This episode is sponsored by Brilliant.org [♪ INTRO].

Put a scorpion under a blacklight and it'll light up like a neon sign. And they aren't the only organisms that glow in this way.

We've found other invertebrates as well as fish, amphibians, reptiles, and birds that do the same kind of thing—there's even a flying squirrel that glows neon pink. All of these are examples of biofluorescence. And this ancient, common, and somewhat mysterious trait might be a signature of life itself.

Normally, when you think of animal colors, you think of pigments in their fur, skin, or feathers, which reflect certain wavelengths of light. Red pigments, for example, absorb blue and green light, but reflect red wavelengths—hence why they look red to us. Fluorescence, though, is different.

Things that fluoresce don't just reflect light, they're actually creating new light of their own. When a fluorescent molecule is struck by the right particle of light, it absorbs it, and because of that little bit of energy, it becomes excited for just a handful of nanoseconds. Then, it relaxes and — this is the important bit — it kicks out a new photon that has less energy than the original.

So the light emitted has a longer wavelength and therefore is a new color. This isn't unique to one single chemical. Lots of chemicals can do this.

There are even rocks and minerals that fluoresce. But many of the fluorescent chemicals we see in animals share a certain structure — several carbon rings combined together. Take the famous green fluorescent protein or GFP—it glows thanks to some rings in the center of the protein.

And how these proteins evolved is a bit of a scientific head-scratcher. Fluorescent proteins can be found in animals that aren't closely related. And that suggests they arose independently multiple times—a phenomenon known as convergent evolution.

On the other hand, many of them look very similar — perhaps a little too similar to be separate evolutionary efforts. So some scientists think fluorescent proteins arose really early on in one of the first animal ancestors. But if that's the case, it's not clear why it was later lost by so many lineages like our own.

Some have even suggested their presence might be explained by multiple events of horizontal gene transfer — a rare phenomenon where a gene jumps from one organism to another. Wherever they came from, these GFPs and other proteins can explain how life fluoresces, but it doesn't really explain why. It's especially hard for us humans to answer that question because we don't see the world in all its colorful glory.

We can't see ultraviolet wavelengths, so we don't realize which things stand out to other organisms because they're glowing certain colors instead of reflecting UV. But, with some help of blacklights, we're starting to see this hidden neon world—and we're learning that all sorts of life is rave-ready. In many cases, like those pink squirrels, animals probably use fluorescence the way they would other kinds of coloration—to camouflage themselves, or to communicate.

But because fluorescence essentially converts light into different wavelengths, it can do much cooler things. Shallow-water corals, for instance, seem to use fluorescent proteins as a kind of sunscreen for their symbiotic algae—they absorb potentially harmful UV rays and emit wavelengths that are less harsh. Deep-water corals also fluoresce.

They don't have to worry about UV damage, though. Their problem is getting enough light in the first place—see, it's harder for light to move through water than air, so as you go further down, you lose lower-energy wavelengths like reds and yellows. Eventually everything starts to look kind of blue.

But that blue light doesn't penetrate as deeply into the corals' tissues. So for deeper corals, fluorescing—especially in shades of orange, yellow and red—can help ensure what little light is available reaches their algal partners. And for other creatures that live down in those depths, the ability to fluoresce can add reds and yellows back to their world.

Some reef fish use these new colors to stand out, while others basically shine them like a special flashlight to reveal hidden prey. But perhaps the coolest fluorescent adaptation is back on land with those scorpions. Some researchers think their eerie glow helps them sense moonlight so they can avoid predators.

Like us, they can't see UV light directly. But, their fluorescent proteins can convert the UV rays from bright moonlight into a color their eyes are more in tune with, letting them see how bright their surroundings are. They might even be able to sense this blue-green light with nerve cells found all over their bodies, essentially turning the entire animal into one big moonlight sensor.

However animals use their neon glows, one thing is clear: lots of living things fluoresce. But that doesn't mean it's always a useful adaptation. Basically all plants fluoresce red when they're making food from sunlight—a fact botanists can use to measure how productive they are.

But the red glow doesn't necessarily do anything for the plants. Similarly, there are some places fluorescence is found in animals, like in crayfish brains, where we're not yet sure how or if it's actually helpful. It's more than likely some of the brilliant neon glow we see when we light up the world with a blacklight is simply a coincidence of the way some molecules are structured.

Still, fluorescing just seems to be something life does. And that might be true elsewhere in the universe, too. In a 2018 paper from the Monthly Notices of the Royal Astronomical Society, two scientists argued that the fluorescence of animals like corals could leave a distinct biosignature in the light reflected from a planet during a UV flare.

And that means the neon glow of life could reveal whether it's out there somewhere else in the universe. Which is just kind of beautiful. In case you haven't noticed, astronomers spend a lot of time examining light from the universe.

And if you want to understand why that is, you might like the course on astronomy from Brilliant.org. It explains the basic toolbox astronomers use to investigate the cosmos, including looking at light. And that's just in the first set of quizzes!

You can also dive deep into the life cycles of stars, or how we look for habitable worlds. And astronomy is one of the interactive lessons and quizzes in math and science Brilliant offers. You can check it and all the others out at Brilliant.org/SciShow.

Right now, the first 200 people to sign up at that link will get 20% off of an annual premium subscription to Brilliant. And in addition to having a great time and learning about the universe, you'll also be supporting SciShow. So, thank you! [♪ OUTRO].