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The Colourful World of Self-Decapitating Sea Slugs

A colourful sea slug. Photo by arhnue from Pixabay.

Scientists from Japan discovered that two species of sacoglossans sea slugs have the ability to self decapitate. Once it removes its head from its body, the body goes through the usual process of decomposing while the head wanders around, eating algae, perfectly fine. 

After a few days, the head starts the process of regrowing its body and all the complex structures it contains such as a heart, its digestive system and all its other organs. Within about 17 days, the sea slug’s head has a brand-new body. While losing body parts and regenerating them (a process known as autotomy) is nothing new in the animal world – lizards and starfish, for example, are well-known for it – being able to exist just as head and regrow an entire body is unheard of.

I spoke to sea slug expert and professor Dr. Angel Valdes to discover more about sea slugs and nudibranchs (a group of sea slug), their unusual behaviour and why some self-decapitate.

(Note: interview edited for clarity).

Sea slugs are often found in the ocean and can be very colourful. Photo by joakant from Pixabay.

Can you describe what a sea slug is and what’s the difference between sea slugs and nudibranchs?

“The term sea slug is generally used to describe any marine snail that either has a reduced shell, or has no shell at all. It’s a very general, informal term. Nudibranch is a very precise term that refers to a clade of sea slugs in which none have a shell when they’re adults. All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.”

What does a sea slug look like?

“Many of them are very colorful, but not all of them are. They’re extremely variable morphologically [the form and structure of an organism]. So there is no one single way to describe them. The general description is a marine snail either with a reduced shell or with no shell, but there are some exceptions to that.”

“Once you get invaded by the parasite, you just become a zombie to the parasite, right?”

– Dr. Angel Valdes

Where do sea slugs live?

“They’re found all over the world, in all oceans, everywhere from shallow waters to the deep sea. There’s a few species that are actually freshwater. They’re normally found either on algae or their prey, if they’re carnivorous, like sponges. You can find them on the rocks, while some of them live within grains of sand. Coral reefs contain the highest diversity of sea slugs. They’re found everywhere, even the greatest depths of the ocean. I think the deepest record, if my recollection is correct, is about 6,000 meters deep.”

Wow, the pressure must be immense.

“You know, pressure is not an issue for them because they don’t have any internal cavities and they’re full of fluid. So pressure is a problem for example, for people because we have an internal cavity, our lungs. So under great pressure, our bodies tend to compress, but water’s incompressible so if your body’s mainly made of flesh and water, as is the sea slug, then pressure is not an issue.”

Sea slugs live in all oceans and can be found in freshwater. Photo by arhnue from Pixabay

What do sea slugs eat?

“Some sea slugs are completely herbivorous. They feed on seaweed or other types of microscopic algae. And then there’s some that are carnivorous. The carnivorous ones tend to feed on sponges, jellyfish or other sea slugs. There are a few exceptions to that, such as the one that feeds on the fins of a kind of fish that makes burrows in the sand. 

“Another interesting aspect of sea slugs is some of them can retain the chloroplasts from the algae they eat and keep them alive in their own bodies. So they’re photosynthetic animals. They have animal cells that have chloroplasts inside. And as far as I know, that is unique to them.”

Sea slugs come in all shapes, sizes and colours. Photo by arhnue from Pixabay.

Is that how sea slugs are able to self-decapitate and exist as heads for a period of time?

“Probably because they already have chloroplasts that are providing them with energy, they’re able to survive without having a digestive system for a while. That’s my guess. That’s why they’re able to regenerate.”

Why do they self-decapitate?

“Possibly when the animal has parasites. Sea slugs can have copepods, which are crustaceans that live in their bodies. They often feed on their reproductive organs and once they have them, it’s impossible to get rid of them. Once you get invaded by the parasite, you just become a zombie to the parasite, right? You just live to feed the parasite. So if you’re able to shed your body and grow a new one, that is an ideal way to get rid of them.”

(To find out more about the research in Japan on these self-decapitating sea slugs, check out the video below)

Discover more about self-decapitating sea slugs. Video by Live Science from YouTube

Have you seen this happen?

“No. I’ve seen walking heads before, but I thought that, you know, the animal was about to die. I never thought of keeping that walking head alive for a few days. The Japanese researchers did that and they found that they actually regrow their bodies.”

So I guess the next time you see you’re walking heads…

“Oh, yeah. I’m going to keep them alive.”

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