Headless? No hindrance…

Move aside salamander, this sea slug is about to blow your regeneration powers out of the water.

Scientists from Nara Women’s University in Japan have discovered two new species of sacoglossan sea slug, Elysia cf. marginata and E. atroviridis, that are capable of regenerating a whole new body, including the heart and other internal organs. 

The slugs are also unique in that they incorporate chloroplasts, which carry out photosynthesis, into their own bodies from the algae they consume.

Reporting in the journal Current Biology, the researchers suggest the slug’s photosynthetic ability may provide them with additional energy, helping them regenerate their new bodies.

“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,” says Sayaka Mitoh, who is a PHD candidate.

The discovery itself was somewhat down to luck. The lab that Mitoh works in raises sea slugs, and one day she noticed something odd: a sacoglossan slug was moving around without a body. 

The researchers noted that the head moved of its own accord immediately after being separated from its body. After a couple of days the wound had healed. Regeneration of the heart had started within a week, and after around three weeks full body regeneration was complete.

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Head of Elysia cf. marginata just after autotomy. Credit: Sayaka Mitoh

Older individuals whose bodies were discarded were unable to regenerate them. Headless bodies did not regenerate new heads, but they did manage to move around and stay alive for days, and in some cases months.

“Normally sacoglossans die within one year,” says Yoichi Yusa, Mitoh’s supervising professor. “The old ones will die soon anyway, and we suspect that their cells have aged and lost the ability to differentiate.” 

Yusa and Mitoh aren’t 100% sure how the slugs pull it off, but they believe there may be some stem-like cells where the separation takes place that have body-regeneration capabilities.

“We think that some multipotent stem cells may be involved in the regeneration process,” says Yusa. “They may be at the neck where the regeneration occurs. But we need to study this from now on.” 

Why would the slugs need to do this? One guess is that it enables them to remove parasites that inhibit their reproduction.

“Other reasons such as predation might be involved,” says Yusa, who says the research team is also unsure what signals prompt the slugs to cast off the rest of their body.

These slugs were already noteworthy for their ability to integrate chloroplasts into their bodies, a technique known as kleptoplasty. They are able to utilise the chloroplasts to carry out photosynthesis, generating an alternate source of fuel for themselves. The researchers think this might help them produce additional energy after casting off their old bodies, thus helping to regenerate new ones.

“The ability of photosynthesis might be involved, especially soon after autotomy [self amputation] and when the food is not available,” says Yusa. “But soon the head starts eating, so the ability of photosynthesis is only one of the reasons that facilitate regeneration.”

The researchers are excited at the prospect of being able to study kleptoplasty in living organisms.

“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,” says Mitoh. “Such studies are almost completely lacking, as most studies on kleptoplasty in sacoglossans are done either at the genetic or individual levels.”

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