In 2015, Professor Jay Dunlap traveled to a Brazilian rainforest to solve a mystery that had puzzled biologists for over a century: why do some mushrooms glow in the dark? He returned with evidence that seemed definitive—the fungi were running on an internal clock, ramping up their luminescence precisely when insects could see it. But within a year, another team working halfway across the world would publish findings that contradicted everything Dunlap had discovered.
The Mushrooms That Tell Time
Dunlap, a circadian rhythm specialist at Dartmouth's Geisel School of Medicine, had focused on Neonothopanus gardneri, a Brazilian species that bathes the forest floor in an eerie green glow. What he found was unexpected: these mushrooms weren't glowing constantly. They were running on a 24-hour biological clock, just like humans.
The fungi produced peak levels of luciferin (the light-emitting molecule) and luciferase (the enzyme that makes it glow) during nighttime hours. Even more telling, this rhythm was temperature-compensated—it maintained the same period length whether the thermometer read 21°C or 29°C, a hallmark of true circadian control rather than simple biochemical variation.
This discovery pointed toward a clear conclusion: the glow served a purpose. Random metabolic byproducts don't get scheduled. The mushrooms were saving their energy for when the light show would actually matter—when insects were flying and darkness made that green glow visible.
The Fake Mushroom Experiment
To test whether insects actually cared about glowing fungi, Dunlap deployed a clever bit of trickery in the Brazilian forest. He built acrylic model mushrooms embedded with green LEDs, then scattered them through areas where N. gardneri grows naturally. Some models glowed. Others stayed dark.
The results seemed conclusive. Illuminated models attracted significantly more insects than the controls—beetles, flies, wasps, and ants all showed up to investigate the fake fungi. Dipteran flies (p = 0.00) and coleopteran beetles (p = 0.02) demonstrated clear statistical preference for the light. The hypothesis made elegant sense: mushrooms glow to attract insects, which then carry spores away on their bodies or in their guts, dispersing the next generation across the forest.
It fit the pattern of 120 known bioluminescent fungal species, all using the same family of chemicals to produce light at roughly 530 nanometers—a green wavelength that travels well through darkness and stands out against the forest background.
What the Ghost Fungus Revealed
Then came the contradictory data. In 2016, Weinstein and colleagues at The University of Adelaide conducted a similar experiment with Omphalotus nidiformis, the ghost fungus of Australia. They set sticky traps on moonless nights in Kangaroo Island, comparing catches near glowing fungi versus controls.
After 480 trap-hours, they found nothing. Glowing traps caught an average of 0.33 insects per night. Dark controls caught 0.54. The difference wasn't just statistically insignificant—it ran in the wrong direction.
The researchers noticed something else: ghost fungi fruit in winter, during Australia's June-July cold snap, when insect populations plummet. And unlike N. gardneri, these mushrooms glow continuously, day and night, with no circadian control whatsoever. If the light was meant to attract insects, the ghost fungus was broadcasting at the wrong time, in the wrong season, to an audience that wasn't there.
A Split in Evolutionary Purpose
The contradiction suggests that bioluminescence in fungi isn't a single story with one explanation. Among roughly 100,000 described fungal species, only about 71 to 80 produce light, all within the order Agaricales. That's a tiny fraction, scattered across multiple evolutionary lineages.
Which parts glow varies wildly between species. Jack o'lantern mushrooms (Omphalotus illudens) and bitter oysters (Panellus stipticus) only illuminate their gills. Other species light up their caps, stems, or all visible parts. The mycelium of Armillaria species can glow so intensely that entire truckloads of infected firewood shine neon green in the dark—a phenomenon that soldiers in World War I exploited to light trenches without attracting enemy fire.
This variation hints that the same biochemical machinery—luciferin plus luciferase plus oxygen—may have been repurposed for different functions as fungi evolved. In some lineages, like N. gardneri, insect attraction appears genuine. In others, like the ghost fungus, the glow might be an incidental consequence of antioxidant metabolism or some other biochemical process.
The Predator's Beacon
Dennis Desjardin, a mycology professor emeritus at San Francisco State University, has proposed an alternative that complicates the picture further. He's observed spiders sitting on glowing mushrooms, using them as illuminated hunting platforms to ambush insects—including cockroaches that come to feed on the fungi themselves.
This raises a darker possibility. What if some fungi glow not to attract helpful spore-dispersers, but to summon the predators of fungal pests? Glowing mycelium threading through wood could function as "a fungal blue light calling in the night guard," attracting beetles and other predators that eat the arthropods feeding on fungal tissue.
The chemical reaction itself is remarkably efficient—nearly all the energy goes into producing visible light rather than heat, making fungal bioluminescence one of nature's best examples of "cold light." That efficiency suggests selective pressure to optimize the glow, though for what purpose may vary.
When One Answer Isn't Enough
The insect-attraction hypothesis works elegantly for Neonothopanus gardneri. The circadian control, the wavelength selection, Dunlap's experimental results—everything aligns. But the ghost fungus breaks the pattern completely, suggesting that a single explanation can't cover all bioluminescent species.
Perhaps the most honest conclusion is that evolution found the same solution—glowing through luciferin-luciferase reactions—and different fungi have kept it for different reasons. Some use it as advertising. Others might benefit from predator attraction. Still others may simply tolerate it as a harmless side effect of necessary chemistry.
Aristotle wrote about glowing wood in the 4th century BCE without understanding that fungi caused it. More than two millennia later, we've identified the molecules and mapped the circadian rhythms, but the ultimate question—why glow at all?—still yields different answers depending on which mushroom you ask.