As It Happens

Meet the sea creature with an anus that disappears when not in use

When the creature isn't pooping, the anus isn't there.

The warty comb jelly's anal opening only appears when it's pooping, says biologist

Mnemiopsis leidyi, the warty comb jelly or sea walnut, is a species of tentaculate ctenophore originally native to the western Atlantic coastal waters. (IrinaK/Shutterstock)

This story was originally published on March 7, 2019. 

Read Story Transcript

The anus of the warty comb jelly is very peculiar. That's because when the creature isn't pooping, its anus isn't there.

At least it doesn't appear to be there, according to Sidney Tamm, author of a study about the warty comb jelly — also known as a  sea walnut — published in the journal Invertebrate Biology

He's spent a lot of time looking at how the warty comb jelly does its business and he spoke with As it Happens host Carol Off about what he's learned. 

Here is part of their conversation.

I know you spent a fair bit of time studying the messy business end of the warty comb jelly. So what's weird about it?

Well, first of all, let me tell you that they have a through-gut. And this has been known for hundreds of years. In fact, probably Aristotle even saw it — the pooping.

They have a through-gut, which means they have a mouth and an anus, and the material, the prey ... that it eats goes all the way through from one opening to the other and it's then excreted after the waste is processed. 

And the strange thing that I found is that the anus ... does not seem to be a permanent structure. It appears before defecation begins, and disappears afterward.

What? So you're saying basically that ... there's an anus only when the critter needs it.

That's right.

At what point does the anus show up?

You can watch it under the microscope without it getting away — what you see first is that before defecation begins, there's no hole there. There's no anal structure visible in the anal canal, which is where the poop accumulates.

And then all of a sudden, an opening appears in the anal canal, which is pressing against the outside skin of the animal — almost like a chimney sticking out.

And this — what I think is tissue fusion, between the outside surface and the anal canal surface — gives you a hole, or an orifice, through which the waste is then expelled.

And you can see it streaming out of the opening pore. And the pore gets larger and larger. Big clumps can now go out from the anal canal until all the waste is expelled.

And then the pore closes, slowly, and seals off the inside digestive compartment from the outside. And you have no anus anymore until it poops again.

It just disappears.


OK. So all this waste builds up, as it does, inside this creature and it has no place to go ... and it starts pushing on the outer skin, and then, poof! There is an anus.

 Yes. That's right.

And then they poop, and then, poof! Anus is gone. 

Yes. And so it's different, you know...


... from all higher animals. You know, even when are when we're not pooping, our anus is still there. It's closed by muscles — sphincter muscles — but the structure is still there.

Besides the warty comb jelly, have you seen this — I guess this intermittent anus — in any other creatures?

I haven't looked.

I've looked at a another kind of ctenophore called pleurobrachia — which is a cydippid ctenophore. It's called the sea gooseberry. I think you have them you have them off both coasts of Canada. And I've seen them poop. They have two anuses instead of one, like in the Woods Hole ctenophore. But I haven't looked to see carefully whether or not they're anuses are intermittent and disappear when they're not pooping.

Were you looking to see if this there was a kind of transient anus on this creature, or did you just happen to notice it?

Oh no, no. I wasn't looking for it, because there's no indication in the literature. All the scientific literature says that the anus is a permanent structure.

And in pictures of ctenophores, the anus is always drawn as a closed hole — even during interludes between defecations. So in a non-defecating ctenophore, in the literature, you see a dot or a circle showing the anus, or the anuses if it has two.

So you were studying this creature and then suddenly you noticed, like, "Whoa! Where did the anus go?"

Yeah. Well, it's that way with everything.

Well, for you maybe.

No, you know, it's like Yogi Berra ... the New York Yankees catcher used to say, he said, "You can observe a lot just by looking."

And that's what I did.

I thought you were going to tell me that Yogi Berra had some comment about about anuses disappearing.

He probably does. But they're unprintable.

Interview produced by Richard Raycraft. Q&A edited for length and clarity.