Luminous frog is the first known naturally fluorescent amphibian

A light in the dark

Carlos Taboada et al.

By Sam Wong

Shine on you crazy frog. The polka-dot tree frog is the first amphibian known to be naturally fluorescent.

Fluorescence, which happens when a substance absorbs light at one wavelength and emits it at a longer one, is known to occur in some parrots and sea turtles, and a wide variety of fish.

The polka-dot tree frog, which is about 3 centimetres long, is pale green and speckled with white, yellow or reddish spots. It is commonly found all over the Amazon basin and is mainly active at dawn, dusk and night.

Julián Faivovich at the Bernardino Rivadavia Natural Sciences Museum in Buenos Aires, Argentina, made the discovery unexpectedly while studying a pigment in the frog. “For some things we were planning on doing, we had to illuminate the frog tissues with UV light. Then we realised the whole frog was fluorescing,” he says.

He and his colleagues traced the fluorescence to a compound found in the lymph and skin glands. They found that this trait enhances the brightness of the frog by 19 per cent on a night with a full moon and 30 per cent during twilight.

The fluorescent compounds absorb light at a wavelength at which frog photoreceptors have low sensitivity, and emit it in a wavelength at which they have high sensitivity. That means it’s likely the frogs themselves can see the fluorescence.

Shedding light

So far it’s unclear what purpose, if any, the trait serves, but it’s plausible that it may have some role in communication, says David Blackburn at the University of Florida in Gainesville. “We think a lot about multimodal signals, both noises and visual,” he says. “Thinking about fluorescence playing a role in that could be really exciting.”

Faivovich and Blackburn both think that fluorescence could be much more widespread than we realise. “In frogs, I don’t know of many people who have gone looking for it,” says Blackburn. “Maybe we should be looking at every species we catch and trying to see if they fluoresce.”

With around 5000 known species of frogs, it’s unlikely that the polka-dot tree frog is the only fluorescent one.

“There’s very few frogs that have a feature that’s not found in any other frogs,” says Blackburn. “So it probably is a trait that’s more widespread, but then the question becomes what are the ecological circumstances that would drive fluorescence? Is it only common in tree frogs? Or is it ever found in other ecological circumstances?”

Journal reference: PNAS, DOI: 10.1073/pnas.1701053114

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