Herpetologists collected frogs in Santa Fe, Argentina to study the biochemical Cloricia found in amphibians. The polka-dot frog, Hypsiboas punctatus, is a species that can be found in South America. Its translucent skin allowed them to see the accumulation of biliverdin.
When they shined a UVA beam on the frogs, they didn’t see the faint red biliverdin they expected. What they saw instead was a beautiful and bright cyan fluorescence. The frogs were so luminous that they shone in the dark under the blacklight, which helped the scientists find specimens. The fluorescence of all the polka-dot frogs was evident.
Researchers from Argentina’s Instituto Leloir Foundation, INQUIMAE, and CONICET, as well as Brazil’s University of Sao Paulo Faculty of Pharmaceutical Sciences of Ribeirao Preto, were part of the team.
Moonlight – Luminous
The translucent skin of a polka-dot frog appears to glow, as it allows high levels of light transmission in the green and the red portions of the electromagnetic spectrum while blocking the information of blue.
The peculiar cyan fluorescence, which we found originated in its skin glands and lymph nodes, belongs to a family of derivatives of the molecule dihydroisoquinolinone. These compounds were given the name “hyloins” after the amphibian Hylidae family, which includes the tree frog.
The polka dot tree frog in daylight and under blacklight. Julian Faivovich, Carlos Taboada, and the Author provided
The frogs can use fluorescence as a biosignal to communicate visually. The brightness of the light depends on a number of factors, including the ratio of photons that are emitted by fluorescence to those reflected by the amphibians, the lighting conditions in the environment they live in, and their sensitivity to color.
We found that between 18% to 30% of the photons (light) emitted by the skin of Hypsiboas punctatus were fluorescent under nocturnal-twilight conditions. This is a significant proportion. It’s enough to add fluorescence to the green coloration (in daylight) of the frog and enhance its visibility.
It is interesting to find fluorescence in land animals because this phenomenon was considered irrelevant until it was found in insects such as spiders, scorpions, beetles, butterflies, moths, dragonflies, and millipedes. The differences in feather fluorescence between the sexes are believed to play a role in mating behavior and attraction in parrotlets.
We expect the fluorescence of the polka-dot frog to play a role in visual communication between species (because its sensitivity matches that of the photoreceptors on the frogs eyes for green and blue). It does not appear to have any significance to mating as the florescence is not different between males and females.
What else glows besides?
The discovery of fluorescent frogs, a species that was previously unknown for exhibiting it, has renewed interest in the search for other amphibians that glow in the dark.
Beetles can also show fluorescence.
This discovery also opens up new avenues of research. For example, a more detailed study of the spectral sensitivities of the eye photoreceptors in Hyperboas Punctatus would allow us to calculate the amount and type of light that reaches each of the polka dot tree frogs’ photoreceptors and help us better understand their visual perception.
We are also interested to evaluate the photophysical characteristics of the purified, free fluorophores that were found in this research. This includes their chemical and biological makeup. These fluorophores could be used in biotechnology or molecular biotechnology as fluorescent labels or markers, allowing microscopic recognition of biomolecules.
This discovery gives scientists a hint at the answer to an important photophysical question: Does naturally occurring fluorescence function as a biosignal, or is it a non-functional result of certain pigments’ chemical structure?
