A few years ago, Sayaka Mitoh, a PhD student at Nara Women’s University in Japan, was studying the huge collection of sea slugs in her laboratory when she came across a gruesome sight. One of the captive-bred sea slugs in the laboratory, an Elysia marginata, had been beheaded.
When Mitoh peeked into his tank for a better look, he noticed something even more shocking: the creature’s severed head was moving around the tank, eating seaweed like it wasn’t unusual to be a disembodied snail.
Mitoh also saw signs that the sea slug had inflicted itself on itself – it was as if the sea slug had loosened the tissue around his neck and ripped off his head. Self-amputation, known as an autotomy, is not uncommon in the animal kingdom. The ability to detach from a part of the body such as the tail helps many animals prevent predators. However, no animal had ever been observed to shed its entire body.
“I was very surprised when the head moved,” said Mitoh, who studies the life cycle characteristics of sea slugs. He added that he hoped the snail would “die quickly without a heart or other vital organs”. But he not only lived on, he regenerated all of his lost body in three weeks.
This prompted Mitoh and his colleagues to conduct a series of experiments to find out how and why some sea slugs are guillotined. The results of their experiments, published Monday in Current Biology, show that Elysia marginata and a closely related species, Elysia atroviridis, are deliberately decapitated to encourage the growth of a new body. Although more research is needed, experts suggest that these marine slugs shed their bodies when infected with internal parasites.
Mitoh and his team have observed different groups of Elysia marginata and Elysia atroviridis in their lives. Not all sea slugs they controlled beheaded themselves, but many did; it has even been done twice. The bodies regenerated from the heads of both species, but the headless bodies remained without them. However, these decapitated bodies reacted to the stimuli for months before they decomposed.
The head wounds caused by sea slugs during the autotomy only took a day to heal. Organs like the heart took an average of a week to regenerate. For most sea snails, the regeneration process took less than three weeks.
“We’ve long known that sea slugs have regenerative abilities, but it really goes beyond what we thought,” said Terry Gosliner, senior curator of invertebrate zoology at the California Academy of Sciences.
Gosliner, who discovered more than a third of all known marine snail species, suspects that these marine snails’ impressive regenerative abilities are 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 them to at least partially stick to the sugars that chloroplasts produce through photosynthesis.
This ability, known as kleptoplasty, could allow these sea slugs to survive for long periods of time without their bodies.
For most animals, and even some sea slugs, it is believed that the autotomy is only used to prevent predators. However, the researchers found evidence that it can also be used to drive away internal parasites. According to the researchers, all of the Elysia atroviridis that split their heads had internal parasites. And by disposing of their infected bodies, they successfully expelled them and regenerated parasite-free forms. No parasites were found on any of the Elysia marginata.
Whatever the purpose, the regenerative ability of these sea slugs is “remarkable,” said Kenro Kusumi, an Arizona State University biologist who studies regeneration in reptiles. According to Kusumi, the marine slugs in question have the best animal characteristics capable of complex regeneration. For example, these sea slugs have a “fracture plane” along their neck that allows them to be shed cleanly. Many lizards, including lizards, share a similar plane of fracture near the base of their tail.
“It is very interesting to see how many regenerative properties in the animal kingdom are brought together in one organism,” said Kusumi.
Much remains to be known about the biological engines and mechanisms that enable Elysia marginata and Elysia atroviridis to cut off their own heads and regenerate their bodies, but 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, Mitoh will continue to spend his days watching his beloved sea snails behead themselves. It’s scary work, but someone has to do it.
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