Self-Decapitating Sea Slugs


Image Credit: Sayaka Mitoh

Introduction


Have you ever heard of an animal that doesn’t end up dying after getting decapitated?


This may be a strange question to ask… but it turns out, getting beheaded doesn’t guarantee death for certain types of sea slugs— more specifically, the Elysia cf. marginata and Elysia atroviridis species.


Earlier this month, researchers published their findings on these sacoglossan sea slugs (otherwise known as sap-sucking sea slugs). They’ve found that they “shed” their body at the neck and can regrow their entire bodies within just a couple of weeks!


What’s up with these weird, yet incredible sea slugs?


Growing New Bodies


This bizarre biological process was first discovered when Sayaka Mitoh, a doctoral student at Nara Women's University in Japan, spotted the detached head of a Elysia cf. marginata circling its detached body in a tank at one of the university’s laboratories. However, instead of the slug dying like Mitoh expected, the wound on the back of its head quickly healed. Furthermore, it was replaced by the beginnings of an entirely new body.


"After a few days, the head started regenerating the body and I could see [the] beating of the heart. It was unbelievable," Mitoh says. About three weeks passed, and the slug had replaced the 80% of its body it had originally lost, including all of its vital organs. The new body was a perfect copy of the original.

Researchers still don’t know how the slugs regenerate their bodies from the neck down after autotomy (the term for self-amputation), but they believe stem cells, special undifferentiated cells that can turn into any type of cell in the body, play an important role.

A Hidden Superpower: Kleptoplasty


Mitoh doesn’t know how the slugs live without some of their vital organs, but shares “they can live without a heart probably because their heads are small" and can intake oxygen from their body's surface.

However, another question arises: the sea slugs’ regeneration takes a lot of energy, which you could imagine would be quite hard for their autonomous head to acquire. So how can they do it with just a head?

Thankfully, they have a special power: kleptoplasty, or the ability to steal chloroplasts from the algae they eat and use those photosynthesizing cell parts themselves. This gives sea slugs the ability to photosynthesize and produce the energy they need to start regenerating their bodies!


They rely on photosynthesis just after decapitation and when food is scarce. The stolen chloroplasts last only for a couple of days within the sea slugs though, so they most likely acquire food or some other energy source to complete the regeneration process.


Why Self-Decapitate?


You might be thinking these sea slugs are cool and all for being able to regenerate their bodies… but why would they decapitate themselves and get rid of their body in the first place?


Scientists don’t actually know for sure, but they have some theories. A leading one is that the slugs do it to remove parasites that have infested their original bodies and are preventing them from reproducing. Another is simply that it is a way to escape from predators by sacrificing their bodies and escaping as just a head.


However, researchers found that only the younger sea slugs are capable of autotomy and regeneration. When older slugs removed their heads, the heads survived for up to 10 days but never started eating and regenerating before dying. It may seem pointless for the older slugs to self-decapitate if it’s likely they won’t survive, but they may choose to do so because they’re aware they’d die anyway. Furthermore, there might be a chance of surviving and regenerating a healthy body.

Conclusion


Other sacoglossan sea slugs are known to be capable of regenerating some parts of their bodies, but the ability of the Elysia cf. marginata and Elysia atroviridis species to regenerate their entire bodies from the neck down, including vital organs, is on a whole new level.

Researchers intend to study whether other species of sacoglossans have this ability in order to study the evolutionary patterns and processes of such extreme autotomy and regeneration. In addition to evolution, they’ll further study the molecular and physiological underlying mechanisms that allow the slugs to regenerate their bodies.


Considering their ability to survive after self-decapitating, I certainly agree that the marvelously complex sea slug has spent just too long out of the biological spotlight.

 

About the Author: Isabel Zhang

Isabel is a senior in high school, and is interested in biology and engineering. In her free time, she loves to bake, sketch, and hang out with her family.

 

References

  1. Baker, H. (2021, March 08). This sea slug can chop off its head and grow an entire new body, twice. Retrieved from https://www.livescience.com/decapitated-sea-slugs-regrow-entire-body.html

  2. Marples, M. (2021, March 08). These sea slugs can self-decapitate and grow a new body. Retrieved from https://www.cnn.com/2021/03/08/world/sea-slug-decapitation-autotomy-scn/index.html

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