Finding scorpions in the desert at night is surprisingly easy. All you need is an ultraviolet torch because scorpions are incredibly fluorescent.

Fluorescence means their bodies absorb ultraviolet light and reradiate it in the visible part of the spectrum. They glow this bright neon green color.

But scorpions aren't the only animals that fluoresce. Jellyfish, amphibians, owls, and even platypus' glow under ultraviolet light.

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I'm out here in the desert near Bakersfield, California, looking for scorpions with Carl Kloock, a professor of biology at Cal State Bakersfield. He's been researching scorpion fluorescence for over a decade.

- If you have a black light it's really pretty easy. If they're there, you're going to spot them.

- [Derek] Does he look like he's in a posture to like defend himself? - [Carl] Yeah, he's not real happy. - [Derek] Okay. (man chuckling)

- [Carl] I'm going to grab my high-tech scorpion catching equipment here. It's a tongue depressor with a yellow highlighter on it. So it fluoresces in black light. So you can see where it is.

- [Derek] Got it. (man chuckling)

- [Carl] This technique is usually to put the vial behind him, then in front of him.

- [Derek] That was impressive. All right. Should we give it a shot?

- [Carl] Just gently pokes him back in. And they usually just kind of do a little side crab walk back. There you go, and just tilt it up.

- [Derek] Unbelievable. (man chuckles) Unbelievable. All right. I'm going to get him. But why are scorpions fluorescent?

- I've got about 12 hypotheses. So you test them one at a time.

- [Derek] You've got 12 Hypotheses about why the scorpions fluoresce?

- More like six actually, I was being a little bit... Almost all scorpions fluoresce. I haven't seen it myself, but some that live at living caves they don't fluoresce but it's only like one very small group. All the other scorpions fluoresce.

One possibility is just, this is a Relic Trait. It's something they developed way back when they first came out on land and just haven't lost. A chemical that has another function that just happens to fluoresce.

There are plenty of chemicals that fluoresce. I mean, we have internal bodily fluids that fluoresce. And clearly those are never exposed to UV light. So the idea of having a function for that fluorescence is kind of silly.

This fluoresce is about the same color as a scorpion, but clearly isn't a scorpion. It's actually a rock. The color is a little off.

- [Derek] But that is like, what is that? - It's plastic from Walpole.

- [Derek] You anted to know if the scorpions were fluorescing in order to attract insects.

- Right.

- [Narrator] So how did you test that?

- What I did was I used preserved scorpions like these ones and I took half of them and dip them in UV blocking marine varnish so they didn't fluoresce. And then I just used basically flypaper and took those things outside and set them side by side and then found the number of flying insects that were caught in each.

- [Derek] Okay.

- [Carl] So we were outside in moonlight and I did the same experiment, both under the new moon and the full moon. And so what I found from that was when there was a new moon, there was no difference in the number of insects caught by the fluorescent and non-fluorescent scorpions. But under the full moon, there's nice, bright UV light available there was a difference and the ones that fluoresce actually caught fewer insects.

- It seems kind of counterintuitive.

- So that tells me that my hypothesis was wrong. Which happens a lot. Basically yeah, they're not using their fluorescence to lure insects. In fact, the fluorescence is a bad thing for them in terms of their ability to catch flying insects, at least.

- Let me ask you this. If you find that fluorescence is counterproductive for the scorpion in some way, doesn't that indicate that there has to be something useful?

- Exactly. There must be something that counteracts that.

- Sort of negative. - That negative, yeah.

Scorpions are really well adapted for what they do. One of the cool things here is it scorpions are actually able to metabolize iron and nickel. And in their pincer here and on the tips of their claws, they actually have basically iron to strengthen that.

- [Derek] Iron and nickel at the end of their tail?

- [Carl] Yeah. Yeah. You can see like the color here's a little bit different and that's because of the iron.

- [Derek] That seems very aggressive to me.

One of the main things we're interested in here is how they see. So I've seen two of their eyes.

- [Carl] So, those are the median eyes right there.

- [Derek] Those two dark spots.

- [Carl] Yeah. There's a cluster of three eyes right there. And of course on the other side, so they're symmetrical. So they have a total of eight eyes.

- Can they detect the light with parts of their body that are not eyes?

- Yeah. Actually they can.

