Many Salamanders and Frogs Glow in the Dark. (We Just Didn't Think to Check)

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glow in the dark Cranwell frog with neon splotches

Lamb, J.Y., Davis, M.P. / Wikimedia Commons / CC by 4.0

During the day, the Cranwell's horned frog is inconspicuous. It's mostly a mottled-brown, striped creature with some dull green stripe highlights. But when researchers recently put the frog under blue light, it came alive with some amazing day-glo hues. The glow show was one of many discoveries unveiled in a new survey published in Scientific Reports.

Above is how Cranwell's horned frog looked under the blue light. This is how it looks in regular daylight:

daytime shot of Cranwell's horned frog
During the day, the Cranwell's horned frog isn't so glowing. Svetlana Mahovskaya/Shutterstock

For the study, researchers from St. Cloud State University in Minnesota tested 32 amphibian species under blue or ultra-violet light. Each one they examined lit up in some way, as their skin, muscles, bones and other body parts glowed in shades of neon green and orange. Their surprising findings show that more frogs and salamanders have the ability to absorb light and re-emit it, a process known as biofluorescence. (This is different from bioluminescence, which is when a living organism produces and emits light.)

It also means that these animals see each other in ways humans don't understand, study co-author and herpetologist Jennifer Lamb tells Discover.

"I'll be careful going forward not to put my own biases of perception on the organisms I study," she says. "We forget to ask if other species might perceive the world in different ways."

In the past, biofluorescence has been observed in many animals ranging from jellyfish and corals to sharks and turtles. Much of the focus has been on aquatic animals until now.

No more 'plain Janes'

The Eastern tiger salamander (Ambystoma tigrinum), shown above right, was the first amphibian the researchers studied.
The Eastern tiger salamander (Ambystoma tigrinum), shown above right, was the first amphibian the researchers studied. Jennifer Lamb & Matthew Davis

Lamb and her colleague, ichthyologist Dr. Matthew Davis, were discussing what other species might share these glowing traits. They commonly work with tiger salamanders so decided to take a look at them under their special lights. When they saw their ordinary yellow spots suddenly become brilliant green, they were intrigued.

"One of the most exciting aspects of this work to us was that with each species we examined we were always discovering something new that could bring novel insights into the life history and biology of amphibians worldwide," Lamb said in a statement.

"The Eastern tiger salamander (Ambystoma tigrinum) was the first salamander species that we surveyed for biofluorescence, and when we saw the bright, intense green light emitted from their yellow spots we each let out a collective Woah! At that point, we were captivated and we set out to investigate how prevalent biofluorescence was across amphibians and the extent of variation in their biofluorescent patterning."

That first salamander really made an impact. After their first foray with their special lights, they went out into the field to see what the could find and made visits to Chicago's Shedd Aquarium.

"When we imaged that species, it was really startling to both of us just how bright and brilliant the fluorescence was," Lamb tells Wired. "We also saw fluorescence in animals that otherwise under white light might kind of look like plain Janes, that were maybe a duller brown or gray."

The frogs, salamanders and caeclians — limbless, worm-like amphibians — they tested all biofluoresced in interesting ways. A few of them had skin that glowed bright under the special lights. Others has fluorescent secretions like urine or mucus. Some, like the marbled salamander, showed off glowing bones.

The researchers were also fascinated to find that some of the brightest parts of the newts were their underbellies. Colorful markings in the daytime can be a sign to predators that animals are poisonous. That's why newts often show their bellies as a warning sign, Lamb tells Discover. Glowing so brightly at night may be a sign that birds or other predators can see.

Why the trait evolved

In other studies, noted in the video above, researchers have found more than 180 species of marine fishes that exhibit biofluorescence. Most of the fish are camouflaged so they need to find each other, including during mating, reports The New York Times.

In the amphibian study, because the researchers found biofluorescence in all the animals they tested, it suggests that the trait likely developed early in their evolution.

They're not exactly sure why it developed, but it was a valuable enough trait that it remained.

The researchers suggest that this glow-in-the-dark ability may help amphibians find each other when light is limited because their eyes have cells that are sensitive to green or blue light. Biofluorescence may help them stand out from their environment, allowing them to be seen more easily by other amphibians. It could also help with camouflage, mimicking predatory actions that other biofluorescent species have used.

"There is still a lot out there that we don’t know," Lamb tells The New York Times. "This opens up this whole window into the possibility that organisms that can see fluorescence — their world may look a lot different from ours."