Bio-fluorescence, that is, when an organism absorbs light, replacing it at a different wavelength (not to be confused with bioluminescence, which is when a living organism produces and emits light), had only been observed in one species of salamanders and three species of frogs. However, a new study by researchers Jennifer Lamb and Matthew Davis of St Cloud State University, Minnesota, U.S.A. have demonstrated that this feature is much more widespread than what was previously thought.
The scientists examined 32 species of amphibians, including salamanders, newts, frogs and caecilians under blue or ultraviolet light, and found that bio-fluorescence was produced in all of them. It is likely to occur in many more, according to the authors of the study published in the magazine Scientific Reports.
Each of the species analysed somehow illuminated, as their skin, muscles, bones and other parts of the body glowed in shades of neon and orange green, which showed that it is also common in amphibians to glow in the dark. They also found the present ability in various stages of life, from tadpoles to adults.
In the past, bio-fluorescence had been observed in many animals, from jellyfish and corals to sharks and turtles; studies have usually focused on aquatic animals, but it is clear that amphibians that generally lead a semi-terrestrial life, although always in humid places, are also included in this group of animals with particular qualities.
The researchers were surprised to find even uncoloured species in normal light reacting similarly under blue light. For example, the caecilian of the Cauca River, a grey aquatic amphibian with no limbs, which shined bright green under blue light.
What practical use can bio-fluorescence have in animals?
Apart from making them beautiful, bio-fluorescence can also have some practical uses for amphibians. It could help them to position themselves in low light conditions, creating more contrast between amphibians and their environment - because their eyes contain bar cells that are sensitive to green or blue light - to serve as a camouflage tool, to imitate the appearance of predators or even to mate, depending on the situation, the scientists explained.
"One of the most exciting aspects of this research was that with each species we examined, we were always discovering something new that could bring new insights into the life history and biology of amphibians around the world," Lamb said. "The eastern tiger salamander (Ambystoma tigrinum) was the first species of salamander that we studied in search of bio-fluorescence, and when we saw the bright and intense green light emitted by its yellow spots, each of us released an expression of collective surprise!".
How do they become bio-fluorescent?
The findings suggest that this occurs through various mechanisms thanks to the presence of fluorescent proteins in the skin, secretions such as mucous membranes, minerals in the bones or even in the urine. Some of the amphibians also have chromophores, or cells that contain pigment and reflect light.
Reference: Salamanders and other amphibians are aglow with biofluorescence, Scientific Reports (2020). DOI: 10.1038/s41598-020-59528-9 , https://www.nature.com/articles/s41598-020-59528-9