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You have one butt, a shark has one butt, even a jellyfish has one butt/mouth, but deep inside sponges of Australia's Darwin Harbor there's a marine worm with BUTTS, hundreds and hundreds of butts. And sometimes those butts have minds of their own.

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Aguado, M.T., Ponz-Segrelles, G., Glasby, C.J. et al. Ramisyllis kingghidorahi n. sp., a new branching annelid from Japan. Org Divers Evol 22, 377–405 (2022).
Ponz-Segrelles, G., Glasby, C. J., Helm, C., Beckers, P., Hammel, J. U., Ribeiro, R. P., & Aguado, M. T. (2021). Integrative anatomical study of the branched annelid Ramisyllis multicaudata (Annelida, Syllidae). Journal of Morphology, 282(6), 900–916.
GLASBY, C.J., SCHROEDER, P.C. and AGUADO, M.T. (2012), Branching out: a remarkable new branching syllid (Annelida) living in a Petrosia sponge (Porifera: Demospongiae). Zoological Journal of the Linnean Society, 164: 481-497.
Aguado, M., Glasby, C., Schroeder, P. et al. The making of a branching annelid: an analysis of complete mitochondrial genome and ribosomal data of Ramisyllis multicaudata. Sci Rep 5, 12072 (2015).
We humans have just one butt.

Now, we may disagree about exactly which portions of our anatomy constitute “butt.” Is butt legs? Is butt torso?

Does ‘butt’ mean only the gluteal muscles or is it really just a synonym for ‘anus?’ These are questions for philosophers. But the fact remains, however we define it, we have only one. And if we pick a definition of ‘butt’ that is reasonably inclusive, like, say, ‘butt’ is the back end of an animal, then having a single butt is actually pretty standard for the animal kingdom.

The exceptions are relatively few and far between, but they do exist. And this marine worm with literally hundreds of butts might just be the most bizarre exception of them all. [ ♪♪ INTRO ♪♪ ] Meet Ramisyllis multicaudata. It’s a marine worm that lives inside of sponges in Australia’s Darwin Harbor.

And we didn’t know this thing even existed until 2003, when divers first noticed it while collecting sponges for another project. The revelation of its many butts did not come for another three years though, when a worm taxonomist dissected a sponge containing Ramisyllis and revealed the strange structure of its body. The worm’s head was buried deep within the sponge, while its body split off into many different branches, each with a butt peeking out of the sponge’s many openings.

Which, if you think about it, probably means that a human’s first glimpse of this worm was of one of its many butts. Now, many animals, including us, are basically a tube that takes in food at one end and excretes waste at the other, encased in some collection of anatomy that helps us do those things. But Ramisyllis doesn’t seem to work like that.

When you look at this weird, branching body, you might think that there’s a normal digestive tract that runs from its head to one of its butts, with all of the side branches acting like our limbs or the legs of a centipede or something. And that would be wrong. Because every single one of those side branches contains part of the worm’s digestive tract that ends with its own anus.

That’s a lot of butts to feed with just one single mouth! And the researchers don’t know how it manages that. They’ve never found any food particles within the worm’s gut.

It may absorb nutrients directly from the water, but we still don’t actually know how this thing eats or poops, or if it poops at all! So, you may be asking yourself, what then does the Ramisyllis do with all those butts? I know I want to know.

First it’s important to understand that the larger group of animals that Ramisyllis belongs to are the annelid worms, also called segmented worms. Being made up of segments means that their bodies are already kind of subdivided into sections. And Ramisyllis and its closest relatives can develop a special segment or set of segments at the butt-end of the worm that they use for reproduction.

This is called the ‘stolon.’ And along with the gonads and gametes they need to make more worms, stolons also have little bristles that they can use to swim and a head with two pairs of eyes and a brain. A stolon is literally a butt with a mind of its own, an independent reproductive unit, a butt on its own quest for booty. When they’re fully-formed, they detach from their original worm and swim off, looking for other stolons to mate with… somehow.

Now this is all unsettling enough when a worm has only one butt from which to make stolons. But Ramisyllis, along with its two branching cousins, is probably able to produce stolons from many of its butts, with multiple stolons being created at the same time. And as far as we know, a single Ramisyllis only produces either male or female stolons, and the males tend to be faster, more active swimmers.

So it seems like the stolons must actually venture forth from their sponge homes, if they want to fulfill their purpose. Now, what larvae are produced by these strange oceanic love matches? We do not know.

The researchers didn’t find any within the sponges they sampled. We also don’t know where mating happens or what happens to the stolons after that, or even how many stolons a single worm can produce. If Ramisyllis is like some of the other worms in its family, its stolons probably swim somewhere, release their gametes into the water column in what’s known as ‘broadcast spawning,’ and then die.

Which feels like a tragic, brief existence for an autonomous butt. Though, I suppose it’s better than getting trapped in the dead end tunnels of a sponge, like some stolons do, never seeing the outside world at all. And it’s definitely better than what happens with my butt, which never gets to do any of this stuff.

But I have to come back to a more practical concern. How does this thing exist? If we know two things about Ramisyllis, it’s that it has a lot of digestive tract, but has not been found to eat anything, yet it somehow generates a fleet of standalone reproductive units from its butts.

Where does it get the energy? And I don’t mean that as a joke, I mean all organisms need to get energy from somewhere to grow and reproduce. Several researchers have suggested that it’s in a symbiotic relationship with the sponges that it lives inside of… but who is getting what out of this?

We just don’t know yet. And that is part of the beauty of exploring the natural world. Sometimes you encounter a living thing that is so strange you have to stop and question how its most basic systems operate.

And when that living thing is a creature with hundreds of butts, well, it’s worth pausing to appreciate that biology and evolution can create both Ramisyllis and an organism capable of laughing at it. The Bizarre Beasts pin club subscription window is open from now through the end of March 13th! The Ramisyllis pin is just very good.

Like it doesn’t necessarily look, when you first look at it, like it’s a creature with a lot of butts. But, if somebody asks, you can tell them. If you’d like to support us another way, we have socks and stickers and a friendly little kakapo and the most beautiful art print by Emily Graslie!

They are all available at And, as always, profits from the pin club and all of our merch go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [ ♪♪ OUTRO ♪♪ ]