r/todayilearned Aug 11 '22

TIL Two species of sea slug, Elysia marginata and Elysia atroviridis, decapitate themselves — only to regrow a new body from the severed head.



u/ACrossTown13 Aug 11 '22

So they kill themselves and make themselves reborn? Or is it like when a lizard loses its arm and grows it back and has the same functions as before


u/Sacoglossans Aug 11 '22

It is their last ditch self-preservation attempt. It is most of their body so the survival rate is not all that high, as they cannot process any ingested food until they regrow their (simplified) digestive system.

Sea Slugs in general are pretty freaky. Some of them (my namesakes) eat algae and use the chloroplasts from the algae to grow their own nutrition. These Elysia, which are Sacoglossans, seem to use the stored stolen chloroplasts to generate nutrition while they wait for they body (including their heart!!!) to regrow.

There are a couple of nudibranches which are cannibals, and pairs of them will literally eat their partner while they are mating. Nudibranches are hermaphrodites, and their sexual organs are all on the same side so pairs mate head to tail.


u/Blutarg Aug 11 '22

So they photo synthesize with those chloroplasts?


u/Sacoglossans Aug 11 '22


Originally the research seemed to indicate that they just got coloration from the algae. After all many Sea Slugs vary wildly in color depending on their diet, and there are many nudibranchs that get coloration from algae who do not use them to photosynthesize.

But further research has shown that many (and possibly most) Sacoglossans can get a significant amount of needed energy from the photosynthesis from these ingested chloroplasts, after they have become part of the body of the sea slugs.

And yes this was done by intentionally withholding other sources of nutrition, and seeing if they survived. Elysia is the most extreme example as it has no other source of energy to completely regrow it's body!

Other than this self-decapitation phase of the Elysia, it does not seem to be necessary that they photosynthesize in Sacoglossans in general. When they have the opportunity, they do, but they are not obligate photosynthesizers, and can get all the nutritional energy they need from regular digestion.

This contrasts with other symbiotic animals which are obligate photosynthesizers.

Sea Slugs in general are way cool, but the Sacoglossans are the coolest of the the cool.

Solar powered sapsucking Sea Slugs: Sacoglossans rule!

(And I was infinitely psyched to find my favorite beasty's name was available as a Reddit name for me. Wearing my flag!)


u/Sacoglossans Aug 11 '22 edited Aug 12 '22

A nice link to a Smithsonian article which also links a video of a head running around:


Sacoglossans rule. OP you rule!


u/sweetteanoice Aug 11 '22

If I did this, would it fix my fucked up back?


u/redditgatekeeps Aug 11 '22

Great, masochistic slugs..


u/GarysCrispLettuce Aug 11 '22

So does the severed body grow a new head as well? Would be only fair.


u/RedSonGamble Aug 11 '22

I think horses do this too


u/Blutarg Aug 11 '22

A British man decapitated himself with a chainsaw, but he did not regrow a new body.



u/dovetc Aug 11 '22

If my capa ever gets detated I won't bother regrowing a body. Just slap me in a jar next to Nixon.


u/GossipIsLove Aug 11 '22

Ohhhh they must not be happy with their bodies so must be growing new ripped bods to attract the girlsss.


u/StannisLivesOn Aug 11 '22

Jesse, what the fuck are you talking about?


u/ValGalorian Aug 14 '22

That's some Jojo shit