“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.