Extraordinary Marine Worm: Multiple Butts Sprout, Each with Independent Eyes and Brain

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

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.