Why Do Those Bugs Glow? For Some, Bioluminescence is Just a Mistake

Bioluminescence. Throughout nature, glowing in the dark could have a number of intriguing purposes, ranging from vision to attracting prey. However, a new study of millipedes suggests that it starts as a simple biological mistake and, for some, can stay that way.

"Living things glow in many different colors and for many different reasons, but now we know that the early evolutionary role of bioluminescence may be completely different than its modern day function," Paul Marek, an expert in life sciences at Virginia Tech University, explained in a statement.

Marek came to this conclusion after taking a look at a rare type of millipede that was "rediscovered" after being forgotten about for half a century. The bug, Xystocheir bistipita, had been collected in 1967 and was not seen again until Marek found new examples of it during fieldwork in the foothills of San Luis Obispo, Calif.

Far shorter and living at far lower elevations in soil compared to most millipede species, it was initially through that X. bistipita was not a glower belonging to the genus Motyxia - the only genus of bioluminescent millipedes in the Western Hemisphere.

Most members of Motyxia glow for a very specific reason - the same reason many poisonous frogs boast vibrant and obvious coloration in the Amazon - as a warming that "if you eat me, you're going to regret it."

However, in its native environs X. bistipita has very few predators, and those that can be found there are often blind to a warning glow. And yet, if you hit the lights, you will find that this little millipede does indeed have its own slight glow.

After sequencing its DNA, Marek found that the tiny millipede was in fact related to its luminous cousins, earning it a name change to Motyxia bistipita.

"After we sequenced them... we demonstrated the faint bioluminescence of the low-lying millipedes represented an older trait and the brighter luminescence of their mountain cousins represented a newer trait," Marek added. (Scroll to read on...)

Unlike fireflies that glow because of an enzymatic reaction between a luciferase and a luciferin, M. bistipita owes its soft green-blue glow to the reaction of a photoprotein that requires magnesium.

Marek and his team suggest then, that the process simply evolved in the tiny millipede's ancestors for its antioxidant properties to help cope with the oxidative stress of living in a low-lying, dry environment. Intriguingly, in a world full of blind neighbors and predators, the consequential glow the reaction produced simply went unaccounted for.

However, when some of those early glowing millipedes moved to higher ground, the bioluminescence slowly earned its own purpose, repackaged as a nocturnal warning signal in a world with more predators.

"Evolution of a complex feature such as bioluminescence is difficult to understand at first glance but totally makes sense when considered in an evolutionary context," Marek said. "We showed that bioluminescence, a trait typically used as a warning pattern evolved gradually and for a different purpose."

Marek explained that today, there is a strong correlation between a large toxic cyanide gland and the glow of a millipede. However, as shown by M. bistipita, there isn't always this direct link. Sometimes, millipedes glow because they just do, without developing any recognizable purpose for the trait overnight.

"This discovery clarifies the evolutionary origins of many complex traits," he added, "not just bioluminescence."

The results of this study were detailed in the journal Proceedings of the National Academy of Science (PNAS).

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