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A sea slug’s detached head can crawl around and grow a whole new body

Losing your body from the neck down can be just another one of life’s annoying, but temporary, setbacks — at least for two kinds of rippling, green-tinged sea slugs.

Heads of young Elysia cf. marginata sea slugs can pull themselves free from their bodies and just keep crawling around while growing a new body, report ecologists at Nara Women’s University in Japan. Within a few hours, some separated heads start nibbling on algae again, Sayaka Mitoh and Yoichi Yusa report March 8 in Current Biology. And within about 20 days, a third of the young sea slugs they watched had grown their bodies back, heart and all.

That’s the first time anyone has reported such dramatic “whole-body” regeneration in any sea slug “as far as we know,” Yusa says.

After disconnecting its head from its body (far left, arrow points to the heart), a process that can take several hours, an Elysia cf. marginata sea slug gradually grows a whole new body.  By day 7 (second from left), the heart has begun to regenerate. The rest of the body follows suit (day 14, second from right) until regeneration is complete (day 22, far right).S. Mitoh and Y. Yusa/Current Biology 2021
sea slug body regeneration on day 0, day 7, day 14 and day 22
After disconnecting its head from its body (far left, arrow points to the heart), a process that can take several hours, an Elysia cf. marginata sea slug gradually grows a whole new body.  By day 7 (second from left), the heart has begun to regenerate. The rest of the body follows suit (day 14, second from right) until regeneration is complete (day 22, far right).S. Mitoh and Y. Yusa/Current Biology 2021

Other creatures can regenerate, too. In one sense, planarians, the little cross-eyed flatworms that biology students mince up to study regeneration, “are better,” Yusa says. They can regenerate the whole body from multiple cut pieces. Yet their body plan is simpler, and they don’t have hearts.

A group of tubelike sea squirts called ascidians might be considered the most complex of whole-body regenerators. Yet ascidians also lack a heart. And vertebrate regenerators, like salamanders regrowing a tail left in the jaws of a predator, don’t regrow a whole body from a severed head.

Mitoh first noticed the sea slugs’ extreme regeneration by chance in some Elysia slugs in the lab. “We were really surprised to see the head crawling,” Yusa says.

Just which Elysia species can turn into crawling heads isn’t clear yet. Marine biologist Sónia Cruz at the University of Aveiro in Portugal works with two other Elysia species but hasn’t seen anything like it. She does caution, though, that she hasn’t done systematic tests.

Three days after the head of an Elysia cf. marginata sea slug separated from its body, the head remains active, creeping around. The body, while also still alive, is more sluggish. It eventually will die, while the head will grow a new body, heart and all.

The head of a sea slug can take several hours to rip itself loose from its body, so Mitoh and Yusa doubt that de-heading helps when predators attack. Instead, a detachable body could give the sea slug a drastic, but effective, way of dealing with parasites. In a batch of wild-caught E. atroviridis sea slugs, the few that ditched their bodies were parasitized by copepods. So were those that just lost pieces of their body, some of which also regrew.

On close inspection, the researchers found that sea slugs have a slight groove looped on the back of the head region that seems to work as a break-here zone. The bodies left behind can still move on their own for days or even months. An abandoned body, however, doesn’t regrow its head. The leaf-shaped remnant instead turns pale and weak and eventually dies.

What might help Elysia slugs manage such extreme regrowth is their ability to steal the tiny green sunlight-trapping energy factories called chloroplasts from plants, the researchers muse. Very young slugs don’t have any chloroplasts. “They need to pierce the cell walls of sea algae and sip the contents,” Yusa says. The grazing slugs can keep the chloroplasts alive for weeks or months.

Biologists debate what the stolen chloroplasts do for their kidnappers besides provide a pretty, green tinge.  Yusa, however, has linked the looted chloroplasts to such consequential matters as improved reproduction. If chloroplasts are more than cosmetic, maybe that energy boost is just what a severed head needs to get (more than) ahead.

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