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The severed heads of at least two species of sea slugs can move, eat and possibly even eliminate waste during the one to three weeks it takes for their bodies – including the heart – to regrow after being detached at the neck.

The headless bodies can also live for up to a few months, with the heart still beating until the flesh begins to decompose, says Sayaka Mitoh at Nara Women’s University in Japan. However, the bodies never regrow heads. “The head has the brain and teeth, or radula, which may be irreplaceable,” she says.

Mitoh and her supervisor, Yoichi Yusa, were raising one species of sacoglossan sea slugs (Elysia cf. marginata) to study the slugs’ photosynthetic abilities when they discovered a living, severed head in their laboratory.

Intrigued, the researchers examined their slugs and found they all had a groove around their necks that they thought might be a “pre-determined breakage plane”. They gently tied a thin string around the necks of six lab-grown slugs at this groove and noted that all six severed their own heads, generally within a day.

“The head becomes green with chloroplasts after it feeds on algae,” both when the body is severed and intact, says Mitoh. The slugs’ digestive glands are thought to be “distributed all over the body surface, including the head”, she says, which might explain how the heads survived.

Meanwhile, the team observed 160 lab-raised and wild-trapped sacoglossans (Elysia atroviridis) daily until their natural death, on average for just under two years.

Five of the 15 lab-raised slugs and three of the 145 wild slugs severed their own heads, while 39 wild slugs amputated smaller body parts like the tail or the feet-like appendages.

Some animals autotomise – shed body parts – to escape predators, so the scientists tried pinching and poking another group of slugs to mimic a predator attack, but none of the animals responded by amputating anything.

Instead, they noted that some of the field-collected slugs had internal copepods, a parasitic crustacean – including all 42 of those that had severed a body part.

“We think that at least this species of sacoglossans autotomises to remove internal parasites which inhibit their reproduction during lifetime,” Mitoh says. “But this is a hypothesis and remains to be tested, and other reasons may also be involved.”

If the goal is to get rid of parasites, the tactic comes at a great cost: older slugs didn’t survive the severing act. “This may seem like a silly choice,” Mitoh says. “But the old ones would die soon anyway, and they might stand a chance of surviving and regenerating a parasite-free body.”

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