Scientists Stunned by the Discovery of Sea Walnuts' Buttholes

Scientists have an evolutionary timeline that explains how and when animals first developed buttholes. But, as Amy Maxmen reports in Science, that timeline was recently called into question by a video of comb jellies pooping. The researcher behind the footage presented his findings at Ctenopalooza (a comb jelly research conference) in St. Augustine, Florida, earlier this month.

Sea walnuts belong to a family of tentacle-less jellies called comb jellies or ctenophores. Their appearance alone makes them stand out: They look like transparent dumplings threaded with strands of flashing, iridescent lights, almost as though someone covered Las Vegas with an enormous, blobby bell jar.

Like many marine invertebrates, comb jellies have an extremely simple body plan: a squishy tube with a single opening for both eating and excreting. Or at least that’s what scientists thought—until they saw William Browne’s video.

The University of Miami evolutionary biologist has been culturing the sea walnut (Mnemiopsis leidyi) and the related sea gooseberry (Pleurobrachia bachei) in the lab. During his research, he captured them on tape, excreting through their buttholes. He was able to trace the path from ingestion to excretion thanks to the red glow emanated by small crustaceans and zebrafish the creatures had eaten, which had been genetically engineered with a fluorescent protein.  

While it may sound trivial (or even juvenile) to you, the discovery of comb jelly buttholes is no small thing to scientists. Anuses represent a split in animal evolution. For millions of years, animals operated without them. Then they evolved, and since then, all new animals have had them. Ctenophores have been around for at least 550 million years—meaning they should have been a part of the group that operated without buttholes. But this finding upends that notion.

When Browne showed the insightful video, the experts in the room were stunned. “Looks like I’ve been wrong for 30 years,” marine biologist George Matsumoto told Science. “If people don’t see this video, they won’t believe it.”

This is not the first time the sea walnuts have shaken the family tree. In 2014, geneticists determined that the jellies seem to have forged their own evolutionary path in developing nervous systems.