A prolonged exposure shows the greenish glow of a Motyxia millipede, which researchers now suggest is a warning sign to ward off predators.
Credit: Paul Marek/University of Arizona
SYDNEY: A mysterious species of bioluminescent millipedes use their unique, eerie glow as a warning signal to deter nocturnal predators, researchers suggest.
For a creature that has lost its ability to see through adapting to primarily subterranean existence, this glowing defence mechanism represents a novel evolutionary innovation that arose inside a relatively small geographic area.
The findings, published today in the journal Current Biology, detail a field trial examining the glowing Motyxia species, which are only found in certain regions of California, U.S.
"Bioluminescence in Motyxia represents an adaptive innovation which is particularly impressive given the small evolutionary and small geographical area in which it evolved," said lead author and entomologist Paul Marek from the University of Arizona.
"By using live and clay millipedes in field trials, half of which with their bioluminescence concealed, we found that non-luminescent ones were preyed upon up to four times more often than their luminescent counterparts," he said.
A rare trait in millipedes
Biologists have discovered and described more than 12,000 species of millipedes and of those known only eight display this glowing trait.
Scientists had previously speculated two things: that the glow served no specific purpose and was simply a result of other physiological functions, or that it served as a warning signal.
To get a better idea of how the ability evolved in millipedes, the team sequenced selected gene regions and estimated their evolutionary history to pinpoint the origin of the glow in millipedes.
"We found that bioluminescence is an unique innovation limited to a single genus of millipedes, and that it functions as an nocturnal warning signal to deter predators," said Marek.
Night time risers
Just like all other millipedes, Motyxia are vegetarians, feeding mostly on decaying plant material and spending most of their time burrowed underground.
Despite being blind, they can sense nightfall, said Marek, at which point they move to the surface to forage and mate.
"When they are disturbed, they ooze toxic cyanide and other foul-tasting chemicals from small pores running along the sides of their bodies as a defense mechanism," Marek explained.
The bioluminescent warning sign
Some millipede species active during the day display bright warning colors to ward off predators, a trait known as aposematism.
This is where an animal that is unprofitable as prey because of physical features such as protective spines or toxins, evolves features to announce this unprofitability to predators - a warning signal.
As the millipedes come out at night and bright colours would not be visible, Marek and colleagues hypothesized that their glow was adapted to serve a similar function. "So predators associate the light generated by Motyxia with a noxious and very much unprofitable meal," said Marek.
Known as bioluminescence, the ability to glow in the dark is remarkably widespread in the animal kingdom, including things like fireflies and ocean animals that reside in the dark depths.
In some of those examples, bioluminescence is thought to help attract sexual partners, communicate messages, or even to attract prey - this is the case with the deep-sea angler fish, which dangles a glowing lure in front of its open mouth.
