Why do scorpions glow under UV light?

All scorpions glow a bluish-green under ultraviolet (UV) light. If you take a “black light” outside on a summer night, you may be surprised about how many scorpions you find. For a more general article on scorpions, see Scorpions, Vinegaroons, and Sun Spiders

Scorpion glowSo, why do they glow? There are really two questions here: 1) what is the mechanism, and 2) does it provide some advantage or protection?

The mechanism:

A hyaline coating (cuticle) on the exoskeleton of a scorpion contains beta-carboline and 4-methyl, 7-hydroxycoumarin which absorb UV light and retransmit it as visible bluish-green light. Young scorpions and recently molted scorpions don’t glow until the cuticle hardens. These chemicals may help the exoskeleton become impermeable.

“According to scorpion expert Dr. Scott A. Stockwell, this could mean that the substance that causes fluorescence is a byproduct of the hardening process itself, or it might be secreted not long after the creature molts.

“Whatever its source, the glowing property is surprisingly long-lasting. When scorpions are preserved in alcohol, the liquid itself sometimes glows under UV light. And the hyaline layer is amazingly durable: It can survive millions of years, Stockwell says; it’s often found in scorpion fossils even when all other parts of the cuticle have vanished. What’s more, even fossilized hyaline fluoresces!” (Source)

The Why (maybe):

Why scorpions developed this trait is subject to much speculation. One thing to note: moonlight transmits UV light.

Is the glow just a random act of nature? California State University arachnologist Carl Kloock thinks otherwise. Over the past few months, Kloock and his colleagues have started unraveling the mystery of why scorpions glow.

“They may be using UV as a way to determine whether or not to come to the surface to look for prey, based on the light levels.”

Scorpions are nocturnal creatures. They abhor the heat and evaporative effects of sunlight, and it turns out they specifically avoid UV light too. In a recent issue of the Journal of Arachnology, the Cal State team reported that the arachnids adjust their activity level depending on the amount of UV shining on them. When flooded in UV, they are less active than when lights are dim.

“My thinking at this point for why they would respond to UV is that there is a UV component in moonlight,” Kloock wrote in an email. If scorpions are hungry, he explained, they’ll come out and hunt regardless of light levels. But if they’re satiated, research shows they tend to lie low on moonlit nights, especially around the time of the full moon. “(Fluorescence) may be part of the mechanism by which the scorpions respond to moonlight.” ( Natalie Wolchover, NBCnews Source)

On the other hand:

Douglas Gaffin from the University of Oklahoma has a more intriguing idea. He thinks that scorpions glow to convert the dim UV light from the moon and the stars into the color that they see best – blue-green. This could explain why scorpion eyes are so exquisitely sensitive, to the point where they can detect the faint glow of starlight against the background of the night sky. They amplify those faint signals by turning their entire bodies into light collectors.

Why bother? In the open, scorpions are vulnerable to rodents, owls and other predators. They like shelter, and they’ll instinctively flee from light in an attempt to find it. In the wild, you’ll often find them in the shade of a single twig or blade of grass. Gaffin thinks that scorpions could easily find such hiding spots by sensing light with their entire bodies. Any object that casts shade upon their skin could reduce its glow and indicate a potential hiding place. (Ed Yong, Discover, Source)

Others speculate that the ability to turn UV light into visible light acts as a sun screen. Not too useful for a nocturnal animal. Some have proposed that scorpion glow is used to “dazzle” predators that are sensitive to UV light. As you can see, the science is not settled.