- [Derek] In a 1968 experiment, researchers put scorpions in half covered Petri dishes. Then they exposed them to bright light and all the scorpions quickly hid under the covered part of the Petri dish. Then the researchers painted over the eyes of the scorpions. So they couldn't see and repeated the experiment. But when they turned on the light, 93% of the time, the blinded scorpions still scuttled over to the covered side. The finding was remarkable. Scorpions don't need their eyes to detect light. They can sense it with their bodies.

- That showed that they have what's called an extra ocular light sense in their tail. So they can detect light with their tail. They can't form images, they can't, but they can detect light.

- The tail of a scorpion can detect light?

- One hypothesis is that they use it to communicate with one another. The idea being that they use it to determine primarily whether or not another scorpion out there is of the same species for mating.

One fairly low probability hypothesis is that they can use it as camouflage because they absorb UV light. If they're sitting on another surface that absorbs UV light and you have an organism that sees in the UV light like some owls and things like that can do, they would tend to blend in very nicely with that. So it's a possibility, but not that many organisms see in UV and not that many that eat scorpion, see in UV so it doesn't seem very likely.

- [Derek] You go for it. - [Carl] You want me to do it upclose? - [Narrator] You do it. - [Carl] Close. Here you go. See? There you go, wow.

One of the ideas is scorpions came out of the water from Silurian period long time ago. And there was a lot more UV just in sunlight and things like that then, because we didn't have an ozone layer, we didn't have, you know, all these things that are blocking UV in our atmosphere now. And so the one of the idea is that it actually acts as a sunscreen. It's way to absorb those damaging ultraviolet photons and convert them away into something and basically keep them from penetrating into the body and causing damage.

- [Derek] Go in your hole. There you go. You got him.

- And then there have been some other ones that are a little bit more out there. One of which being the one that I actually settled on at the end is that they use it as part of their sensory system to detect the presence of light in the environment.

So this was the later experiment after I gave up on the initial idea that said that didn't work.

So, all I really did in that one, very simple, put a scorpion in each one of these things here and then put this under ultraviolet light and exposed it to ultraviolet light.

And so I measured how long they spent exposed versus unexposed and how many times they went back and forth.

- [Derek] And you were testing scorpions that were fluorescent and not fluorescent.

- [Carl] Right.

So what I did with that was I developed a technique to remove, or at least reduce the ability of scorpions to fluoresce simply by exposing them for long periods of time to ultraviolet light. So basically, you're just kind of taking the chemical that is causing fluorescence in their exoskeleton and your photo bleaching.

- [Derek] So you're kind of breaking it down?

- [Carl] So basically, you're making the chemical not be able to function properly anymore. What we found is the activity levels changed significantly. So when you exposed to UV light, the non-fluorescence scorpions acted like they were in the dark. There was no difference between their behavior and the dark and in UV light. Whereas when UV light was present for the ones that can fluoresce, they reduced their activity levels.

- [Derek] What's the conclusion from that?

- [Carl] The conclusion from that is that the fluorescents itself is acting as a way to, for them to detect the presence of ultraviolet light. Another researcher by the name of Douglas Gaffin, came up with a great phrase for it. I wish I'd come up with the phrase, but I gotta give credit. He calls it, a whole-body photon detector. So it's part of their sensory system.

What we don't know is why they want to detect ultraviolet light so badly that they've turned their whole body into a photon detector.

The suspicion is that it has to do with determining whether or not they should come out at night.

- [Derek] The idea is when UV light hits a scorpion's body, it fluoresces and the tail detects this emitted green light alerting the scorpion that it is exposed so it searches for cover.

- [Carl] Scorpions are good at starving for long periods of time. They don't need to eat that often. And coming out and foraging is dangerous for them. And so they don't like to come out when it's a moonlit night. Typically the only ones you'll find out in the full moon are the ones that are really hungry, really need food. If they're well fed, they'll stay down. So they're using that as basically their cue in the environment and saying, you know, okay, this is how bright it is. So that's a cue as to how likely I'm being preyed upon. And then here's how hungry I am. That's a cue how badly I need to get out and get some food.

- [Derek] Right. - [Dr. Carl] And then the trade off between those two is basically should I go out tonight or should I stay in my hole. - [Derek] Yeah. (techno sound)