While scorpions are often dark in colour (black, brown, grey) but can also be translucent in daylight, they are completely different when exposed to ultraviolet light. In these conditions they fluoresce (with a few exceptions) in the yellow, blue and green range. This fluorescence doesn’t occur among juveniles or recently moulted adults. In this case, it takes up to two days after moulting for the fluorescence to reappear.
Scorpions living in desert environments seem to fluoresce more intensely than those living in less dry areas.
This property has been long known and has been observed in other animals and in certain plants, but its origin and role remain mysterious. Several theories have been proposed:
- It could be a way for scorpions to recognise each other, but this theory has proved controversial because scorpions are unable to distinguish colours.
- It is believed that this property coincided with the scorpions’ marine ancestors (eurypteroids) “emerging from water”. This would have allowed them to be protected from the sun’s rays which they’d never been exposed to. This explanation remains controversial, however, as scorpions are primarily nocturnal.
- The scorpion’s exoskeleton could act as a light receptor, enabling the animal to detect refuges by reducing the intensity of light in shaded areas, “synonymous” with a refuge. The fluorescence would then vary depending on the animal’s habitat area.
- The molecule responsible for UV fluorescence could play a role in protecting scorpions against parasites.
All of this research could lead to medical applications: one of the molecules responsible for scorpion fluorescence (β-carboline-3-carboxylic acid) is believed to be produced in the scorpion’s exoskeleton but also in the lens of the human eye, which becomes opaque during the cataract process.
And let’s not forget that scorpions are among the most resistant animals in the world: some can fast for several years, others can withstand radioactivity levels 250 times higher than doses lethal for humans, and they can withstand very large temperature fluctuations, whether very high or even negative.