These Sea Slugs Can Lose Their Heads Then Grow New Bodies

It only takes about three weeks.

head and body of a sea slug
Head and body of a sea slug.

Sayaka Mitoh

Sea stars can grow new arms. Crayfish can grow new claws. There are interesting creatures that can grow back body parts if accidents happen.

That's child's play after what researchers saw a sacoglossan sea slug do. The slimy mollusk managed to lose its head and then began crawling around. Eventually, it grew an entirely new body.

"We were surprised to see the head moving just after autotomy," said Sayaka Mitoh of Nara Women's University in Japan. "We thought that it would die soon without a heart and other important organs, but we were surprised again to find that it regenerated the whole body."

Mitoh, a PHD candidate, raises sea slugs from eggs to adults in order to study their life cycle. One day she happened to see a slug head moving around without its body.

Although it was detached from the body and heart, the head moved on its own around the bottom of the tank. Within a few days, the slug began to regrow its body. The regeneration was complete after about three weeks.

Mitoh and her colleagues reported the discovery in the journal Current Biology.

After the initial discovery, the researchers observed what they called "extreme autotomy" (self-amputation) and whole-body regeneration in two species of sapoglossan sea slugs.

They found that for younger slugs, the detached heads began eating algae within hours. The wound typically closed at the back of the head within days. They started regrowing a heart within a week and the regeneration of the entire body was finished after around three weeks.

Older slugs weren't as fortunate. Often the heads didn't feed, so they died in about 10 days.

Young or old, the headless bodies never generated a new head. But they did move around and respond to being touched for several days and sometimes months after losing their heads, the researchers said.

Understanding Why and How

The researchers aren't sure why the sea slugs shed their heads or how they're able to grow new bodies.

They may have parasites that can hurt reproduction so they are removing their heads in order to also remove the parasites. But the researchers aren't sure what prompts them to know when to cast off the body when they do it.

And there's the question of how.

Mitoh says they believe there must be cells located in the neck that are similar to stem cells. These are able to regenerate a new body.

How the heads can survive without a heart and other vital organs is another puzzler. The researchers say these sea slugs fuel their bodies by photosynthesis. They rely on chloroplasts from algae in their own bodies when other food isn't available, a process known as kleptoplasty.

They think this might help them survive long enough after autotomy to regenerate a body.

"As the shed body is often active for months, we may be able to study the mechanism and functions of kleptoplasty using living organs, tissues, or even cells," Mitoh said. "Such studies are almost completely lacking, as most studies on kleptoplasty in sacoglossans are done either at the genetic or individual levels."

View Article Sources
  1. "These Sea Slugs Sever their Own Heads and Regenerate Brand-New Bodies." Eurekalert!, 2021.

  2. Mitoh, Sayaka, and Yoichi Yusa. "Extreme Autotomy and Whole-Body Regeneration in Photosynthetic Sea Slugs." Current Biology, vol. 31, no. 5, 2021, pp. R233-R234, doi:10.1016/j.cub.2021.01.014