June 3, 2023

A few years ago, Sayaka Mitoh, Ph.D. The candidate at Nara Women’s University in Japan was looking at the huge collection of sea slugs in her laboratory when she came across a gruesome sight. One of the lab’s captive-bred sea slugs, an Elysia marginata, had somehow been beheaded.

When Ms. Mitoh peeked into the tank for a better look, she noticed something even more shocking: the creature’s severed head moved around the tank, chewing seaweed like there was nothing unusual about being a disembodied snail.

Ms. Mitoh also saw signs that the sea slug had self-inflicted: it was as if the sea slug had loosened the tissue around its neck and tore off its own head. Self-amputation, known as an autotomy, is not uncommon in the animal kingdom. The ability to throw away a part of the body like a tail helps many animals prevent predators. However, an animal had never been seen to throw its entire body overboard.

“I was really surprised and shocked when I saw the head move,” said Ms. Mitoh, who studies the life history traits of sea slugs. She added that she expected the snail “to die quickly without a heart and other vital organs”. But not only did it live on, it also regenerated the entire lost body within three weeks.

This prompted Ms. Mitoh and her colleagues to conduct a series of experiments to find out how and why some sea slugs guillotine themselves. The results of their experiments, published Monday in Current Biology, provide evidence that Elysia marginata and a closely related species, Elysia atroviridis, deliberately beheaded themselves to encourage the growth of a new body. Although more research is needed, the researchers suspect that these marine slugs leave their bodies when infected with internal parasites.

Ms. Mitoh and her team monitored several groups of Elysia marginata and Elysia atroviridis throughout the life of the creatures. Not all of the sea slugs they monitored beheaded themselves, but many did – one even did it twice. Bodies regenerated from the heads of both species, but the headless bodies remained headless. However, these dumped bodies responded to stimuli for months before decomposing.

It took only a day for the head wounds of the sea slugs created during the autotomy to heal. Organs like the heart took an average of a week to regenerate. For most marine snails, the regeneration process took less than three weeks.

“We have long known that marine snails are regenerative, but it really goes beyond what we thought,” said Terry Gosliner, senior curator of invertebrate zoology at the California Academy of Science.

Dr. Gosliner, who discovered over a third of all known marine snail species, suspects that the impressive regenerative ability of these marine snails is related to another impressive biological talent they possess.

Elysia marginata and Elysia atroviridis are often referred to as “solar powered sea snails”. They are one of a small number of snails that can ingest chloroplasts into their bodies from the algae they eat. This allows the snails to feed at least partially on the sugars that the chloroplasts produce through photosynthesis.

This ability, known as kleptoplasty, could allow these sea slugs to survive without their bodies for long periods of time.

For most animals, and even some sea slugs, it is believed that the autotomy only serves as a means of preventing predators. However, the researchers found evidence that it can also be used to drive away internal parasites. All of the Elysia atroviridis that parted their heads had internal parasites, according to the researchers. And by dropping their infested bodies, they successfully expelled them and regenerated parasite-free forms. Parasites were not found in any of the Elysia marginata.

Regardless of purpose, the regenerative abilities of these sea slugs are “remarkable,” said Kenro Kusumi, a biologist at Arizona State University who studies regeneration in reptiles. According to Dr. Kusumi, the marine snails in question have the best properties that can be found in animals that are capable of complex regeneration. For example, these sea slugs have a “fracture surface” along their neck that allows for a clean break. Many lizards, including geckos, have a similar fracture surface near the base of their tail.

“It is so interesting to see how many features of regeneration in the animal kingdom are brought together in one organism,” said Dr. Kusumi.

Much remains to be learned about the drivers and biological mechanisms that enable Elysia marginata and Elysia atroviridis to sever their own heads and regenerate their bodies, but Ms. Mitoh and other scientists believe that our understanding of this strange phenomenon will one day improve Advances could lead to regenerative medicine and other areas.

Until the secrets of solar powered snails are revealed, Ms. Mitoh will continue to spend her days watching her beloved sea snails behead themselves. It’s gruesome work, but someone has to do it.