"Like an Avatar character": in the moonlight the common salamander becomes fluorescent

It is one of the most iconic animals on a European scale and surely one of the most representative and studied amphibians in the world. And yet, until now it was unknown that common salamanders (Salamandra salamandra) have a very special property, not at all common in nature and invisible to the human eye: under ultraviolet light they are fluorescent.

This has been discovered by a team of Catalan researchers from the Natural Sciences Museum of Barcelona and the Institute of Evolutionary Biology (CSIC-UPF), who have found for the first time that when this amphibian is exposed to UV radiation it emits a turquoise light. They now publish the finding in the journal Royal Society Open Science.“It was a huge surprise”, recalls Bernat Burriel, a researcher at the Natural Sciences Museum and co-author of the study. “It had nothing to do with what we had seen before, it had some dots similar to the characters in the movie Avatar”, summarizes this biologist, referring to the first salamander in which they detected this phenomenon one night of sampling in La Garrotxa.

Until now it was known that some marine animals, such as ctenophores –gelatinous bodies that inhabit the depths of the oceans–, or scorpions, some parrots, and also platypuses –fascinating mammals capable of laying eggs–, also glowed in green and bluish tones under ultraviolet light. And, in fact, in recent years, the list of species that exhibit this phenomenon has been growing, but until now it had never been described in these amphibians.

Biofluorescents

Catalan researchers, in collaboration with a team from the German Max Planck Institute, have found that upon receiving ultraviolet light, the chemical substances present in the salamander's skin transform this radiation and re-emit it in the visible spectrum, in green and blue tones, a phenomenon known as biofluorescence.

The fluorescence points coincide with the cutaneous glands and those that excrete venomous secretions located in the ventral and lateral area of this vertebrate. Likewise, they have found fluorescent compounds in the blood, which indicates that the molecules are very likely distributed throughout the body.

“Salamander secretions are very well studied and characterized, we know the toxic compounds they secrete very well. However, until now we had not identified any molecule with fluorescent properties”, highlights Burriel, for whom “it is fascinating that such a well-studied species still contains unknown phenomena like this. It reminds us that even the most familiar organisms can hide secrets that are only revealed when observed with new tools”.

To communicate or pair up

Now the challenge for these researchers is to discover whether this fluorescence serves any ecological function. For example, in the case of scorpions, which are blind, it is purely coincidental: the cuticle of the skin contains crystals that emit fluorescence, without any function behind it.

In the case of salamanders, researchers suggest as a plausible hypothesis that it could be used for communication between individuals. The turquoise they emit contrasts sharply with the dark colors of moss and leaf litter at night, and at night the only illumination that reaches the forest floor comes from the stars and the Moon, the light of which contains a greater amount of UV wavelengths than daylight.

Also, they point out, it could be a way to attract females, as males have more yellow spots on their bodies, which is where this fluorescence is most evident, and furthermore, during courtship, males tend to raise their torsos.

Other options could be related to orientation, migration, or even predator deterrence. This latter possibility, moreover, would be supported by the fact that the toxic secretions of salamanders are fluorescent for at least 24 hours.

A movie story

The story of how Catalan researchers discovered this characteristic of salamanders, amphibians that in Catalonia mainly inhabit humid forests, streams and mountain areas and are listed by the International Union for Conservation of Nature (IUCN) as vulnerable in its red list of threatened species, is as unique as the fluorescence itself.

The team of biologists had incorporated an ultraviolet light into the material they were carrying for sampling. And already in Saudi Arabia two years ago they had discovered that a desert newt showed fluorescence on its sides. Returning to Catalonia, one evening after a rainy day, they went out to sample amphibians in Garrotxa. "We had already looked at many amphibians with the ultraviolet light and nothing. But, suddenly, from the car we saw a salamander crossing the road; we stopped the car, we illuminated it with the ultraviolet light, and bingo!", recalls Burriel.