To avoid the hungry mouths of predators, members of the animal kingdom have evolved some fascinating “don’t eat me” signals; some of which are truthful, flagging to enemies that they are poisonous, whereas others are a clever ruse. Some cyanide-stuffed millipedes, for example, emit an eerie teal glow to warn of their toxicity. But interestingly, it turns out that, for at least one particular species, this bioluminescence did not in fact evolve as a defense mechanism, but rather to help them cope with the stresses of living in a hot, dry environment. This intriguing find has been published in Proceedings of the National Academy of Sciences.
Found exclusively scuttling around three mountain ranges in California, Motyxia, or the Sierra luminous millipedes, is a group of blind, cyanide-producing arthropods known for their ability to emit light. These bioluminescent bugs produce their distinctive glow by means of molecules called photoproteins which generate light upon combination with oxygen or other oxidizing agents - things that contribute oxygen or steal electrons in chemical reactions.
Research has shown that not only does the luminescence intensify when the animals are handled, but also that some glow brighter than others. By examining how much this brightness varies between species, National Geographic points out, then it might be possible to trace the evolutionary origins of this trait. So, entomologists from Virginia Tech and the University of Arizona began collecting specimens from the wild, including non-glowing controls for comparison, in order to assess their luminescence levels.
One species they examined, Xystocheir bistipita, had not been seen since its discovery back in 1967 and was assumed to be non-luminescent. But to their surprise, the bug began glowing in their laboratory, arousing suspicion that this millipede may actually be a mislabeled member of the Motyxia group. Later genetic analysis by the team revealed that this was indeed the case, so the enigmatic species was renamed Motyxia bistipita.
Image credit: National Geographic Society Expeditions Council.
The researchers then began comparing the genomes of the members of the Motyxia genus, including its newest member, in order to examine genetic relationships. After constructing an evolutionary tree, the researchers measured the brightness of their characteristic glow photographically and then investigated their toxicity levels by seeing how much cyanide they possessed in their specialized glands.
Interestingly, they found that the millipedes’ bioluminescence originated in Motyxia’s common ancestor and then grew brighter over time. Furthermore, species living at lower elevations, such as M. bistipita, glow less than their relatives at higher elevations. Since M. bistipita’s habitat is much hotter and drier than bugs living further up the mountains, the researchers reasoned that their glow may not have in fact evolved as a warning signal for predators, but instead to help them deal with the stresses of such a climate.
At higher temperatures, animals struggle to control the levels of highly reactive, oxygen-containing molecules called reactive oxygen species, such as peroxides, which can lead to cellular stress. However, photoproteins can actually mop up these harmful molecules, reducing the damage to the cell that would otherwise ensue whilst also producing light.
But when the members of the Motyxia group began to migrate to higher elevations with a greater risk of predation, the bugs repackaged this luminescence system as a warning signal. This was supported by the observation that species containing larger volumes of cyanide were brighter than their slightly less toxic counterparts.
“This discovery clarifies the evolutionary origins of many complex traits, not just bioluminescence,” lead author Paul Marek said in a statement.