A Blog by Ed Yong

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

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.

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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14 thoughts on “If you see a glowing millipede, best not to bite it

  1. The phenomenon of poisonous creatures advertising their danger has always struck me as strange. In order for it to work, the creature’s predator would have to evolve an aversion to the bright colors or glow. But it would take a while for such an adaptation to occur in the predator. In the meantime, what favors the bright colors in the prey?

  2. I haven’t read the paper (and I hate when people write that, but I just saw the article), but GFP is a fluorescent protein, while luminescence is a different phenomenon. Luminescence is a chemical reaction that releases photons, while fluorescence is the absorption of light at a specific higher energy (sometimes more than one) and the release of light at a specific lower energy (longer wavelength). The proteins that do these two different things are not related to one another structurally, AFAIK. I could be wrong, tho :)

  3. I wonder if the “burglar alarm hypothesis” might be an alternate explanation. It’s thought that some plankton and fish light up when attacked to bring attention to their attackers, putting them at higher risk of getting eaten.

  4. Woah, these are real. I must have seen some of these in my childhood, I vividly remember small things with that very color, moving quickly out of sight, but at one point I concluded it might just have been a trick on the eye or the mind.

  5. @Russ, it’s usually targeting a learned response, as far as I can tell; a predator will try one of them, maybe two, and then won’t go near them again. It’s a bit of a medium-term strategy; doesn’t rely on the predators evolving an aversion, but does need a few to take one for the team. These are high enough density that it apparently makes sense!

  6. @Russ, Chris Yes aposematism is strange, and at first counterintuitive. But totally makes sense after some thought. Aposematism is a biological phenomenon where a feature (appearance, sound, smell) deters predators because it denotes something noxious, or unpleasant. E.B. Poulton coined the word in the book “The Colours of Animals” in 1890. Any signal can be aposematic, and it doesn’t have to be garish, or even conspicuous. Although often times in nature aposematic signals are conspicuous. There’s empirical evidence that demonstrates conspicuous aposematic signals work better because they are more quickly learned, easier to remember, and so different looking as to reduce errors in discrimination against edible prey. Anyways, learning (at some point in establishing avoidance) is a big component of aposematism. Predators, over time, learn that some appearance denotes something unpleasant. And learning a signal that is associated with something unpleasant is very rapid. In some instances, after some time, avoidance becomes fixed, this is referred to as innate avoidance. (It’s believed that humans have an innate, or unlearned avoidance to snakes, as a result of an ancient evolutionary association with them.)

  7. One thing that confuses me about aposematism is why it isn’t vulnerable to cheaters. There are mimics, certainly, take advantage of other species’ learned aversion to the actually toxic animals, but why doesn’t this happen intra-species? It seems to me that a mutation that knocked out the toxin would be adaptive – such an organism would be spared the cost of producing the toxin while displaying the “I’m toxic” signals and avoiding being eaten. So, why doesn’t non-toxicity spread, especially once would-be predators have evolved innate avoidance? Or *does* that happen, making aposematism an evolutionarily short-term phenomenon? I.e., toxin evolves, warning displays evolve, and then non-toxicity spreads, initially benefiting from innate avoidance until the predators lose that innate avoidance, possibly starting the cycle over again?

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