Worms that can regrow complete head found

Washington: Scientists have found at least four species of marine ribbon worms that have independently evolved the ability to regrow a head, including brain, after amputation.

Regeneration of amputated body parts is uncommon but does exist throughout the animal world, said researchers at the University of Maryland (UMD) in the US. Salamanders, spiders and sea stars can regrow appendages while a species of ribbon worm can regenerate an entire individual from just a small sliver of tissue, they said. However, regenerative abilities were broadly assumed to be an ancient trait that some species managed to hold on to while most others lost through evolution.

The study, published in the journal Proceedings of the Royal Society B, turns that assumption on its head. In a survey of 35 species of marine ribbon worms, the researchers found that the ability to regenerate an entire head, including a brain, evolved relatively recently in four different species. "This means that when we compare animal groups we cannot assume that similarities in their ability to regenerate are old and reflect shared ancestry," said Alexandra Bely, an associate professor at UMD. All animals have some degree of regenerative ability. Even humans re-grow damaged skin over a wound. However, animal lineages that diverged very early in evolutionary history -- such as sponges, hydroids and ctenophores -- are often able to regrow entire individuals from even small amputated parts.

As animals evolved greater complexity, regenerative abilities have become less dramatic and common, researchers said. The research presents the clearest documentation of animals gaining regenerative abilities and could shed light on the characteristics necessary for the trait to evolve, they said. The researchers collected ribbon worms along coasts of the US, Argentina, Spain and New Zealand from 2012 to 2014. They performed regeneration experiments on 22 species, bisecting them front to back and observing their ability to regenerate. They also obtained information on 13 other marine ribbon worm species from previous studies.

All of the species were able to restore themselves to complete individuals by re-growing back ends. Only eight species were able to regrow their heads and restore an entire individual from just the back portion of the body. Four of these were known from previous studies and four were new, according to the study. More surprising than the number of ribbon worms that could re-grow heads was that the majority of them could not.

"The ancestor of this group of worms is inferred to have been unable to regenerate a head, but four separate groups subsequently evolved the ability to do so," Bely said. "One of these origins is inferred to have occurred just 10 to 15 million years ago," he said. In evolutionary terms, that is recent history given that regenerative abilities are thought to have first evolved before the Cambrian Period more than 500 million years ago.