Under an ultraviolet flashlight, one of Europe's most familiar amphibians turns into something out of a science fiction film. The black-and-yellow fire salamander glows with turquoise and green speckles along its belly and sides, and the toxic fluid it oozes from its skin glows too. That secretion kept fluorescing for more than 24 hours after leaving the animal's body, according to research published in Royal Society Open Science.

The finding is striking because fire salamanders are among the most studied amphibians in Europe, and their toxins have been analyzed chemically for decades. Yet no one had reported the glow.

The researchers still do not know which molecule produces the light, or what, if anything, the salamander uses it for.

A Glow Hiding in Plain Sight

The discovery began by chance — on a wet road. Lead author Bernat Burriel-Carranza of the Museum of Natural Sciences in Barcelona was carrying an ultraviolet light during fieldwork in Spain when his team spotted a fire salamander crossing a road on a rainy night. When they lit it up, the flanks lit up with bright speckles, National Geographic reported.

The team found the teal speckles on 10 of the 12 salamanders they examined in Spain's Catalonia region. Glowing spots were rarer in a subspecies surveyed in Germany, but every secretion sample collected, including from the German animals, fluoresced.

"It is fascinating that such a well-studied species still harbors unknown phenomena like this," Burriel said, according to the Max Planck Society.

This is biofluorescence, not bioluminescence. Fireflies make their own light through chemical reactions. Fluorescent animals absorb light at one wavelength, often ultraviolet that humans cannot see, and re-emit it as visible color. The salamander glows only when ultraviolet light strikes it.

The Poison Is the Source

The glow follows the salamander's glands. It is concentrated on the yellow underside and sides of the body, and it comes mainly from skin glands and the secretions they release, the researchers reported. Those secretions are the animal's chemical defense, containing steroidal alkaloids that deter predators.

When the team examined tissue from two preserved museum specimens, they also found fluorescent compounds in the blood and granular glands. That kind of body-wide distribution had previously been documented only in some tree frogs, Smithsonian Magazine reported.

The surprise for chemists is that the known toxins are not thought to fluoresce. "We still don't know what the compound responsible for this fluorescence is," said Salvador Carranza of the Institute of Evolutionary Biology in Barcelona, a co-author. The team believes it is a molecule not previously identified in this species and is now working to isolate it.

Two Leading Ideas, No Answer Yet

Why a salamander would glow is an open question — and a tantalizing one. One hypothesis is communication. Fire salamanders are active at night, when the only light reaching the forest floor comes from the moon and stars. Animals with sensitive vision might detect the faint cyan-green glow, helping salamanders find each other or assess potential mates.

"It could help salamanders detect each other in nocturnal or particularly dense environments," said Martin Kaltenpoth, a director at the Max Planck Institute for Chemical Ecology in Jena, Germany.

A second idea is warning. Bright yellow-on-black coloring already signals to predators that the salamander is toxic. A glow that comes from the poison itself could reinforce that message for predators able to see it.

The timing of the salamander's life cycle adds an intriguing detail. The amount of moonlight reaching the forest floor tends to rise in autumn, when fire salamanders typically breed, which could make any glow more useful during courtship. That link is speculative and would need field experiments to confirm.

Neither idea has been tested. The study documents that the glow exists and where it comes from. It does not show that salamanders or predators can see it in the wild or change their behavior because of it.

Part of a Wider Wave of Glowing Amphibians

Biofluorescence was long thought to be mostly a marine phenomenon, with scorpions as a rare land exception. In recent years, researchers have found it in reptiles, birds, and many amphibians. A 2020 study provided the first broad evidence of fluorescence in salamanders and other amphibians, and the list keeps growing as more scientists carry ultraviolet lights into the field. Courtney Whitcher, an independent researcher who studies fluorescence in tree frogs and was not involved, told National Geographic that the finding should encourage scientists to re-examine even well-known species.

Fire salamanders are listed as vulnerable by the International Union for Conservation of Nature, threatened by habitat loss and fragmentation. In parts of northern Europe, they have also been hit hard by an introduced fungal disease that attacks salamander skin. Learning how the animals use their skin and secretions could eventually help conservation, especially if the glow plays a role in breeding.

Fire salamanders live in Europe, not in the wild in the United States, but North America is home to its own diverse salamanders. Anyone tempted to look for glowing amphibians should avoid handling wild animals, since salamander skin is delicate and many species secrete irritating toxins. Never shine ultraviolet light into human or animal eyes, and follow local rules protecting amphibians.

The bottom line: the fire salamander's poison glows under ultraviolet light and keeps glowing for more than a day, a trait missed for centuries. The molecule behind it and its purpose remain unknown, making this a reminder that familiar animals still hold surprises.

What Readers Want to Know

What did scientists discover about fire salamanders? They glow turquoise and green under ultraviolet light, mainly on the belly and sides, and their toxic secretions glow too.

How long does the glow last? The secretions kept fluorescing for more than 24 hours after leaving the salamander's body.

What makes them glow? An unidentified compound in the skin glands and secretions. Researchers are working to isolate it.

Is this the same as fireflies? No. Fireflies produce their own light. Fluorescent animals glow only when external light, such as ultraviolet, strikes them.

Why might salamanders glow? Researchers suggest it could help them find each other at night or reinforce warnings to predators, but neither idea has been tested.

Can I find fire salamanders in the United States? Not in the wild. They are native to Europe, though North America has many other salamander species, which should not be handled.