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Researchers in Japan have discovered two species of sea slug that can grow a whole new body complete with heart and other organs.

An individual creature was seen moving around without its body after cutting off its head, and one was even seen doing this twice.

This process of dropping off a body part called autotomy is common in the animal kingdom, for example, lizards losing their tails to escape predators.

However, this extreme form of it has never been seen before and was only spotted by chance, while scientists observed the animals for other studies.

Scientists found that the head moved on its own immediately after being separated from the body, and within days the wound at the back of the head had closed.

The report also found that the heads of relatively young slugs started to feed on algae within hours, according to the study published in Current Biology.

Sayaka Mitoh of Nara Women's University in Japan, said: "We were surprised to see the head moving just after autotomy.

"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."

The slugs started regeneration of the heart within a week, and within about three weeks the regrowth of a new body was complete.

The cast-off bodies did not regenerate a new head, but the headless bodies did move and react to being touched for several days or even months.

Researchers are not sure how the sea slugs manage to regenerate bodies, but they suspect there must be special cells at the cut end of the neck that are capable of regenerating the body.

They also say the reasons why they would do this are unclear, but one possibility is that it helps to remove internal parasites that slow down or affect their reproduction.