The severed heads of at least two species of sea slugs can move, eat and possibly even eliminate waste for one to three weeks: that's how long it takes for their bodies, including the heart, to grow back after being detached from the neck.

Headless bodies also live, only for a few months, with a beating heart until the flesh begins to decompose, says Sayaka Mitoh of Nara Women's University in Japan. However, the heads do not grow back on the bodies.

"The head has brains and teeth, or radula, that can be irreplaceable," she says.

Discovery made about sea slugs

Mitoh and his supervisor, Yoichi Yusa, were creating a kind of sacoglossan sea slug ( Elysia cf. marginata ) to study the photosynthetic abilities of slugs when they discovered a severed head alive in their lab.

Sea slugs (Elysia cf. marginata) can regenerate from the head.

Intrigued, the researchers examined their slugs and noticed that they all had a groove around their necks. So they tied a thin rope around the necks of six lab-grown slugs in this groove and noticed that they all cut their own heads off within a day.

When the body is cut open and intact, the head turns green with chloroplasts after feeding on algae, Mitoh says. It is believed that the digestive glands of sea slugs "are distributed over the entire surface of the body, including the head," according to the researcher. That would explain why the head survives.

Meanwhile, the team observed 160 sacoglossans ( Elysia atroviridis ) bred in the laboratory and in the wild on a daily basis until their natural death, on average for just under two years.

In conclusion, 5 of the 15 lab-created sea slugs and 3 of the 145 wild slugs cut off their own heads, while 39 wild slugs amputated smaller body parts, such as the tail or foot-like appendages and then were able to regenerate.