Previous: Why Are These Fish Walking?
Next: Horses Are Weirder Than You Thought



View count:170,653
Last sync:2024-06-02 16:45
Visit to get started learning STEM for free. The first 200 people will get 20% off their annual premium subscription and a 30-day free trial.

There are many creatures on the internet that do not actually exist, even though we would like them to. Like, would you believe there is an animal that can photosynthesize, decapitate itself, and is incredibly cute? They are the sacoglossan sea slugs.

Get your own fantastic sea slug shirt here:

Subscribe to the pin club here:
This month's pin is designed by Emily Alvarez. You can find out more about them and their work here:

You can cancel any time by emailing

Host: Hank Green (he/him)
Follow us on socials:
#BizarreBeasts #SeaSlugs #leafsheep

Thanks to Brilliant for supporting this video!

Because you watch Bizarre Beasts, Brilliant is offering you a 30 day free trial and 20% off an annual premium subscription at There are many creatures on the internet that do not actually exist, even though we would like them to, and even though people would like for you to think they exist. And then there are creatures on the internet that seem like they should not exist, but do, like the colugo, the Christmas Island coconut crab, and the leaf sheep.

This adorable little creature is a member of the group of sea slugs known as the sacoglossans, sometimes also called ‘sap-sucking sea slugs.’ And it, along with many of its relatives, are thieves… and that’s not even the weirdest thing about them. [♪♪ INTRO ♪♪] If you want to support this show, the Bizarre Beasts pin club is open for subscriptions again from today through January 15th! Sign up now and the first pin you will get will be one of these Bizarre Beasts. Sacoglossans are gastropods, which is the taxonomic class that the things we call slugs and snails belong to, whether they live on land or in the water.

Some sacoglossans lack shells and are more slug-like, while others have shells and are more snail-like. They’re also often confused with nudibranchs, because both groups tend to be colorful, but while all nudibranchs are sea slugs, not all sea slugs are nudibranchs. Sacoglossans live in the shallow parts of temperate and tropical oceans worldwide, though they’re more diverse in tropical waters, and they’re usually small and difficult to find.

And there are at least 300 species of sacoglossans, ranging from orange to bright green to striped, to these incredibly cute little yellow guys we couldn’t identify to the species level over on our sister channel Journey to the Microcosmos. The whole group is beautiful and fantastical and weird-looking. And how they get the energy they need to live is just as strange… for an animal.

I mentioned in the intro that these guys are sometimes called ‘sap-sucking sea slugs’ and that’s because most of them primarily feed on green algae. But they don’t eat the algae, really… Instead, they use a tongue-like structure covered in tiny tooth-like projections called a radula to rasp or bore a hole into the cells of the algae, and then they suck out their contents. And for some of these sea slugs, the goo inside of the cells of algae is enough to subsist on, they just digest it and everything it contains.

But for others, extracting a cell’s contents is only the beginning. Because, that goo contains chloroplasts, the organelles that are responsible for photosynthesis in plants and algae. And some sacoglossans can incorporate those stolen chloroplasts into their own tissues.

This process is called ‘kleptoplasty.’ There, the chloroplasts just seem to keep doing what they were doing for the algae: turning sunlight into energy that the sea slugs can use. Some sacoglossans can survive on this energy for a matter of days, while others can go for up to a year without eating, relying solely on their stolen chloroplasts. Like, imagine being able to functionally turn into a plant for a year.

And their ability to live off of the photosynthetic energy produced by their stolen chloroplasts might also be what allows some of them to do another incredibly bizarre thing. Two species in the genus Elysia can literally decapitate themselves and grow new bodies. They just, pinch off their bodies at a little groove in what passes for their necks.

This is a pretty extreme example of what’s called autotomy, when an animal can voluntarily shed part of its body. A more common example being like a lizard shedding its tail to avoid being eaten by a predator. But these sea slugs can shed their entire bodies, one individual in a study from 2021 even did it twice.

And it’s not like they have all of their organs packed into their heads and their bodies are just a weird fleshy accessory. The shed bodies contained the sacoglossan’s entire heart, along with most of their reproductive organs, kidney, and intestine. And yet, the heads stayed alive… somehow.

Or, at least, some of them did. The heads of three relatively young sea slugs started moving around and feeding on algae again within a couple of hours. And they started to regrow their hearts within a week and had finished regrowing their entire bodies within about 20 days!

The heads of older individuals didn’t regenerate though, so it’s clearly not a foolproof plan. And none of the shed bodies regrew their heads. But why?

Why do they do this and how do they survive? Well, losing their bodies doesn’t seem to be an anti-predator defense for these sea slugs. The process takes hours and when researchers tried to simulate a predator attacking one of these little guys, it didn’t trigger the body loss.

So, it seems more likely that it is an anti-parasite defense. In one of the species the researchers studied, all of the sea slugs that lost their bodies were infected by a parasitic copepod, a type of tiny crustacean. Their presence in the sea slugs can interfere with their reproductive success– and all of the sea slugs that got rid of their bodies also got rid of the parasites.

So, from an evolutionary point of view, it might make sense to just grow a whole new body if the old one can’t do one of the most basic things an animal has evolved to do. Losing their bodies might also help the sea slugs avoid getting tangled up in algae and get rid of any toxins they might accumulate from their food, but these explanations are not mutually exclusive. As for how the sea slugs survive without their bodies, the researchers suspect that those stolen chloroplasts could be playing a key role.

See, in this genus of sacoglossans, the digestive system isn’t just a single, simple tube. Instead, it’s a branching gland that extends over most of the surface of their bodies… including their heads. And the cells that line that gland are where the sea slug keeps its stolen chloroplasts.

So these sacoglossans basically have a backup power supply in their heads, the chloroplasts there can continue to photosynthesize and produce energy for the sea slug when it’s just a head. And that might be why they can survive until they can regenerate their bodies and the rest of their organs. In the grand scheme of what makes an animal weird to us, sacoglossans really tick a lot of boxes.

Some of them can be solar-powered, some of them can lose their bodies, and some of them can be extremely beautiful… but, collectively, they are always bizarre.

Sarah: Hi! Guess what?! If you like these weird little guys, we made shirts! You can get this month's pin art on a t-shirt for a limited time.

It's only available this month, January 2024. If tiny sea slugs can figure out solar power, I bet you can too, especially with help from this Brilliant course all about Solar Energy. This online course takes you all the way from photons to spectral selectivity and the workings of electrical grids.

And that’s just one of the many courses you can explore with Brilliant, the online learning platform with thousands of interactive lessons in science, computer science, and math. Brilliant lets you learn anywhere at any time and if you aren’t sure what course to take, Brilliant has a quiz you can take when you sign up to be matched with content that fits your skill level and interest. You can try it for free for 30 days at or by clicking the link in the description down below.

That link also gives you 20% off an annual premium Brilliant subscription. Thanks to Brilliant for supporting this episode of Bizarre Beasts! In addition to being chloroplast thieves, some sacoglossan sea slugs can also steal other useful stuff from the algae they feed on in the form of toxic compounds.

These sea slugs can concentrate or modify these chemical defenses from the algae to deter potential predators in a process is called kleptochemistry. [♪♪ OUTRO ♪♪]