Even a salamander scientists have been studying for more than 250 years can still throw up a surprise.

Fire salamanders, one of Europe’s most-studied amphibians, are biofluorescent, according to a study published May 27 in Royal Society Open Science. The researchers found the animals emit a bluish-green glow after exposure to ultraviolet light.

Biofluorescence means an organism absorbs light from an external source at one wavelength and re-emits it at another. Scientists once thought the trait was limited to marine animals and arthropods, but in recent decades they have found it in some reptiles, birds and amphibians.

“We are in a thrilling period of discovery in terms of biofluorescence in amphibians and other [four-limbed vertebrates],” Jennifer Lamb, a biologist at St. Cloud State University who was not involved with the research, told National Geographic’s Jack Tamisiea.

Studies like this one, she said, “help fill some of the gaps in our understanding, both in terms of what species fluoresce and in terms of the mechanisms likely responsible for that fluorescence.”

The finding started with a field sighting by Bernat Burriel-Carranza, an evolutionary biologist at the Natural Sciences Museum of Barcelona, who had begun carrying an ultraviolet flashlight on evening expeditions. On a rainy night in Spain, he saw a fire salamander crossing the road and shone the light on it, revealing a bright, speckled pattern along its flanks.

Fire salamanders are common across Europe. They are black-and-yellow amphibians measuring about 15 to 30 centimetres long. They are nocturnal and usually live in cool, damp forests near bodies of water, where they eat worms, slugs and other insects.

If threatened, they can defend themselves with toxins in their skin or by spraying poisonous liquid from glands behind their eyes. They also breathe through their skin, can regrow limbs and tails, and give birth to live young.

After that first observation, Burriel-Carranza and colleagues examined fire salamanders in Spain and Germany between April 2024 and November 2025. They illuminated the animals with a UV flashlight and photographed the speckled glow.

The fluorescence appeared to come mostly from the yellow spots on the salamanders’ skin, especially along their sides and stomachs. The researchers also swabbed the animals’ skin to collect toxic secretions. Under UV light, the slime glowed too, suggesting the fluorescence may come from the glands that produce the poisonous substance.

The team then dissected two preserved fire salamanders. Under a microscope, they found fluorescent chemical compounds in the glands and bloodstream, suggesting the substances circulate through the animals’ bodies.

The study says that had previously only been observed in some tree frogs, which use fluorescent compounds called hyloins to illuminate their translucent skin.

“We still don’t know what the compound responsible for this fluorescence is, but everything indicates that it is a molecule unknown until now in this species,” study co-author Salvador Carranza, a biologist at the Institute of Evolutionary Biology in Spain, said in a statement.

“Identifying it will be key to understanding its origin and function.”

The ecological role of the glow remains unclear. The researchers suggest the salamanders may use it to communicate, choose mates or warn off predators.

The study says one possible explanation is that the animals fluoresce so they can see each other better at night. The researchers said full moonlight contains more UV and violet wavelengths than daylight, and those are the wavelengths the salamanders absorb and re-emit. They also noted that moonlight reaching the forest floor peaks in autumn, when the salamanders usually breed.

The scientists also think the fluorescence may strengthen the warning effect of the animals’ bright yellow markings and toxic secretions.

Burriel-Carranza said in the statement that he finds it “fascinating” that such a well-studied species could still hold undiscovered traits.

“It reminds us that even the most familiar organisms can hide secrets that are only revealed when they are observed with new tools,” he added.

Read more from Smithsonian Magazine.