That’s Not Algae Swirling on the Beach. Those Are Green Worms.

https://www.nytimes.com/2018/08/01/science/plant-worm-circles.html

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Between April and September, just minutes after waves splash over beaches along the eastern Atlantic coast of England, Wales, France or the Channel Islands, tiny, green worms emerge from their sandy hide-outs. They gather in small pools, often near small boulders.

This is Symsagittifera roscoffensis, the plant-worm. Some call it “mint-sauce” because of its vibrant color. And if you happened to be walking where they emerge just after high tide on a sunny day, you’d probably think they were algae.

But if you stuck around and watched patiently, you’d see something strange happen.

One by one, the worms accumulate, getting denser until a swirling mass forms, just more than an inch wide. Movement stops. The pools start drying up. And up to a million worms, collected as one, become a verdurous mat, bathing like a beach blanket beneath the sun. When the tide returns, the worms retreat back into the sand.

“They’re really quite fascinating,” said Ana Sendova-Franks, a biologist who is studying the worms and collective behavior (usually in ants) at the University of the West of England, Bristol. “I suppose when people stumble across them, they don’t recognize them as what they are.”

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Since the late 19th century, this peculiar marine flatworm, just about the size of a pencil tip, has intrigued scientists. They’ve studied it to understand regeneration, photosymbiosis and climate change. But the plant-worm’s collective behavior has only recently captivated scientists like Dr. Sendova-Franks.

At first glance, it looks and acts like a plant. But it has a rudimentary brain and nervous system, and if you chop off its head, it rejuvenates. It also has gravity sensors and eye-like photoreceptors. As a juvenile, the worm swallows solar-powered algae that lose their cell walls, eyes (yes, the algae have eyespots) and wiggly tails.

The algae gets incorporated into the worm’s body, between its skin and muscles, and becomes the source of all the nutrients it needs to stay alive. Like corals that host their own algae, the worms bleach in acidic environments, expelling their algae and dying.

After observing worms swirling in the lab, Dr. Sendova-Franks and her colleagues at the University of Bristol ventured to the northern shores of Guernsey, in the Channel Islands, to see if this circular milling occurs in nature.

After five days, they observed four circular mills, including a twinsie that turned like cogs in opposing directions. They reported this “by no means rare” behavior in the wild in a paper published last week in Royal Society Open Science.

After a few worms move in parallel, they start rotating suddenly at a certain density. The team thinks the worms enter the spiral like cars all moving toward the center of a roundabout. As traffic increases, the center comes to a halt. Then mats form.

The worms stay together to maximize their algae’s ability to harness solar power. But too much light is bad for the algae. Rotating, the researchers speculate, may prevent a single plant-worm from getting all the exposure. The worms also secrete a mucus which may reflect away some sunshine — and possibly aid in travel.

“For me, the most amazing thing is how they keep together given how precarious the environment is,” said Dr. Sendova-Franks.

Circular mills have been observed in birds, fish and other animals. But for many organisms, they’re a sign of trouble — like lost army ants following a chemical signal in a circle until they die of exhaustion.

Next, the researchers hope to learn if this collective behavior truly is adaptive, and if so, how it evolved. They want to know how the worms communicate and travel to the most advantageous or safest mingling locations.

Until then, if you’re lucky enough to stumble upon plant-worms in a circular mill, Dr. Sendova-Franks suggests that you look but don’t touch. Until we get to know them better, we don’t want to harm them.