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This sea slug cut off its own head—and lived to tell the tale

It was “like a horror movie.” That’s how ecologist Sayaka Mitoh describes the scene at her lab at Nara Women’s University in Japan one day when she was studying sea slugs. One of the slimy aquatic critters had lost its body, and its head was crawling around the bottom of a tank. “We thought that it would die soon without a heart and other important organs,” she says.

But it didn’t. Within days, the slug began to regenerate its entire body. By the end of the month, it was back to normal.

The remarkable recovery, reported today in Current Biology, has been observed in simpler life forms such as hydra and flatworms—but it’s practically unheard of in complex animals like sea slugs. “It underscores the fact that still in the 21st century, we truly do not know what is possible in biology,” says Alejandro Sánchez Alvarado, a molecular biologist at the Stowers Institute for Medical Research.

After her initial observation, Mitoh and her colleagues took a closer look at two species of sea slugs: Elysia marginata, grown in the lab—and the first the team observed ditching its body—and E. atroviridis, collected from the wild. Over the course of the study, five of 15 E. marginata decapitated themselves, a behavior known as autotomy. The neck wound usually closed within 1 day, and the heads, especially in younger specimens, began to feed on algae within hours (as seen in the video above). Twenty days later, an entirely new body had regrown, the team reports. (The discarded bodies never regrew heads.)

In the E. atroviridis, three of 82 autotomized, and two of the three eventually grew new bodies. All of these animals were infected with small crustaceans known as copepods. In another group of 64 E. atroviridis without parasites, none self-decapitated, leading the researchers to hypothesize that animals cast off their bodies as a means to get rid of parasites.

Another possibility is that the slugs autotomized to escape predators. But when the researchers tried to mimic an enemy’s attack by pinching and cutting the creatures, none cast off their bodies. And the process itself takes several hours, which the scientists say would make it ineffective as means of escape.

How the slugs survive without a heart and other vital organs for nearly 1 month remains a mystery. Mitoh and her colleagues suspect it may be tied to their ability to survive using the photosynthetic algae in their diet while other energy sources are unavailable.

The find is “another example of how biology can come up with clever solutions to challenges that threaten survival,” says James Godwin, who studies animal regeneration at the Jackson Laboratory. Although he cautions that regeneration of this magnitude may never be possible in vertebrates, like humans, Godwin says the slugs could provide a valuable testing ground for understanding the genetics behind remaking whole body segments.