The Mysterious Case of the Disappearing Anus

Disappearing Anus

You’ve probably never heard the phrase “transient anus” before. At least, there is no obvious reason why you would. 

The phrase is new, even to biology. It describes an anus which appears only when the animal needs to poop. Then it vanishes as soon as the deed is done.  

The proud owner of this unique feature is the warty comb jelly or “sea walnut” – a sea creature a bit like a jellyfish. Comb jellies have existed for almost 500 million years. Scientists think that studying these jellies could provide important clues about how the permanent anus, like the one that you and I are lucky enough to have, came to exist.  

Sidney Tamm, a scientist who spends a lot of time studying comb jellies, made the discovery. When he first looked at the sea creature under the microscope, he couldn’t see an anus at all. It was strange, because scientists have known for a long time that most species of comb jelly have a through gut – a digestive system which allows food to enter through one hole and leave through another.

Sea sponges
Sea sponges (NOAA)

A through gut is a step up from that of jellyfish and sea sponges. These poor animals have to use the same hole for both eating and pooping. 

To learn more, Tamm took videos with a microscope to watch the comb jelly over a longer time. To his surprise, the jelly formed a new anus each time it needed to poop. And apparently it pooped a lot – once an hour for the adults and up to once every ten minutes in the young. As waste built up, the gut of the jelly began to extend until it merged with the outer layer of the organism, called the epidermis. This allowed a temporary anal pore to form. As both the gut and the epidermis are made up of just a single layer of cells, this didn’t take long. Then the hole could disappear just as quickly as it emerged.  

“That is the really spectacular finding here,” Tamm told the New Scientist, “there is no documentation of a transient anus in any other animals that I know of.” 

“It is not visible when the animal is not pooping,” he continued. “There’s no trace under the microscope. It’s invisible to me.” 

Tamm suspects that a system like this could have been a stop on the way for the evolution of the permanent anus. If true, studying it could provide interesting clues about the origins of our own rectums 

For larger animals like humans, the through gut is a very useful adaptation. It allows us to get more energy from food because there is no need to bring it all the way back up to the mouth after the gut absorbs the useful nutrients. Using two holes also makes it possible to eat a second meal while still digesting the one before. All of this is important to make sure that our large brains – the most fuel-hungry organ in the body – have enough energy to work properly. In short, we probably owe a lot of our success as a species to the humble anus. 

The warty comb jellyfish or sea walnut
The warty comb jellyfish or sea walnut at the New England Aquarium, Boston MA, Credit: Steven G. Johnson


Transient anus: This is an anus (or, in less polite terms, a “bum hole”) that comes and goes. Instead of being a permanent feature like the one that you are lucky enough to own, the transient anus only forms when strictly necessary before disappearing again. 

Through gut: This is the scientific name for a digestive system where food goes in through one hole and comes out of another. Humans and many other animals have a through gut, but some primitive species like sea sponges or jellyfish use the same hole both to eat and poop. Not a very appealing thought.  

Epidermis: This is the outer layer of cells in an organism, which provides a barrier to the outside world. In humans, the epidermis is the outermost section of the skin. 

Evolution: Evolution is how different species of animals develop over many millions of years. Over time, small changes are passed on from parents to children which make animals better adapted to their environment. When all of these small changes add up over a long time, it can lead to a new feature or a new species.   

Rectum: A fancy science word for anus. 

Anal pore: Yet another fancy science word for anus.


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