Fluorescence is defined as the ability to absorb light from a higher energy (shorter) wavelength (energy of light decreases across the spectrum from blue to red) and re-emit some of that light energy at a lower energy (longer) wavelength. It is important to keep in mind that this is not the spontaneous generation of light, it is the absorption and re-emission of light. Meaning that some source of light is required for fluorescence.

Transgenic Fluorescent Organisms

By now I imagine many of you are familiar with scientists creating transgenic organisms that can glow, by expressing a fluorescent protein in them. It's been done in so many different types of animals:

We have created glowing fish:

Source

glowing mice:

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even glowing rabbits:

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However the genes that allow for these glowing characteristics were not created artificially in the lab. It is derived from an animal, which naturally glows like this. The protein that everyone plays around with to make these recombinant species glow green is called Green Fluorescent Protein (GFP)...yeah I know, more super creative science names...

GFP was extracted from the jelly fish Aequorea victoria:

Source

which looks normal in the light, but it also fluoresces green, because of its expression of GFP.

Come to your deaths my little food creatures... mua hahahahahaha

Why does it do this? Well it lives in a fairly dark aquatic spot and uses the extra green light to attract unsuspecting prey to eat....

Natural Fluorescence In Animals

It's pretty rare, especially in animals which live on land. However an interesting paper was recently published in the journal: Proceedings of the National Acadamy of Sciences of the United States of America (PNAS) titled "Naturally occurring fluorescence in frogs". This article describes the identification of an unusual South American tree frog that also has this fluorescent property.

The publication concerns the frog Hypsiboas punctatus which is unusual (as far as frogs go) as it has a translucent skin (you can sort of see through it). Under normal light, it looks like a little yellow frog, but shine UV light on them and...

They glow green!

Yet they don't express any GFP, so they are not using the same mechanism as the previously discussed jellyfish, or any of the transgenic glowing animals created in the lab.

How Do They Do This?

The authors examined a variety of the frogs tissues and found that the majority of the fluorescence comes from the animals skin. After further investigation they identified that frogs have glans in their skin which secrete compounds called hyloins, which are what result in the fluorescence of the frogs.

What Is The Advantage For The Frogs?

The authors hypothesize that it may actually be to help the animals see at night. They determined that under dark twilight conditions that the fluorescence would make up as much as 24% of the total light available to the frog for vision, that's actually a pretty sizable amount! This discovery opens the door for the discovery of other frogs/amphibians that also produce these fluorescent molecules, and may be a common mechanism for how these animals see under low light conditions.

TL;DR

Researchers found a species of frog that produces fluorescent compounds that they hypothesize that it uses to turn itself into its own flashlight to better see in times of low light.

Sources

1. https://en.wikipedia.org/wiki/Fluorescence
2. https://en.wikipedia.org/wiki/Transgene
3. https://www.jic.ac.uk/microscopy/more/T5_9.htm
4. http://www.pnas.org/content/early/2017/03/07/1701053114

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