Recently, I ventured to the cliffs overlooking the Pacific Ocean in San Diego, CA to watch one of nature’s coolest phenomenons — the glowing blue waves. These waves look like blue lightning bolting across the surface of the ocean and are the result of microscopic organisms called dinoflagellates emitting light as a defense mechanism triggered by the motion of the waves. However, these wacky microorganisms are not the only lifeforms that glow in the dark.
Glowing is more common than you’d think
One way animals glow is called biofluorescence. Biofluorescence has been documented in fungi, plants, and even animals. Researchers at St. Cloud State University recently documented wide-spread biofluorescence in salamanders for the first time.
Biofluorescence occurs when high energy wavelengths of light such as ultraviolet light, are absorbed by the organism then re-emitted with lower energy wavelengths; this causes the organism to glow in blue, green, or even red! This is different from fire flies or the glowing dinoflagellates I saw in the ocean which are bioluminescent. Bioluminescent organisms have a light producing organ with an enzyme called luciferase which starts a chemical reaction and produces a glow (Hadock et al. 2009). So biofluorescence is physically altering a wavelength producing a glow and bioluminescence is creating a chemical reaction to glow. However, both of these mechanisms may have similar benefits to the creatures that fly, scurry, or swim around in the darkness.
Salamanders are a group of amphibians that live on land and in the water. The researchers in this study found that in general, these amphibians glow green and yellow in response to blue and ultraviolet light! The eastern tiger salamanders produce yellow blotches of light while the three-lined salamander has glowing yellow stripes. The aptly named Chinese fire-belly newts glow a fiery orange. In some species, even the mucus secreted from the skin and the urine glowed!
Based on the prevalence and the diversity of the luminous salamanders, the researchers concluded that biofluorescence is not restricted to a single lineage but is likely present throughout the entire salamander family and occurred early on in the evolutionary history of amphibians.
Why do they glow?
This is the first documented case of wide-spread biofluorescence across an amphibian lineage, meaning prior to this researchers knew about some glowing amphibians but didn’t realize how common it was. The authors suggest that the physiology of amphibians may be why this phenomena may be so prevalent. Some pigments and reflective structures that are known to fluoresce are present in light producing cells, called chromatophores, on the skin of amphibians and in other parts of the body. Additionally, other fluorescent compounds may be found in the mucus-like secretions salamanders exude to keep their skin moist. This is the case of the Neotropical tree-frogs, cousins of the salamander. Some sharks have a metabolite in their skin that glows, this may also occur in amphibians. And in some cases, even the bones of amphibians contain fluorescent pigments.
Is there a benefit for being glow-in-the-dark?
Many salamanders are active at dawn, dusk, and night when it may be advantageous to glow as a form of communication, to attract a mate, or to use the light to see. But this all depends on the ability of the salamander to see the light they are emitting, and since we can’t walk up to a salamander and ask it what it sees, scientists attempted to deduce what they see based on the structures in their eyes.
Salamanders and many other species active at twilight have “red rods” which are light receptors in the eyes that are sensitive to green-light. This is also the reason why as humans, we are drawn to green glow-in-the-dark stars on the ceiling, and use green light in night vision goggles. Therefore, the salamanders should be able to see the glowing green fluorescence and help individuals see each other in low-light conditions.
Salamanders are also known for having complex reproductive behaviors based on visual cues. The patterns on the salamanders may help them find mates. One part of the salamander that glows is the the cloaca– a one stop shop for bodily excretions and sex. The green glow emitted by the cloaca may be a sign to the other salamanders luring them in.
But really, we don’t know why they glow. As the author’s so cleverly stated, “our findings shine a new light on how much more we have to learn about the biology of these fascinating vertebrates.” Maybe it is time to replace the glow-in-the-dark stars on the ceiling with glowing salamanders as a constant reminder that there are some crazy creatures on this planet and we still have so much more to learn about them.
Lamb, J.Y., Davis, M.P. Salamanders and other amphibians are aglow with biofluorescence. Sci Rep 10, 2821 (2020). https://doi.org/10.1038/s41598-020-59528-9
Haddock, S.H.D., Moline, M.A., and Case, J.F. Bioluminescence in the sea. Annual Review of Marine Science 2, 443-493 (2010).
Reviewed By: Kristen Brown, Carrie McDonough , Laura Mast