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The Spontaneous Anus: First Animal On-Demand Anal Opening Discovered

EnolaGaia

I knew the job was dangerous when I took it ...
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You've probably heard the well-worn quip "Opinions are like assholes - everybody's got one."

It's now been discovered that at least one ctenophore - the warty comb jelly or sea walnut - doesn't have one - at least not a permanent one.

Sometimes This Comb Jelly Has An Anus. And Sometimes It Doesn't.
An anus is a gateway for solid-waste removal from an animal's digestive system; in most animals, the anus is reliably found in one location all the time. But Mnemiopsis leidyi, a jellyfish relative that is also known as a warty comb jelly or sea walnut, is not "most animals."

M. leidyi's anus isn't fixed in place on its gelatinous body. Instead of a permanent opening, a so-called anal pore appears when the jelly needs to defecate and then disappears immediately afterward, leaving unblemished skin behind, according to a new study. ...

M. leidyi belongs to a group of marine invertebrates called ctenophores (TEEN-oh-fours). Unlike such close relatives as sponges and jellyfish, ctenophores — especially their bodily functions — are poorly understood, Sidney Tamm, a researcher with the Marine Biological Laboratory in Woods Hole, Massachusetts, wrote in the study.

In fact, prior studies had concluded that M. leidyi had a permanent anus. But when Tamm used video microscopy to closely examine M. leidyi larvae and adults, he discovered that their anuses were intermittent, and that the jellies' defecation took place through an opening "which appears and disappears" in a regular rhythm ...

After M. leidyi gulps down prey, the meal travels through a six-part digestive system. Eventually, the food ends up in a central stomach that feeds into canals for pooping, which dead-end at the body's surface as lobes ...

Tamm observed that when a jelly was ready to defecate, the shape of its stomach would change — narrowing into a rectangular box — and its anal canals would widen. Two minutes later, the esophagus "crumpled," preventing more food from entering the stomach. Lobes at the ends of paired anal canals filled with waste particles and began to swell, with one lobe protruding significantly.

As that lobe reached "maximum volume," a pore opened and released a stream of poo as particles and clumps ... But before the pore opened, the skin of that lobe appeared "uniformly smooth," and there was no sign that the pore had opened there before.

Then, when all the waste had been released, "the pore closed completely and disappeared" ...

M. leidyi is to date the only known animal with a "now-you-see-it-now-you-don't" anal pore. Further investigations of its elusive anus may help to explain how permanent anuses evolved in other animals, according to the study.

SOURCE: https://www.livescience.com/64956-ctenophore-sometime-anus.html

ABSTRACT of the Published Study: https://onlinelibrary.wiley.com/doi/abs/10.1111/ivb.12236
 
Then, when all the waste had been released, "the pore closed completely and disappeared" ..
This surely is a remarkably-subjective non-scientific statement. What if the anus of this creature is simply identical in external appearance to other peridermal pores that exist for other forms of biostatic transjunction?

Or is just very small?
 
This surely is a remarkably-subjective non-scientific statement. What if the anus of this creature is simply identical in external appearance to other peridermal pores that exist for other forms of biostatic transjunction?
Or is just very small?

According to the article there is no persistent pore at the location where the anal pore forms, and the anal pore does not form in the same location on subsequent rounds.
 
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