Some sea slugs sever their own heads and grow new bodies

Scientists have discovered that some sea slugs can sever their own heads and regenerate an entirely new body that is complete with a heart and other organs. In a new study published by Cell Press, scientists could hardly believe what they were witnessing among two types of sacoglossan sea slugs.

To survive long enough for regeneration, the researchers propose that the sea slugs may capitalize on the photosynthetic ability of chloroplasts they incorporate in their diet.

“We were surprised to see the head moving just after autotomy,” said study lead author Sayaka Mitoh of Nara Women’s University. “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.”

Mitoh is a PhD candidate in the lab of Yoichi Yusa, where experts are raising sea slugs from eggs to study their life history traits. Just by coincidence, Mitoh saw something unexpected in the lab one day – the head of a sacoglossan was moving around without its body. 

The experts report that the head of the sea slug, completely separated from its heart and body, moved on its own immediately after the separation. Within a few days, the wound at the back of the head closed. 

The scientists found that the severed heads of young sea slugs started to feed on algae within hours, regenerated their hearts within a week, and completely regenerated their bodies in about three weeks. On the other hand, the heads of older sea slugs did not feed on algae and died in about ten days.

While the bodies did not regenerate a new head, they moved and reacted to being touched for several days or even months.

It is not yet clear how the sea slugs manage to pull off such an unexpected technique. According to Mitoh, the team suspects there must be stem-like cells at the cut end of the neck that are capable of regenerating the body. 

One possibility for why the sea slugs use this unusual strategy could be that it helps to remove internal parasites that inhibit their reproduction. Going forward, further investigation is needed to determine what cue may prompt the sea slugs to cast off their body.

One thing that makes sacoglossan sea slugs unique is that they incorporate chloroplasts from the algae they eat into their own bodies, a strategy which is known as kleptoplasty. As a result, the animals can fuel their bodies through photosynthesis. 

According to the study authors, this photosynthetic ability may help the sea slugs survive long enough to regenerate a body.

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

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer