It’s the sort of thing you’d expect to see in a David Cronenberg film: sacoglossans, a type of sea slug, are able to exist as detached heads for several weeks after shedding their entire bodies, including the internal organs, before regenerating a new one.


The discovery was made entirely by accident by researchers at Nara Women’s University in Japan. The team were studying the life cycles of sacoglossans, a type of sea slug, when PhD student Sayaka Mitoh noticed that one of them was moving around without its body.

The head had separated from the heart and body and had begun moving on its own immediately afterwards, she said.

Within days, the wound at the back of its head closed up. Its heart started to regenerate within about a week. And after three weeks it had completely regenerated its body.

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“We were surprised to see the head moving just after autotomy,” said Mitoh. “We thought that it would die soon without a heart and other important organs, but we were surprised again to find that it regenerated the whole body.”

In further experiments, the team noted that the heads of older sacoglossans didn't resume feeding and died after about 10 days. None of the cast-off bodies regenerated a new head, but they did move and react to being touched for several days.

It’s as yet unclear how the sacoglossans accomplish this nightmarish feat, but the researchers say it may have something to do with stem-cell-like cells at the cut end of the neck that are capable of regenerating the body.

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As if this wasn’t all strange enough, they also have another unique ability that may help them survive long enough to completely regenerate, the researchers say. They are able to incorporate chloroplasts from the algae they eat into their own bodies and use them to provide energy via photosynthesis – a habit known as kleptoplasty that has led to them being nicknamed ‘solar-powered sea slugs’.


The researchers are now planning new studies to further investigate this bizarre behaviour.

Reader Q&A: How do corals eat?

Asked by: Mary Leonard, Chelmsford


Corals are communal animals related to sea anemones and jellyfish. Like their cousins, they catch tiny animals (called zooplankton) using stinging tentacles that surround the single body opening that acts as both a mouth and anus.


Many corals also contain symbiotic algae called zooxanthellae. The zooxanthellae allow them to get some extra energy through photosynthesis.

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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.