It’s the 21st Century and ‘glow-in-the-dark’ is all the rage. We put plastic stars on our ceilings that shine in the night to mimic the constellations and we insert genes into our zebrafish to make them glisten in their tanks. From nail polish and glow sticks to now even puppies, we’re fascinated by our ability to manipulate objects otherwise unseen in the dark. But long before we artificially created glow-in-the-dark objects, a few fungi evolved to use this characteristic we so desire.

The phenomenon in nature is called bioluminescence and is restricted to a small group of species. While most fungi don’t possess this ability, there are some 71 known species of bioluminescent mushrooms contained within three groups— the Omphalotus, Armillaria, and Mycenoid lineages. Their degrees of light intensity differ; while many of the Australian species are very luminous, North American species tend to emit less light and require adjustment to the dark before they can be seen.2 Reports of these fungi date back to Pliny the Elder in the first century, who described luminescent white wood-decaying mushrooms in France. Even earlier, Aristotle had commented on glowing rotten wood, now known to be a product of the luminescent mycelium within, a subject which continued to be one of great mystery throughout European history2,4. In 1555, Swede Olaus Magnus published A Compendious History of the Goths, Swedes, and Vandals and Other Northern Nations, which mentioned numerous luminescent mushrooms such as the “Agarick” and their connection to wood decay; he also described the practical use of mycelia-infested bark (often called “Foxfire” or “Faerie fire”) by Scandinavians during long winter nights. The practical uses of these mushrooms extended to other areas of the globe as well; in the late 17th Century in Herbarium Amboiense, Dutch physician G.E. Rumph commented on how Indonesian natives used bioluminescent mushrooms as primitive flashlights. And even in 20th Century Micronesia, these special mushrooms were incorporated into ritual headdresses and warfare face paint.4

Various other organisms also bioluminesce. Many people are most familiar with the firefly, which emits flashes of light produced by the pigment oxyluciferin. Numerous studies have revealed their flickers act as a form of communication between individuals and are vital in the selection of mates. Firefly larvae similarly glow–perhaps in order to advertise their unpalatability and so discourage predation.1,5 The femme fatale Photurus firefly is a particular case— she uses her light to attract and consume eager male fireflies of other species.1 Another fluorescent insect is the fungus gnat (whose light abilities are unrelated to the mushrooms in which they reside). These flies bioluminesce primarily to attract prey to their webs, although other uses include deterring predators, displaying larval strength to neighboring aggressive larvae, and locating mates.7 A handful of marine animals incorporate symbiosis with fluorescent bacteria into their predatory behavior. Deep sea flashlight fish and female anglerfish use this light to attract prey in the dark depths of the oceans.3 Cookiecutter sharks use their luminescent undersides in the opposite manner; the emitted blue-green camouflages with the surrounding ocean so only a small black patch is visible. Other predatory fish, such as tuna, perceive the shark to be much smaller than it really is, and when these other fish approach their ‘prey’, the shark attacks (and subsequently consumes) its predator8.

In comparison, bioluminescence in mushrooms is often assumed to play roles in spore dispersal and increased survival. Unlike some marine species, fluorescence in mushrooms is produced without endosymbionts. One fundamental effect of mushrooms’ light, as shown by Sivinskif, is its ability to more successfully lure arthropods, in particular Collembola and Diptera. These bugs may aid in spore dispersal, much as the stench of stinkhorns does. Appealing to arthropods at night could give bioluminescent mushrooms an additional half day advantage over their non-luminescent counterparts, assuming insects would eat or pick up the spores and then drop them elsewhere. Alternatively, perhaps the mushrooms are unsavory to insects and luminescence emphasizes their toxicity at night. Illumination may also discourage negatively phototropic fungivores, especially those found in the soil (hence one explanation for why the mycelia of these species also glow). Luminescence might also attract carnivores to eat arthropods on the mushrooms, with the assumption that if more carnivores than insects are attracted, then predators of the fungus are limited overall.6,7 But not all hypotheses are centered around the relationships of the fungus with other organisms. All known glowing mushrooms are wood decayers that can digest lignin, a particularly big and difficult molecule that helps bind the cellulose fibers of wood together. An alternative idea is that bioluminescence is a side-product of lignin degradation: Reactions that lead to light production may generate antioxidants to protect the fungus from toxic peroxides released during lignin digestion.2

The next time you step out in the woods at night, leave your flashlight home–if you look closely enough, you may find yourself face-to-face with one of these fascinating bioluminescent mushrooms.