A biological phenomenon called autotomy is the voluntary shedding of a body part. It is common to distantly-related animals such as arthropods, gastropods, asteroids, amphibians, and lizards, and is generally followed by regeneration of shed terminal body parts, such as appendages or tails. In a new study published this week in the journal Current Biology, researchers in Japan identified a new type of extreme autotomy in two species of sacoglossan sea slugs: Elysia marginata and Elysia atroviridis. Surprisingly, the slugs shed the main body, including the whole heart, and regenerated a new body; in contrast, the shed body did not regenerate the head; the animals may use the photosynthetic ability of chloroplasts they incorporate from the algae in their diet to survive long enough for regeneration.
This image shows the head and the body of Elysia marginata, a day after autotomy; the shed body is much heavier (over 80% of the total weight) than the head. Image credit: Sayaka Mitoh.
“We were surprised to see the head moving just after autotomy,” said Sayaka Mitoh, a Ph.D. candidate at 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.”
In their lab, Mitoh and Nara Women’s University Professor Yoichi Yusa raise Elysia sea slugs from eggs to study their life history traits.
One day, they saw something unexpected: a sacoglossan slug moving around without its body. They even witnessed one individual doing this twice.
The slug’s head, separated from the heart and body, moved on its own immediately after the separation.
Within days, the wound at the back of the head closed. The heads of relatively young slugs started to feed on algae within hours.
They started regeneration of the heart within a week. Within about three weeks, regeneration was complete.
The heads of older individuals didn’t feed and died in about 10 days.
In either case, the cast-off bodies didn’t regenerate a new head. But the headless bodies did move and react to being touched for several days or even months.
“We aren’t sure how the sea slugs manage it,” Mitoh said.
“But we suspect there must be stem-like cells at the cut end of the neck that are capable of regenerating the body.”
“It’s also unclear why they would do this. One possibility is that it helps to remove internal parasites that inhibit their reproduction.”
“We also don’t know what immediate cue prompts them to cast off the rest of the body. These are areas for future study.”
Elysia marginata and Elysia atroviridis already were unique in that they incorporate chloroplasts from algae they eat into their own bodies, a habit known as kleptoplasty. It gives the animals an ability to fuel their bodies by photosynthesis.
“This ability might help them survive after autotomy long enough to regenerate a body,” the scientists said.
“These findings in sea slugs represent a new type of autotomy in which animals with complex body plans shed most of their body.”
Sayaka Mitoh & Yoichi Yusa. Extreme autotomy and whole-body regeneration in photosynthetic sea slugs. Current Biology, published online March 8, 2021; doi: 10.1016/j.cub.2021.01.014