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This Marine Worm Sprouts Hundreds of Butts—Each With Its Own Eyes and Brain

When it’s time to reproduce, each of the worm’s many rear ends will swim off to get fertilized

Just one section of a marine worm with a strange, branching body. This species usually lives inside the many-chambered body of a sea sponge (Ponz-Segrelles, Aguado & Glasby)
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Humans spend a lot of time and money working to add or subtract tissue from their posteriors, but where humans obsess over size and shape, one species of marine worm instead focuses on sheer numbers. That’s right, these critters grow multiple butts—and not just three or four, we're talking hundreds. And, eventually, each grows a set of eyes and a brain and swims away on its own to spawn the next generation.

In a paper published last month in the Journal of Morphology, scientists describe the unique anatomy of Ramisyllis multicaudata, an annelid worm that lives inside the swiss-cheese bodies of sea sponges, and, more importantly, has one head and more than 100 butts, reports Jake Buehler of Gizmodo.

“We were able to count more than 500 [branches] in one specimen, but we think that they can easily reach 1,000,” M. Teresa Aguado, an evolutionary biologist at the University of Göttingen and co-author of the study, tells Gizmodo.

For their study, researchers collected specimens of Ramisyllis multicaudata and their host sponges from waters near Darwin, Australia, and examined them using microscopes, X-ray computed microtomography scans, histology and other techniques. In combination, these multiple analyses provided a 3-D picture of the worms’ internal organs as well as the structure of the sponges the worms inhabit, according to a statement.

Peering inside Ramisyllis m. revealed that each time its body branches in two, the internal organs—from the nerves to the guts and muscles—are also duplicated, according to Gizmodo. Each split, the researchers discovered, is encircled by a band of muscle. When the team took a closer look at the structure of these rings of muscle, they could actually tell which half of the bifurcated body came first and which was a new addition.

When it comes time for these worms to reproduce, things take another odd turn. Each of the animal’s many terminal openings forms something called a stolon that grows eyes and a brain, reports Mindy Weisberger for Live Science. When the stolon is ready, it detaches and swims off, guided by its rudimentary nervous system so that it can get fertilized.

Guillermo Ponz-Segrelles, a zoologist at the Autonomous University of Madrid and lead author of the study, tells Live Science that researchers in the 19th century had actually suggested that these stolons might have their own brains, but this study is the first to confirm their existence.

Part of what makes this creature strange is the branched body structure that gives rise to its menagerie of rears-ends, Kevin Thiele explains in a blog post for Taxonomy Australia from 2019. He writes:

Plants branch. Fungi branch. Animals don't. Most plants (and fungal mycelium) are indeterminately modular—that is, they are made up of modules (branches) that can make more modules (more branches) more or less without stopping. Most animals, however, are unitary—they comprise a single module. At most, this may comprise a set of segments (think insects and most worms), but even so the set is determinate, and doesn't branch… Ramisyllis multicaudata is an animal doing something that plants do. That's weird.

Though the new study has provided answers to some questions we may not have known needed answering until now, it has also raised some new ones.

“This study has concluded that the intestine of these animals could be functional, yet no trace of food has ever been seen inside them and so it is still a mystery how they can feed their huge branched bodies,” says Aguado in the statement. “Other questions raised in this study are how blood circulation and nerve impulses are affected by the branches of the body.”

Aguado tells Gizmodo that her team is working to figure out what the worm eats given that its labyrinthine guts always appear to be empty as well as genetic studies of the species’ relatives in hopes of revealing the DNA that underlies its panoply of posteriors.

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