Why are Scorpions Fluorescent
Why are Scorpions Fluorescent?
Fluorescence means bodies absorb ultraviolet light and re-radiate 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.
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. I'm going to grab it with my high-tech scorpion catching equipment here. It's a tongue depressor with a yellow highlighter on it. So it fluorescence in black light. So you can see where it is. Got it. This technique is usually to put the vial behind him, then in front of him. All right. Should we give it a shot? Just gently pokes him back in and they usually just kind of do a little side crab walk back.
But why are scorpions fluorescent? - I've got about 6 hypotheses. Almost all scorpions fluoresce. I haven't seen it myself, but some that live in living caves don't fluoresce, but it's only like one very small group. All the other scorpions fluoresce.
One possibility is that 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 aren't 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.
You wanted to know if the scorpions were fluorescing in order to attract insects, right? So how did you test that? You ask? What I did was I used preserved scorpions like these ones and I took half of them and dipped them in UV blocking marine varnish so they didn't fluoresce.
And then I just used basic 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.
So we were outside in the moonlight and I did the same experiment, both under the new moon and the full moon. And so what I found was that 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 was nice, bright UV light available. There was a difference between 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's negative, yeah.
Scorpions are really well adapted to what they do. One of the cool things is that scorpions are actually able to metabolize iron and nickel. And in their pincers here and on the tips of their claws, they actually have basic iron to strengthen that.
So yeah, then you can see the color is a little bit different and that's because of the iron. 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 those are the median eyes right there. Those two dark spots. 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. So you may ask can they detect light parts of their body that are not eyes? Answer is actually, they can.
In a 1968 experiment, researchers put scorpions on half-covered Petri dishes. Then they exposed them to bright light and all the scorpions quickly hid under the cover of the Petri dish. Then the researchers painted the eyes of the scorpions. So they couldn't see repeat 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 eyes to detect light. They can sense it in 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, but they can detect light.
The tail of a scorpion can detect light? Sounds vague right? One hypothesis is that they use it to communicate with one another. The idea is 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 it absorbs UV light. If they're sitting on another surface that absorbs UV light and you have an organism that sees UV light like some owls and things like that can do, they would tend to blend very nicely with that. So it's a possibility, but not that many organisms see UV and not that many scorpions see UV, so it doesn't seem very likely.
One of the ideas is scorpions came out of the water from the Silurian period a long time ago. And there was a lot of 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 one of the ideas is that it actually acts as a sunscreen. It's a way to absorb those damaging ultraviolet photons and convert them into something else, and basically keep them from penetrating into the body and causing damage.
And then there have been some other ones that are a little bit more out there. One of which is 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 didn't work. So, all I really did was one thing, very simple. I put a scorpion in each of these things and then put them under ultraviolet light and exposed them to ultraviolet light. And so I measured how long they spent exposed versus unexposed and how many times they went back and forth.
So what I did with that was develop 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 causes fluoresces their exoskeleton and your photo bleaching.
What we found is that the activity levels changed significantly. So when you are exposed to UV light, the non-fluorescence scorpions act like they are in the dark. There was no difference between their behavior in the dark and in UV light. Whereas when UV light was present for the ones that can fluoresce, they reduced their activity level. The conclusion from that is that the fluorescent itself acts as a way 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 got to 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 bodies into photon detectors. The suspicion is that it has to do with determining whether or not they should come out at night. The basic idea is that 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 and searching for cover.
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 at the full moon are the ones that are really hungry and 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 of how badly I need to get out and get some food. And then the trade off between those two is basically, should I go out tonight or should I stay in my hole? So yeah that's the basic idea behind it.
Conclusion
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