Jellyfishes, fireflies and many deep-sea creatures are famously bioluminescent, harnessing specialized chemical reactions to emit an eerie glow. The biological flashlights may be involved in camouflage, hunting and even enhanced communication in low-light environments. However, other animals use a different mechanism to produce light – biofluorescence – by absorbing high-energy wavelengths of light (blue and UV) before re-emitting them as a blue, green or even red glow. Recent work has shown that a smattering of creatures, including chameleons, parrots, penguins and even some rodents, have added biofluorescence to their palette of colours. However, instead of looking for individual examples of glowing animals, Jennifer Lamb and Matthew Davis from St Cloud State University in Minnesota, USA, took a different, broader approach. They conducted a survey of amphibians, focusing on newts and salamanders, to find out how widespread the phenomenon is.
Lamb and Davis scoured the pet trade, the natural environment and the Shedd Aquarium in the USA for as many amphibians as they could find. Then, they gave each species its moment in a blue or UV spotlight, beaming the animals with enough light to bring out their hidden biofluorescent colours. The researchers then viewed their subjects through a filter that blocked light bouncing off the skin, revealing the subtle, inner gleam of biofluorescence.
Every amphibian examined (mostly salamanders, but also some frogs and a caecilian) glowed under the right conditions, even the aquatic larvae. Bold markings and colours, like the orange tummy of fire-belly newts or the yellow blotches of the tiger salamander, glowed bright green or greenish orange. Animals with demure patterns had a more subtle sheen to them. In a few cases, species fluoresced from unusual places: some had glimmering bones and others shone from a coating of glowing mucous.
Taking stock of their diverse collection of shimmering specimens, Lamb and Davis reasoned that biofluorescence is widespread in amphibians and probably appeared early in their evolutionary history. The next logical questions are exactly how do salamanders and their relatives glow and why did they do it in the first place?
The authors suggest that the amphibians’ light shows might originate from pigments in the skin, such as carotenoids, pterins and structures with guanine crystals, which all fluoresce. Alternatively, the glow could have nothing to do with pigments, relying instead on something like the green fluorescent protein common in jellyfish and molecular biology labs worldwide, or hyloins – compounds produced by the mucous glands of neotropical frogs. Which, if any, of these mechanisms apply to salamanders and their cousins remains uncharted territory.
And the team suspects that these amphibians probably use their glowing abilities in much the same way as other biofluorescent animals. Like a neon sign, the green blotches and markings could send messages. Perhaps female salamanders judge their suitors by their ‘glow up’, tolerating only the brightest mates. The fact that some species show strong biofluorescence around the cloacal region, a part of the anatomy that many species investigate during courtship, makes this possibility especially intriguing. Bizarrely, glowing in the dark could also be a form of camouflage, if the patterns mimic those of other fluorescent predators, yet how biofluorescence impacts the daily life of amphibians remains a mystery.
As is often the case, good science can lead to more questions than it answers and the results of this study reveal how little we know about the hidden world of amphibians.