Meet the Sea Slugs That Chop Off Their Heads and Then Grow New Bodies

Since the days of BBC’s original Planet Earth, nature documentaries have brilliantly revealed the bizarreness of life. But, the behavior of a sea slug observed in a Japanese lab might be the one to steal the prize and leave you speechless.

As remarkable as dancing birds of paradise, as strange as bioluminescent sea life, as startling as young goslings diving off a cliff—nothing will make your jaw drop like this video of a sea slug decapitating itself before cruising around as if nothing had happened.

Researcher Sayaka Mitoh discovered that among her university’s extensive collection of sacoglossan sea slugs—the largest in the world—one of the elysia marginata had mysteriously had its head separated from its body.

Oddly, its head was moving around on its own, seemingly without issue. Odder still, within hours it began eating—despite the fact that its kidneys, heart, sexual organs, and digestive equipment, were all absent.

Then, oddest of all, within weeks it had regenerated an entirely new body.

The findings of this extraordinary regenerative ability have been described by Ms. Mitoh in a paper titled Extreme autotomy and whole-body regeneration in photosynthetic sea slugs.

Autotomy is the scientific term for self-amputation, a not uncommon trait found most commonly in lizards such as newts and geckos, and also crustaceans like crabs and lobsters. Purposely dissolving the bonds between a lizard’s tail or a crab’s leg allows them to escape predators who manage to grab them by the appendage.

However, autotomy of the head, the ditching of the entire body, has never been observed in the animal kingdom.

Autotomy in sea slugs: by the numbers

Five of 15 laboratory-bred individuals of E. marginata, and one wild caught individual, autotomized their head at some point during their lifetime. One even did so twice.

In all cases, all the major organs were left behind. In most cases the head moved immediately, began feeding on algae within a few hours, and the neck wound healed completely within one day.

Young individuals regenerated their entire body in an average of 20 days, while older individuals never regenerated and eventually died. Their discarded bodies reacted to tactile stimuli immediately, and continued to do so for months in some cases. Eventually though they would decompose, though the heart would continue to beat until moments before the highest stage of decomposition.

Ms. Mitoh recorded a video of the entire process here, featuring groovy music to make it less disturbing.

But… why?

Two theories categorized the study of this remarkable phenomenon. The first was the constant co-factor of internal parasites present in the body at the time of autotomy.

The purpose behind the ditching of the body is presumed to be a way to prevent parasites from destroying the organism completely, as in every case that members of another species E. atroviridis, autotomized their bodies, they were infected with internal parasites.

It’s not a perfect hypothesis, as E. marginata never had any parasites in the cases when it autotomized. It’s unlikely, Ms. Mitoh states in her paper, to be related to predation, since “adult sacoglossans generally have few predators due to their cryptic coloration andani presence of toxic chemicals incorporated from their food”.

“Our experiments also showed that autotomy of E. marginata took several hours, which is not effective to avoid predation, and that imitated predator attacks did not induce autotomy,” writes Mithoh.

As for how they survive and regenerate so quickly, the answer requires expanding your idea of what an animal is. The digestive gland in the slugs is actually a single organ spread across the majority of the surface of their bodies, including over their heads, which is lined by cells that actually commit theft of algal chloroplasts—cells that react to light.

The theft of the plant cells lead to the creation of a new kind of cell known as a kleptoplast, that allows them, once decapitated, to continue living on by generating energy like plants through photosynthesis, even when they have no heart, and there’s no physical way for them to digest food.

The discovery must be one of the most striking sets of evolutionary equipment ever observed, and for Ms. Mitoh, her job watching slugs decapitate themselves, strangely represents the frontiers of biological science.