Why do scorpions fluoresce under UV light?

A guest blog on scorpions by my good friend, Geoff Oliver.

Having had a couple of Emperor Scorpions as pets (“Grabbit” & “Scarper”), I was familiar with the fact that they glow eerily under ultraviolet (UV) light. After they died I dried and mounted one in a picture frame (you have to have a hobby after all!), and even now it will still glow under UV light. Apparently scientists have discovered that fossilized scorpions can still glow even after millions of years in rock.

Other related Arthropods such as Horseshoe Crabs also fluoresce, which suggests that they all share a common ancestor which had this trait.

So that begs the question “Why do scorpions fluoresce under UV light?”

The outer layer or cuticle of a scorpion, has a thin section called the hyaline layer. It is this hyaline layer that reacts to UV light. The fluorescence is due to the chemical compounds beta-carboline and 7-hydroxyl-4-methylcoumarin (bit of a mouthful but apparently non-toxic so you could put it in your mouth!) in this hyaline layer. The wavelengths of ultraviolet light which are absorbed by this layer are re-emitted at different wavelengths that are visible in the dark as a blue-green glow. When a scorpion is preserved in alcohol, the alcohol itself fluoresces! 

One hypotheses that has been suggested for the glowing phenomenon is that the scorpion uses its exoskeleton to detect UV light and so recognise when it’s night and time to eat. One problem I can see with this theory is that scorpions live by day in dark places, under stones, deep cracks in trees (boots if you are not careful where you leave them) etc, places were UV light can’t penetrate. They would have to venture out of their safe refuges to detect the UV. Scorpions also have two eyes on the top as well as between two and five pairs along the front corners of the cephalothorax. They are unable to focus sharp images but their central eyes are amongst the most light sensitive in the animal kingdom. Some species also have light receptors in their tails. This again suggests that they don’t need to use their fluoresce to determine if it is day or night.

Another suggestion is that they use their exoskeletons to detect UV light because they want to avoid it. Exposure to ultraviolet light can cause blindness, stress, and even death to scorpions. To be able to detect UV and so avoid it would seem to be advantageous to them, but once again as they avoid daylight anyway they would automatically avoid UV too. The moon reflects sunlight and so there is some UV present at nighttime. However the light reflected by the moon is about 400,000 times dimmer than the sun. So although UV is reflected off the moon and can reach the Earth, it is very dim and far too weak to be any real threat to scorpions.

It has also been suggested that their bright blue-green glow confuses prey and makes it easier to catch, or that their glowing exoskeleton makes it easier for scorpions to recognize one another. Since most insects tend to avoid fluorescence, there are some doubts as to whether glowing can help scorpions catch more prey. Also glowing scorpions would themselves be easy targets for predators seeking a good meal.

When a scorpion sheds its skin, it temporarily loses its ability to fluoresce. This doesn’t reappear until the new cuticle hardens. It may be that the fluorescing substance could be just be chemicals produced in the hardening process, and is a random act and so has no function. This would probably be the most disappointing conclusion, knowing that the glow is purely accidental and serves no real purpose. 

Nature however usually does things for a reason, and perhaps we just haven’t discovered what that reason is yet? I think I’ll stick with that conclusion for the time being!

Photo: Grabbit (or was it Scarper?) glowing under UV light. (Interestingly the camera sensor was far more sensitive to the UV than the eye was, revealing a much brighter image than could be seen by eye. Further experimentation in photography for another time perhaps?)