Slugs that can survive decapitation, regrow new bodies discovered in Japan

slug axe
The slugs can lose their bodies and regrow them. Photo credit: Current Biology/Getty

Scientists in Japan have discovered two bizarre creatures with abilities that wouldn't be out of place in one of the nation's classic monster movies. 

Both are sea slugs that can not only survive being decapitated, but regrow entire new bodies - including a new heart and all the other necessary organs.

But that's not all - they appear to do it using photosynthesis, like plants. 

"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," said Sayaka Mitoh of Nara Women's University in Japan, who made the discovery.

Mitoh, a PhD candidate, didn't deliberately chop off a slug's head to see what would happen, at least at first - the discovery was pure serendipity. One day in the lab, where she studies their life cycles, she saw a slug head moving around by itself, without a body attached. 

Further research involving two species - Elysia cf. marginata and Elysia atroviridis - found if you tie a piece of nylon string around their necks, at some point in the next few days the slug would 'autotomise' the rest of its body - just let it go, like some lizards do with their tails when a predator grabs it. 

If the slug is young and healthy, the head goes on with life as if nothing has happened, continuing to feed on algae. Within a day its wound has healed, and a week later regeneration of the heart is underway. Three weeks later, they've fully regenerated their entire body. Older slugs didn't seem to have this ability, their disembodied heads refusing to eat, eventually leading to their deaths. 

One slug they studied survived being decapitated twice, regrowing its body both times. 

Photographs of the slugs, showing them without bodies, with regrowing bodies, and a parasite commonly found in them (bottom right).
Photographs of the slugs, showing them without bodies, with regrowing bodies, and a parasite commonly found in them (bottom right). Photo credit: Current Biology

It's not clear why the slugs would have this ability - attacking the slug like a predator didn't trigger autotomy, like it does in lizards. 

"Our experiments also showed that autotomy of E. cf. marginata took several hours, which is not effective to avoid predation, and that imitated predator attacks did not induce autotomy. Therefore, their autotomy is unlikely to function as predation escape."

The scientists suspect instead it's to get rid of a body that's been infected with parasites.

"The parasites occupy most of the main body of E. atroviridis and strongly inhibit its reproduction during lifetime," the study, published in Current Biology, reads. "Thus, removing such parasites by autotomy likely enhances the host's reproductive success." 

Nor is it understood how they do it - but a process known as kleptoplasty is suspected.

"In Elysia, a highly branched digestive gland is spread over the majority of its body surface, including the head, and the gland is lined by cells that maintain ingested algal chloroplasts," the part of a plant cell which allows it to extract energy from sunlight.

"Thus, these sacoglossans can obtain energy for survival and regeneration from photosynthesis by kleptoplasts, even when they cannot digest food." 

None of the discarded bodies regrew a head, but bizarrely were able to keep on living and reacting to stimuli for up months afterwards. 

"As the shed body is often active for months, we may be able to study the mechanism and functions of kleptoplasty using living organs, tissues, or even cells," said Mitoh.