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Weird Worms Swim in Huge Green Circles

If you find yourself walking along the beaches of the English Channel, you might come across a mat of green goo, as if someone had tipped a jar of mint sauce onto the beach. But if you get down on your hands and knees, and stare at the goo with a magnifying glass, you’ll see its true nature.


Little green worms, each just a few millimetres long, writhing together in their millions.

Their formal name is Symsagittifera roscoffensis, but Channel Islanders call them mint sauce worms for obvious reasons. Their green colour comes from the algae in their bodies, which provide them with nutrients by harnessing the sun’s energy. The algae also gave the creatures their other name, favoured by early 20th century biologists: “plant-animals.”

One of those biologists, Frederick Keeble, wrote a whole book about these creatures in 1901. Eight decades later, Nigel Franks from the University of Bristol picked up a copy at a second-hand bookstore for the princely sum of 50 pence. Intrigued, he set out to find them, and it took him several trips to the island of Guernsey to do so. “It’s hard to find a patch of them but once you find them, they’re there in huge numbers,” he says. “They’re hugely impressive. When you see them, you’ll think they’re algae.”

Circular Mill

Franks scooped up a group of them and started studying their behaviour. He quickly realised that if he added enough of them to the same pool of shallow water, they’d start swimming in mesmerising circular mills, with hundreds of individuals in rotating green rings. “They did them at the drop of a hat,” says Franks. “It was such a telltale symptom of really strong social interactions.”

Having spent most of his career studying the collective behaviour of ants, Franks was the perfect audience for the worms’ display. He knew that many other social animals will produce circular mills, for varying reasons. Army ants do it if you isolate a group of them, slavishly following each other’s chemical trails until they die from exhaustion. Fish do it when confronted by predators. Even virtual animals will form mills if you program them with simple rules. So what about the worms?

By studying videos of the animals, Franks and his colleagues showed that they interact strongly with one another, often swimming in parallel with just a millimetre between them. As their densities increase, they grew disproportionately closer. The mills, it seems, arise from these close-quarter interactions, and from the worms’ tendency to curve in a particular direction. (It’s notable that almost all the mills spin clockwise.)

Using this information, Franks’ colleague Alan Worley built a computer simulation in which virtual worms behaved according to simple rules, and yet spontaneously produced circular mills just like their real counterparts.

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Green mint sauce worms. Photograph by Stevie Smith

“But many different models of individual interactions can reproduce the same kind of collective patterns,” says Guy Therauluz, another collective motion researcher at Paul Sabatier University. “Deciphering the real social interactions at work between worms is a task that remains to be done.”

Franks plans to do that. It’s possible, he says, that the worms are dribbling some kind of chemical behind themselves, which the others follow. Or perhaps they are reacting to turbulence in the water. “The rules have yet to be worked out,” he says.

He also wants to know why the worms behave in this way and he has a fascinating suggestion. Perhaps the worms are social sunbathers. By gathering in large mats of biofilms, bound together by mucus that they themselves secrete, they can stabilise their position on sandy beaches or tidepools. In Franks’ words, they “behave collectively as a social seaweed”.

Individual worms are also known to head towards light sources that are almost too bright for them, and would max out the abilities of their algal partners. They could deal with that problem in a mat by ducking down into the darker centre once they’ve had their fill of light. Franks compares them to emperor penguins huddling against bitter Antarctic winds: these flocks continuously rotate as individuals at the edge wheedle their way into the centre.

Dora Biro from the University of Oxford praises Franks’ attempt to explain not just the how of the collective motion, but also the why—something that has been overlooked by scientists in this field.  “The hypothesis is very interesting,” she says. “It would be great to find support for it through future work, including observations in the wild on the formation of the biofilm, and the role of milling in the process.”

Franks is just getting started with the mint sauce worms, but he sees them as great subjects for understanding collective behaviour in animals. Other scientists have analysed their bodies, the way they grow, and their powers of regeneration—but only ever one at a time. “I suspect that if people looked at populations more, as we have been fortunate enough to do with these things, they’d see more and more examples of strange microscopic organisms showing social behaviour like this,” Franks says.