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The mysterious case of the sea walnut’s disappearing anus

ByEmma Clarke

Mar 13, 2019

It is a fair assumption that you have not come across the phrase “transient anus” before. At least, no obvious context comes to mind.

The phrase is new to biology and has been coined to describe an anus which appears only when it is required for defecation, vanishing as soon as the deed is done. The proud owner of this rather unique claim to fame is the warty comb jelly or “sea walnut,” a jellyfish-like creature native to the western Atlantic. Comb jellies belong to a family that is 500 million years old, and it is possible that this intriguing phenomenon represents an intermediate stage in the emergence of the permanent anus sported by more recently evolved organisms.

The person to make this discovery was Sidney Tamm, the leader of a research group based at the Marine Biological Laboratory in Massachusetts. When looking at the sea creature under the microscope he had initially been unable to identify any sign of an anus whatsoever. This was strange, as scientists have known for years that most species of comb jelly have a “through gut”, the term for a digestive system which allows food to enter through one hole and leave through another. (Incidentally, this type of gut is a step up from that of jellyfish and sea sponges, which endure the unfortunate situation of having to use the same hole both for eating and excreting waste).

To investigate further, Tamm used video microscopy to watch the comb jelly over a longer period. Surprisingly, he observed that the jelly was forming a new anus each time it needed to defecate, which it turns out is pretty frequently – once an hour in the adults and up to once every ten minutes in the larvae. As waste builds up, the gut of the jelly begins to extend until it fuses with the outer layer of the organism, called the epidermis. This allows the formation of a temporary anal pore. As both the gut and the epidermis are made up of just a single layer of cells, the process is brief, and the hole disappears as quickly as it emerged.

“That is the really spectacular finding here,” Tamm told 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 says. “There’s no trace under the microscope. It’s invisible to me.”

Tamm suspects that a system like this could have been a pit-stop on the evolutionary journey to the through gut, and therefore could provide intriguing clues about the origins of our own anus.

For larger animals like humans, the through gut is a decidedly useful adaptation. It allows the consumption of a greater amount of fuel for energy because there is no need to bring food all the way back up to the mouth after the extraction of nutrients in the lower gut. Moreover, when waste has a separate exit point it becomes possible to eat a second meal while still digesting a previous one. This efficiency in digestion is vital to ensure that our large brains, the most fuel-hungry organ in the body, have enough energy to function. In essence, we likely owe a good deal of our evolutionary success to the humble anus.

So far, it appears to just be the warty comb jelly which makes use of a transient anus. Tamm is looking for evidence of a similar digestive system in other species of comb jelly, but has so far not seen anything like it. It will be interesting to see what further insight about our own evolutionary past we can gain from this fascinating creature and where it can lead to.

Image credit: Pixabay

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