Scientists have discovered that sea slugs from two different species can sever their own heads and then regenerate their entire bodies, including the heart, within weeks or even days.

It's not clear yet how the slugs manage this unique feat, but the heads might survive on photosynthetic energy from the algae they ingest, the researchers reported March 8 in Current Biology.

Many animals, from lizards and salamanders to insects and crustaceans, have the ability to shed and regrow body parts such as tails, limbs and eyes. Often, this ability helps them distract and escape from predators. Scientists have long been interested in understanding how animals grow and regenerate body parts with the hope of figuring out how to do the same for humans.

Many sea slugs can regenerate their tails, tentacles and other appendages, Yoichi Yusa, a professor of ecology and aquatic biology at Nara Women's University in Japan and a coauthor of the new paper, told The Academic Times. Until now, though, none were known to take regeneration to such extremes.

Yusa and her colleague Sayaka Mitoh, a doctoral candidate in ecology and marine biology at the Japanese institution, stumbled across this behavior in sea slugs they were raising in the laboratory. One day, Mitoh noticed that the head of one of their slugs had become separated from its body and was moving on its own.

"It was a serendipity," Yusa said. "Such a great ability to regenerate really surprised us."

Yusa and Mitoh observed five captive-bred slugs from this species, known as Elysia marginata, and one specimen captured from the wild detach their head over their lifetimes. The whole process took several hours, and the wound where the head came away from the body closed within a day. The slugs left their kidney, intestine and reproductive organs behind, and one slug even detached its head twice.

Several of the sea slugs dined on algae within a few hours of their voluntary decapitation, began regenerating their hearts within a week, and finished regrowing their bodies within about three weeks. However, the heads of two older slugs did not feed and died within 10 days. The abandoned bodies moved and reacted to touch but didn't regrow a head, and over several days to months became pale and shrunken before decomposing.

The team also collected more than 140 slugs belonging to another closely related species, Elysia atroviridis, from the wild. Eighty-two of the slugs were infested by tiny crustaceans called copepods. Three of these slugs beheaded themselves and two regrew their bodies within a week. None of the 64 parasite-free slugs shed any appendages.

Yusa and Mitoh speculate that the slugs shed their bodies to rid themselves of internal parasites, although the ability may also have other purposes such as allowing the animals to escape if they become tangled in algae.

The researchers noticed that both slug species had a small groove running across their necks. When they gently tied nylon string around the grooves of six slugs, five of them detached their heads from this area within a day. 

By contrast, when the researchers imitated an attacking predator by pinching the animals' heads or cutting their appendages, the slugs kept their heads firmly in place.

The group of sea slugs these species belong to — known as sacoglossans, or sap-sucking slugs — generally have few predators because of their camouflage and ability to absorb toxic chemicals from their food. 

Sacoglossans have another unusual skill that may help the heads survive after being liberated from their bodies. The slugs are able to incorporate chloroplasts — organelles in which photosynthesis takes place — from the algae they eat into their own cells. Drawing energy from the chloroplasts they consume might sustain the slugs while they regenerate, even though they cannot digest food.

The team plans to investigate whether other sacoglossans can also sever their heads. 

"Moreover, we will further explore the mechanisms underlying this phenomenon at the tissue and cellular levels," Yusa said.

The study, "Extreme autotomy and whole-body regeneration in photosynthetic sea slugs," published March 8 in Current Biology, was authored by Sayaka Mitoh and Yoichi Yusa, Nara Women's University.