It was an ordinary day in the Yoichi Yusa laboratory at Nara Women's University in Japan, where scientists examine the life cycle of sea slugs. PhD student Sayaka Mitoh was conducting her usual work at the lab when she spotted something truly bizarre: the head of one of the slugs, a member of the sacoglossan family, was moving around without its body, as if that was a totally normal thing to do.
While many species have evolved the ability to cut off and regenerate non-essential body parts, such as limbs or tails, it is very rare to see an animal do away with its entire body, complete with vital organs like the heart, intestines, and reproductive system.
But this extreme form of “autotomy,” or self-amputation, is exactly what Mitoh witnessed multiple times in slug species from the genus Elysia, according to a study published on Monday in Current Biology.
“We were really surprised to see the head moving just after autotomy,” said Mitoh and co-author Yoichi Yusa, an ecologist who heads the lab, in an email. “We thought that it would die soon without a heart and other important organs, but we were surprised again to find that it started to regenerate the body.”
“This is the first example of such extreme autotomy as far as we know,” the team added. “Some other animals such as planarians, starfish, and polychaetes have greater abilities of regeneration, but most of them do not have a heart (in polychaetes, blood vessels with muscular pumps are sometimes called the heart).”
The researchers observed a total of nine sea slugs spontaneously discard their bodies, one of which then regenerated its body only to self-decapitate a second time.
Here’s how it worked: the heads of younger individuals (less than one year old) moved around and began eating algae again within hours; days later, the wound where the head was severed had healed and new bodies, complete with vital organs, were grown back over the course of one to three weeks.
Older populations, meaning individuals that hatched 480 to 520 days before the autotomy event, did not survive the amputation. The heads of the elder slugs were not able to feed or grow back the body and died within 10 days.
In a macabre twist, the bodies did not grow back their heads, but reacted to tactile prompts, sometimes months after the decapitation.
“The bodies gradually shrank and became pale, apparently from losing chloroplasts, and eventually decomposed,” the team said in the study. “The beating of the heart was visible just before the body decomposed.”
To investigate this ungodly process further, Mitoh and Yusa lightly tied strings around the necks of six sea slugs, which prompted all of them to toss out their bodies and grow new ones.
The new study raises many questions about this weird behavior, including how the slug heads survive for so long without vital organs. One possible explanation is that these animals can use chloroplasts from the algae they eat to temporarily power their heads with photosynthesis until their bodies take over again once they are regrown.
“We imagine that they can live without a heart as they are small and the stolen chloroplasts can provide energy and oxygen for the head,” Mitoh and Yusa said. “But this needs further study.”
The team is also not sure what purpose this adaptation serves. Some animals swiftly amputate their limbs as a tactic to evade predators, but the sea slugs require several hours to remove their bodies, which would not be helpful in the thick of an encounter with a hunter. Plus, adult sacoglossans don’t attract many predators in the wild, and the lab animals did not amputate their bodies in response to simulated predator attacks from the researchers.
Ultimately, Mitoh and Yusa concluded that the behavior is likely motivated by parasitic infections in the body. All of the sea slugs that performed this extreme form of autotomy were infected with some form of parasite, which they successfully removed after the regrowth of their bodies. In contrast, none of the parasite-free individuals were observed totally ditching their bodies. This motivation could explain why older slugs performed the extreme autotomy, even though it resulted in their deaths in every case.
“We think this autotomy requires a great cost (losing 80% of the total weight) and the older ones simply cannot stand the burden,” Mitoh and Yusa said. “Then the autotomy may appear a ‘silly’ choice, but the old ones will die soon anyway and there may be a chance of surviving and regenerating a parasite-free body.”
The serendipitous discovery will require further investigation to shed light on its function, underlying dynamics, and its evolutionary origin, along with how widespread the behavior is among other sacoglossans.
“We want to study whether other species of sacoglossans have this ability, to study the evolutionary pattern and process of such extreme autotomy and regeneration,” Mitoh and Yusa said. “The function of the autotomy is also worth studying. Moreover, we will further explore the mechanisms underlying this phenomenon at the tissue and cellular levels.”
While researchers at Yoichi Yusa laboratory continue their work, the disembodied heads and twitching bodies will just have to remain nightmare fuel for the rest of us.