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These chopped off heads chomp on algae and grow new bodies5 min read

March 11, 2021 4 min read

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These chopped off heads chomp on algae and grow new bodies5 min read

Reading Time: 4 minutes

You can only imagine Sakaya Mitoh’s surprise when she returned to her lab to find that one of her sea slugs was decapitated. There lay its lifeless body, but also, surprisingly, its very alive and active head! What was going on?

Well, it turned out that the sea slug had just brutally mutilated itself. No big deal. This awesomely gruesome act was, in fact, an act of self-preservation. So, what happened and why are bodiless slugs going to haunt your dreams? Let’s find out.

Autotomy

“We were surprised to see the [sea slugs] head moving just after autotomy.”

Sakaya Mitoh

Regenerating body parts -replacing damaged or lost body parts with an identical replacement- may seem like something out of a science fiction movie. However, it is a pretty run-of-the-mill process for many creatures. Even the house lizards that you’ve seen lurking on your walls can regenerate their tails when they get cut off. The act of regrowing severed body parts is known as autotomy.

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A gecko grows back its tail. Image: Youtube

But these two species of sap-sucking sea-slugs take autotomy to a whole new level. These Sacoglossans, as they are known, lose almost everything, including their heart and other vital organs! Other creatures can indeed lose and regrow life-sustaining organs as well, but never has a severed head survived for weeks with over 80% of their body missing! 

Ciao Parasite Ciao

So, why do they do it at all? Well, Mitoh and her colleagues believe that it was to get rid of the parasites that commonly prey on these sea slugs bodies. Most creatures practice autotomy to escape predators or survive after a predator has attacked them. However, if you were to just cut off a sea slugs limb, much like a predator would, it won’t grow back. 

Why? Well, because their autotomy seems to have evolved solely for the parasites that infest them. Their necks have a specific severance point that has been appointed for the gory business of decapitating and bidding adieu to both their bodies and the entire parasite that regularly infests them.

The head of Elysia cf. marginata just after autotomy.
The head of the sea slug just after decapitation. Image: Scientific American

This groove on their necks is teeming with stem-cells. Stem-Cells are a type of primitive cells that can reproduce to create any kind of cell under the sun. They can generate everything from heart tissue to skin cells. That’s how these stem cells, which are present at the line of decapitation, quickly healed the slugs initial wound. They simply formed skin cells and performed some patch work. Then, voila! Within three weeks, they had regenerated the slug’s entire body.

Owliver’s Obscure Observation: Humans have very few and hard to reach stem cells. However, they can be found in embryos and our bone marrow. Scientists are now using stem cells to regenerate organs for patients with their own DNA!


Infographic: How cloning produces stem cells that can be used to create organs or other body parts as needed.

But how did the head survive?

The head of the sea-slug roams around three days after decapitation. Image: Scientific American

But there is one mind-boggling question that is yet to be answered in full? How in the name of God was it possible for the sea slug heads to survive for almost three weeks with no heart or digestive system. One potential solution is so brilliant, it’ll knock your socks off. You see, Mitoh mentioned that once the heads were severed, they were very active. Actively eating algae. It seems that these creatures may have been using the photosynthesis process of the algae as their own digestive system and energy resource. This is because the sea slugs heads have the ability to line themselves with chloroplast cells. They are the very cells that are responsible for photosynthesis and energy production in Algae. So, lined in the slugs heads was a borrowed digestive system that kept them bodiless and around for three odd weeks. Mind-Blowing!

What’s left to say other than:

“Move over humans, silence! until you learn to pop your brains off your bodies.” 

The Saccoglossan sea-slug whose head can regenerate its body, first communicated in slug tongue

Sayaka Mitoh and Yoichi Yusa at Nara Women’s University in Japan have published their research on sea slugs in the journal Current Biology. They were the first people to identify this fascinating behaviour in sea slugs, and then even artificially influenced slugs to perform autotomy. They did that by tying a nylon string around the sea slugs’ necks at the line of severance. Watch this video to observe the sea slugs entire decapitation and regeneration process.

With Excerpts From: The Guardian, Scientific American, Sciencemag, Nature, New York Times, Current Biology, Livescience

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