Heads will roll

These sea creatures use a zombie-like trick to grow back their bodies

“We were very surprised to see the head moving.”

It’s like something out of a B-grade horror movie.

A severed head separates from its body. An unsuspecting scientist assumes a tragic fate, until the head twitches, moves, and regrows its body. Its Hellraiser meets Night of the Living Dead.

However, this is no mere Hollywood spectacle. Instead, it’s the brutal, real-life world of sea slugs, according to a study published Monday in the journal Current Biology.

What's new — Scientists from Nara Women's University in Japan discovered two types of sea slugsElysia cf. marginata and Elysia atroviridis — are capable of regenerating themselves after voluntarily shedding their original body.

The researchers define the process of shedding one's own body part as autotomy. They refer to this study's specific finding — including the severing of the sea slug's head and regrowth of its body — as "extreme autonomy" in their research. The process can take several hours.

Co-author Yoichi Yusa describes the astonishment he felt seeing the sea slugs' severed heads “crawling like [in] horror movies."

"We were very surprised to see the head moving and could not believe it,” Yusa tells Inverse.

Watch a video of the head moving on its own below:

A video showing the severed sea slug's head moving on its own.Mitoh et al.

How they did it — The sea slugs have a groove in their neck, which the scientists call a "breakage plane." This is where the slug can detach its head from its body.

Scientists gently tied a fine nylon string around the breakage plane of six of the Elysia cf. marginata sea slugs. Of those sea slugs, five autotomized — removed their heads from their bodies — within a day. One of the slugs studied even autotomized twice.

Why exactly this triggers separation is not known, but it’s possible the string experiment replicates a cause for extreme autonomy sea slugs might encounter in nature — like becoming tangled up in algae.

Digging into the details — The slugs shed approximately 80 to 85 percent of their body weight over the course of several hours. The shredded body parts contain the kidney, intestine, reproductive organs, and most importantly, the heart.

The study authors write: "The head, separated from the heart and body, moved autonomously, even immediately after autotomy."

Here's a simple timeline of what happened after the sea slug severed its own head:

  • Within a day, the head wound closed
  • After the head wound closed, young sea slug heads fed on algae within hours
  • Within 7 days, the heart began to regenerate
  • Within 20 days, a whole new body had been regenerated
  • Finally, the original body decomposed, and its heart stopped beating

However, not all of the severed slug heads thrived. The heads of older sea slugs, in particular, had a hard time feeding on algae, often dying within ten days.

The study authors found similar autotomy in three out of 82 field specimens of Elysia atroviridis, which, unlike the other sea slug specimen, contained parasites.

The severed sea slug head and body.Mitoh et al.

Why it matters — Sea slugs are far from the only animals — or even invertebrates — to shed their body parts.

These body parts are usually minor, like tails. In some cases, animals like amphibians shed and regenerate more complex organs, such as the eyes and the heart ventricle.

But this study gives us a much deeper look at extreme autonomy — and how extreme it can go. The study team writes:

“However, autotomy in this study is remarkable in that animals with complex body plans can survive even if they lose the main body, including the heart, and subsequently regenerate the whole lost area.”

How are these two sea slugs able to regenerate body parts like a zombie villain from a horror movie? Photosynthesis, suggests the study authors.

The slugs incorporate chloroplasts —photosynthesis-conducting organelles in plants — from algae into their bodies in a process known as kleptoplasty. A digestive gland covering the slug's entire body — including its head – contains cells with chloroplasts that the slugs have consumed.

The scientists hypothesize that the nutrients absorbed during kleptoplasty enable sea slug heads to survive and regenerate their bodies.

A video explaining the autotomy process in sea slugs. Mitoh et al.

What's next — Alongside the gruesome appearance, the evolutionary mechanics behind autonomy still puzzle researchers — especially in these sap-sucking sea slugs, known as sacoglossans.

"The mechanism is currently unknown. But we think that there are stem cells for regeneration," Yusa says. The team plans on examining this “phenomenon at the tissue and cellular levels” in the future.

The study authors suggest three possible evolutionary explanations for autonomy:

  • To eliminate or remove parasites within the shredded body
  • To remove toxic chemicals from the body
  • To escape from a tangle of algae

But, ultimately, more extensive animal research will be necessary to fully understand the meaning behind the sea slug's severed head.

"We want to study whether other species of sacoglossans have this ability [in order] to study the evolutionary pattern and process of such extreme autotomy and regeneration," Yusa says.

Summary: Autotomy, the voluntary shedding of a body part, is common to distantly related animals such as arthropods, gastropods, asteroids, amphibians, and lizards. Autotomy is generally followed by regeneration of shed terminal body parts, such as appendages or tails. Here, we identify a new type of extreme autotomy in two species of sacoglossan sea slug (Mollusca: Gastropoda).
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