If you see a glowing millipede, best not to bite it

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If you go down to the woods of California today, you might be in for a big surprise. At night, the forests crawl with sinuous shapes that glow with an eerie greenish-blue colour. They are Motyxia millipedes and they shine brightly whenever they’re disturbed. “If you go to the right forest and you let your eyes get adjusted to the night, then you can see them everywhere,” says Paul Marek from the University of Arizona. Some big oak tress can shelter 1 glowing millipede in every square metre. They look like fields of stars.

There are around 12,000 known species of millipedes, and only the eight Motyxia species glow. Marek says, “[They] would definitely be on my top 10 for my imaginary “millipede biodiversity global tour” (along with the shocking pink millipede in Thailand & the longest millipede in Africa).”

But why do the Californian ones glow? Marek knows the answer. With hundreds of millipedes, some clay, and a bit of paint, he has shown that they light up to ward off predators. You might expect that the light shows would make the millipedes easier to find and eat. In fact, it deters hungry mouths.

The ability to make your own light, known as bioluminescence, has evolved around 40 to 50 times in the history of life. Hundreds of animals can do the same thing, from fireflies to squid to deep-sea fish. They use this ability to attract their prey, to recognise their mates, and to hide from predators. Motyxia millipedes are part of this extensive club, but they’re unusual in one important respect: they’re blind. They can’t see their own glows; their light shows are aimed at a different audience.

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Marek collected 164 Motyxia millipedes from California’s Giant Sequoia National Monument and painted half of them to cover up their nightly glow. He then tethered them with a gently knotted string to specific places throughout the forest. Marek also built 300 clay millipedes using a bronze cast made by his wife. He covered half of them with the same obscuring paint as the live millipedes, and the other half with a glow-in-the-dark hue. He scattered the fake millipedes throughout the forest just like the real ones, and waited.

The next morning, he found that rodents like grasshopper mice, had savaged around a third of the millipedes. The dull ones took the brunt of the attacks – they had between two and four times as many bite marks as the glowing ones. Nearly half of the dull clay millipedes bore the wounds of a rodent attack compared to just 22 percent of the glowing models. Similarly, rodents had attacked around 18 percent of the painted live millipedes but only 4 percent of the glowing ones.

The experiment showed that the millipedes’ glow repels predators, and the models proved that it’s the light, rather than the smell or taste of the animals, that puts off attackers. The glow sends a clear message: “Don’t eat me. I’m dangerous.” And they are – the millipedes create cyanide in their bodies and secrete the poison through pores along their flanks. They make for an unpleasant and possibly lethal mouthful.

If doesn’t matter that an animal is poisonous if its predators have to bite it to find that out. The predator would get a mouthful of poison, but the prey would incur a serious wound. This is why many poisonous animals advertise their toxic payloads with bright colours.

Many millipedes also have bright colours, but these would be useless to Motyxia species. They spend the daytime buried in the leaf litter, emerging only at night to feed on decaying plants. “Night is an excellent time to do millipede things like eating detritus and mating,” says Marek. When they’re active, predators wouldn’t be able to see bright colours anyway. As such, Motyxia millipedes are a dull orange, and they publicize their defences by glowing in the dark. “I think that Motyxia is better able to exploit this nighttime niche if bioluminescent & toxic,” says Marek.

Now, Marek wants to find out more about how the millipedes got their lights. By analysing the genes of all 8 species, he found that bioluminescence has evolved only once in this group. While many animals glow by harnessing luminous bacteria, the millipedes rely on their own light-producing protein. What that protein is, and how it’s related to those of other glowing animals, is still a mystery.

Reference: Marekt, Papaj, Yeager, Molina & Moore. 2011. Bioluminescent aposematism in millipedes. Current Biology. Current Biology; citation tbc.

Images by Paul Marek

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