Why on earth would a mushroom glow at night?
The existence of bioluminescent mushrooms has been noted since the time of the ancient Greeks, but less is known is about why a small percentage of fungi would feel the need to glow like fireflies.
Or at least until now.
In research published Thursday in the journal Current Biology, scientists said it's likely that the mushrooms are attempting to attract beetles, flies, wasps and other insects to help spread fungal spores.
"It appears that fungi make light so they are noticed by insects who can help the fungus colonize new habitats," said senior author Jay Dunlap, a biologist at Dartmouth's Geisel School of Medicine.
Dunlap specializes in the study of circadian rhythms -- biological processes that run according to a 24-hour clock. He and his colleagues focused on the glowing mushroom Neonothopanus gardneri because it turns on its "exceptionally intense" light at night, and shuts it off during the daytime.
"Regulation implies an adaptive function for bioluminescence," Dunlap said in a statement.
Though there are more than 100,000 known species of fungi, N. gardneri is just one of 71 that can emit a ghostly green glow. It was believed that they all emitted this light around the clock in a process that involved oxygen and energy, but the researchers noted that N. gardneri had its own glow schedule that was regulated by temperature.
N. gardneri, or what Brazilians call flor de coco (coconut flower), prefers to hang out at the base of young palm trees in coconut forests, where there is little wind. Consequently, it needs some help spreading its fungal spores, scientists reasoned.
Mushroom spores can adhere to the bodies of traveling arthropods and get deposited in other areas.
To test their hypothesis, the researchers constructed fake mushrooms from resin, wired them with green LED lights and placed them in the wild.
The prosthetic mushrooms attracted far more staphilinid rover beetles, flies, wasps, ants and "true bugs" than non-glowing control traps, the researchers said.
"Circadian control may optimize energy use for when bioluminescence is most visible," study authors wrote.
Bioluminescence has been observed in a variety of insects, sea creatures, bacteria and fungi, and can serve very different purposes in each. Some organisms use it to attract dinner, while others use it to attract a lover. And some use it to light the way or as a form of defense.
In the case of the coconut flower, its nocturnal lighting is particularly suited to reproduction.
"Spore dispersal in canopy forests is greatest at night or early in the morning, when environment humidity and spore germination are highest," authors wrote.