This Jelly’s Butthole Only Appears When It Poops

Imagine if your butthole materialized when you needed to poop, and vanished when you were done. That sums up a normal bathroom break for Mnemiopsis leidyi, a comb jelly species with a “transient anus” described in a recent paper published in Invertebrate Biology.

Colloquially known as the warty comb jelly or sea walnut, M. leidyi belongs to a group of marine invertebrates called ctenophores. These creatures are descended from ancestors that lived in the Cambrian period 525 million years ago, making them one of the oldest animal lineages.

Unlike jellyfish, which eat and poop out of the same orifice, ctenophores consume food through an oral opening and defecate through at least one anal pore. However, the exact process by which food waste is transported through the jelly’s body to the anus is not known.

Sidney Tamm, a biologist at the University of Chicago’s Marine Biological Laboratory in Woods Hole, Massachusetts, shed some light on this mystery in the new study. Using a technique called differential interference contrast microscopy, he zoomed in on comb jellies as they pooped and discovered that the anal pore is not a permanent anatomical fixture on M. leidyi, as some studies had previously reported.

“The anus is invisible between defecations,” Tamm told Motherboard over the phone. “You cannot see it either with your eyes or through a microscope. No anus or anal pore in any other animal is like this.”

Tamm also noticed that the jellies had a rigid poop schedule that correlated to body size. The small larval ctenophores he studied defecated roughly once every ten minutes, while the adult jellies, which were five or six centimeters in length, dumped waste on an hourly timetable.

“The smaller the ctenophore, the more frequent the defecation,” he said. “This is evidence that there’s an internal clock because there’s no outside stimulus that’s occurring—either every ten minutes with the young ones, or every hour with the bigger ones—that could be doing it.”

Tamm thinks the likely process behind this amazing transient anus is a “ring of fusion” created when the jelly’s anal canal swells with waste, causing it to press against the animal’s external skin tissues. At the contact point between the inner and outer surfaces, an orifice dilates so that the jelly can poop.

“It looks like a hole opening up getting bigger and bigger, and then you see the waste coming right out,” Tamm said. “Then, the ring of fusion decreases and it seals again and there’s no longer any opening.”

Tamm plans to build on the research by examining ctenophore defecations in more detail using electron microscopy. He also wants to study the digestive process in other comb jelly species, such as Pleurobrachia pileus.

Because ctenophore lineages are so ancient, their adaptations can shed light on larger evolutionary questions. For instance, the jellies could be a kind of missing link between species that never evolved a separate anus and the animals that did develop firm poopers, including humans.

“It might be an evolutionary step toward a permanent anus in higher animals,” Tamm said of the transient ctenophore bum.

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