Scientist Discovers Jellyfish-Like Creature Has Disappearing Anus

Comb Jelly in Dark Water
File photo. A comb jelly in dark water. iStock

A scientist has discovered an unusual feature on the morphology of a jellyfish-like creature—an "intermittent anus" that disappears after it defecates.

Mnemiopsis leidyi is a type of ctenophore—also known as a comb jelly. While native to western Atlantic coastal waters, it is one of the world's most invasive species and has established itself in regions around Europe and Asia.

The species has been the subject of much scientific research. Comb jellies, such as Mnemiopsis leidyi, evolved more than 500 million years ago. The first animals on Earth did not have a dedicated anus—a separate opening for waste turned up far later in the evolutionary timeline. Instead, these ancient creatures had one opening that served as both their mouth and anus. Many of their descendents alive today still have this digestive set-up.

Having a separate mouth and anus is advantageous evolutionary speaking—it allows an animal to eat and digest at the same time, rather than having to finish a meal before starting the digestive process.

In 2016, scientists discovered that unlike other similar creatures, Mnemiopsis leidyi did have an anus, Science magazine reports. At a meeting dedicated to ctenophores, William Browne of the University of Miami in Florida revealed videos of comb jellies defecating—shocking evolutionary biologists in attendance.

In a study published in the journal Invertebrate Biology, Sidney Tamm, from the Marine Biological Laboratory in Massachusetts, has made an even stranger discovery about Mnemiopsis leidyi. In the paper, he wrote: "Contrary to the scientific literature, individuals defecated through only one of the two anal canals which possesses the only anal pore. The anal pore was not visible as a permanent structure as depicted in textbooks, but appeared at defecation and disappeared afterward."

He found that timing between defecations varied depending on the size of the creature, ranging from less than 10 minutes through to around an hour.

Microscopic images revealed that the "opening and closing of the anal pore resembled a reversible ring of tissue fusion," concluding that individuals "appear to have an intermittent anus and therefore an intermittent through‐gut that reoccur at regular intervals." As New Scientist explains, this suggests there is no permanent connection between the gut and anus—as waste accumulates, the gut expands and, when it touches the epidermis, it forms an anal opening. After the waste is disposed of, the gut deflates and the anus disappears.

"That is the really spectacular finding here," Tamm told the magazine. "There is no documentation of a transient anus in any other animals that I know of."

The discovery of Mnemiopsis leidyi's intermittent anus could help us understand why some animals—including humans—ended up with a permanent one. "Such temporality of the anus might provide clues for understanding evolution of a permanent anus and through‐gut in animals … Further investigations using more incisive approaches are required to settle this matter," he wrote.

In an email to Newsweek, Tamm said there may be other early animals with related waste disposal systems: "Nature is full of marvels if you look," he said, adding "my next step will be to use an electron microscope to see the ultrastructure and cellular basis of anal pore dynamics—how it works."

This article has been updated to include quotes from Sidney Tamm.