This sea slug can lose its head and regenerate a new body in three weeks.

At least two species of sacoglossal sea slugs are capable of severing their own heads from their bodies and then developing an entirely new body, which includes a heart and other internal organs. The authors of a new study published in the journal Current Biology postulate that the secret to the survival of decapitated slugs could lie in the algae that make up the majority of their diet.

It is a type of self-amputation known in biological circles as “autotomy”, and many species exhibit some form of the phenomenon, especially lizards and salamanders, which shed their tails (caudal autotomy) to evade predators (the tails generally revert to grow) . Sea cucumbers can respond to stressful situations by expelling their internal organs, which then regenerate. Starfish can shed their arms; sometimes those arms turn into new starfish. It is much rarer in mammals, but there are two species of African spiny mice that can shed their skin to escape the grasp of a predator, regenerating all damaged tissue (including hair follicles, sweat glands, fur, and cartilage. ).

Other species of sea slugs, in addition to those used in this last study, will respond to being manipulated by dropping their “mantle skirt” (which forms a large part of the slug’s dorsal surface. But the ability to separate the head from all the body) —and not just surviving, but regenerating — is an extreme form of autotomy that has not been observed until now.

Co-author Sayaka Mitoh is a graduate student at Nara Women’s University in Japan and works in Yoichi Yusa’s lab. Researchers breed sea slugs and study them throughout their lab-centered lives. One day a few years ago, Mitoh was observing sea slugs and noticed that one of the species Elysia marginata“I had lost my mind.” He was surprised to see that the head was still moving in the tank, feeding on algae as if nothing was wrong. Even more surprising was that the wound appeared self-inflicted: the slug had removed its own head, apparently after dissolving the tissue around its neck.

“We were surprised to see that the head was moving right after the autotomy,” Mitoh said. “We thought he would die soon without a heart and other important organs, but we were again surprised to find that he regenerated the whole body.” And it only took three weeks to do it.

Fascinated, she and Yusa decided to investigate further. They selected adult specimens from Elysia marginata that had been cultured in the laboratory, descendants of sea slugs collected off the Japanese coast of Domori in October 2018. They also selected subadult specimens of related species, Elysia atroviridis, collected from April to July 2019 on the island of Mukaishima. About 33 percent of the Elysia marginata specimens lost their minds during their lifetime, and one even did so twice after regrowing his body the first time.

Heads continued to move and function despite lacking a body (or kidneys, or heart, or intestines, or most reproductive organs), regularly feeding on algae. The neck wound closed in one day and all the decapitated specimens recovered their bodies in about 20 days. The severed bodies also continued to move and react to touch for several days or even months. But they paled and shrunk over time and eventually broke down. “The heartbeat was visible just before the body decomposed,” the authors wrote.

Mitoh and Yusa reported similar findings for the Elysia atroviridis specimens. Many of them harbored a copepod parasite (Arthur sp.), and of those that changed their heads, two regenerated their bodies in one week. Another 48 percent of those specimens lost part of their bodies over time and eventually shed the parasites. About a third of those who regrown their bodies; the rest died. The specimens without the parasite did not shed their bodies at all.

As a complementary experiment, for six of the Elysia marinata In the samples, Mitoh and Yusa induced autotomy by gently tying a nylon rope in a groove in the neck, the site they suspected was the “plane of rupture.” The tissue began to noticeably decompose at the site within 16 to 18 hours, and all but one of the specimens had their heads cut off after 18 to 22 hours. (The final specimen finally shed its head after nine days, perhaps because the rope had slipped out of the breaking plane.) They also tried to mimic a predator attack by pinching the heads of another sample of sea slugs, but this did not induce autotomy.

“The heartbeat was visible just before the body decomposed.”

Unlike autotomy in other species, these two types of sea slugs do not appear to molt their bodies in response to predatory threats. The authors note that adult sea slugs don’t have as many natural predators, and given the time it took for specimens to separate their heads from their bodies, escaping a predator would not be a useful strategy anyway.

Based on the findings of their experiments, the authors suggest that the real function of this behavior is to expel parasites, at least in Elysia atroviridis. A similar function has also been observed in earthworms infected with a protozoan parasite. “Alternatively, but not mutually exclusive to this idea, the autotomy of sacoglossans may work to escape entanglement in algae or to remove accumulated toxic chemicals,” the authors wrote.

Regarding whole body regeneration and the ability to continue eating algae without a functional digestive system, Mitoh and Yusa note that these two species have digestive glands that cover most of the body surface, including the head. This gland is lined with special cells capable of maintaining the chloroplasts (photosynthesis centers) of the algae consumed by the severed head. The authors suggest that the energy for survival and regeneration is the result of internal photosynthesis (kleptoplasty), which keeps the heads of sea slugs alive even without the ability to digest food.

However, “the reason why the head can survive without the heart and other organs is unclear,” the authors wrote, and is therefore the subject of further study. “Since the molt body is usually active for months, we may be able to study the mechanism and functions of kleptoplasty using living organs, tissues, or even cells,” Mitoh said. “Those studies are almost completely lacking, as most studies on sacoglossan kleptoplasty are done at the genetic or individual level.”

Sayaka Mitoh listing image