors, there are molecules called photopigments.

Pigments absorb specific wavelengths of light, and whenever some light gets absorbed, it sets off a chain reaction inside the cell that ends with an electrical impulse. These electrical impulses then go to the brain, where they get transformed into some image of the world, and voila! You've got sight. Different pigments have different shapes, which lets them absorb different wavelengths; so the types of pigments you have can change the range of your vision and the colors that you see. Overall, that's vision in a nutshell whether you're a person or a salmon.

The thing is though, over a century ago, scientists found that some species were able to change their pigments and expand the range of their vision. Animals like salmon and leopard frogs could extend their vision further into the red end of the spectrum, even into infrared wavelengths. Infrared light radiates from anything with a temperature, so it's visible even in what we think of as total darkness. In other words, these animals were basically turning their eyeballs into night vision goggles on the fly.

Now, it was easy enough to see why they would do this; the fish and amphibians with this ability were known for moving between environments with different conditions. For instance, when salmon migrate, they travel from the ocean to rivers and streams. These freshwater environments tend to be murkier than the open ocean, so they filter out a lot of visible light, especially blue wavelengths. Most of the light that makes it through is red or infrared, so by expanding their visual range, salmon can pick up more of the wavelengths that are actually available to them in these places.

But scientists still had no idea how they were doing it, and it took decades of research before they finally figured it out; the secret turned out to be a single enzyme (Cyp27c1). Enzymes are proteins that jumpstart chemical reactions in the body, and this enzyme is involved in the processes that make the eyes pigments.

One key ingredient you need to make pigments is vitamin A, but there are different kinds of vitamin A. The stuff we get from carrots and lots of other foods is vitamin A1; that's the main form of vitamin A that we have in our bodies. But this particular enzyme converts the vitamin A1 in salmons' bodies into a slightly different molecule - vitamin A2. These two forms of vitamin A generate different pigments. The one generated by vitamin A2 is especially sensitive to long wavelengths, so it enhances salmon's vision on the red end of the spectrum. So thanks to one enzyme, salmon can tune their vision by balancing the amount of vitamin A1 and A2 in their eyes. It's not like they're just flipping some vision switch at will, but their bodies seem to react this way automatically in response to changing light in their environment.

Now, long before scientists understood how salmon and other animals managed to see in the dark, they knew it had something to do with the presence of vitamin A2. So naturally, scientists wondered if they could give humans night vision by getting some A2 into their systems, and some of them actually gave it a try. Back in the 1940s, a group of scientists fed eight med students a bunch of pike liver, which is high in A2, and then they measured the effect on their vision compared to a control group. And the experiment actually worked... A little.

They were able to see a little bit beyond the usual end of the visible spectrum, but not enough to actually give them night vision. So sadly, there's no superpower I can turn on to see where I'm going when I'm driving through Montana in the dark. But the fact that salmon basically *can* turn on night vision shows that it's not some completely mysterious superpower; it's the same combination of biology and physics that lets us see the world at all.

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