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Most animals’ rear ends are at their…rear end.
But in the grand scheme of things, anuses didn’t necessarily
have to evolve to be where they are.
Just think of all the options we have for where to put them!
And some kinds of organisms, like bryozoans, have quite a variety of
anus locations across their different species.
And for these animals, their poop chutes
come with an added bonus: they are retractable.
Bryozoans are microscopic animals that live underwater in colonies,
connected to each other by a structure made of calcium carbonate.
These creatures have been on Earth for 485 million years.
And because they’ve been around for so long,
their fossils are studied pretty frequently.
But studying fossils can’t tell you about how an animal moves when it’s alive.
And since studies are typically done with their fossils,
we’re just now starting to learn about how living bryozoans poop.
Spoiler alert: a variety of ways.
They share a lot of commonalities, though.
They have a central trunk with a retractable tentacle crown
that they stick out to eat and poop.
These telescoping tentacles also have tiny moving parts that create currents
to bring food closer to their mouths.
And when they’re done, they can retract their tentacle crowns
to keep them safe from predators.
Now you might have noticed a major problem with this body plan from a mile away.
Because we all know not to poop where we eat.
But do not worry, they don’t eat and poop out of the same hole.
On a single bryozoan, the anus is far enough down the retractable segment
that it probably won’t eat its own waste.
But these creatures tend to live in groups.
So one bryozoan pooping could have some unfortunate
consequences for another community member.
Their waste floats in the water right over to the next bryozoan,
who’s trying to eat food.
This is more of an issue for some species than others.
Among bryozoans, there are differences in where the anus is
situated along that retractable segment.
A paper published in 2020 by the title, and I’m not kidding here,
“O Anus, Where art Thou?”
found that even within a single bryozoan genus,
there can be quite a variety of anus locations along the retractable segment.
And they found that these different locations can come with serious disadvantages.
When there are a lot of other organisms around, pooping out of an anus
that extends farther out of the body, like the Amathia caudata does,
is more likely to contaminate water that your neighbor consumes.
So the anus location that contaminates less water for your friend,
like what H. expansa has, is one that seems to have evolved
over and over again for different bryozoan species.
And you might think they could just avoid each other’s poop
if they were farther apart, but that kind of behavior
is evolutionarily selected against.
Bryozoans tend to have more success getting food when they are closer together.
Some bryozoan colonies even divvy up tasks.
In those cases, certain bryozoans are the eaters
for other colony members that focus on reproducing.
And that kind of labor division is a way to grow a stronger community as a whole.
Researchers even found some organisms, like gymnolaemates, that can
vary how far they extend their retractable anus.
And those bryozoans usually prefer to extend it farther out,
which could mean that pooping in a way that contaminates
your neighbor’s water happens to be better for an individual.
It’s possible that the tissue attached to this retractable anus is also a long channel.
And this would mean that the individual bryozoan needs to extend even farther out
to clear that tissue and avoid accidentally pooping its bryozoan pants,
even if it’s not great for its neighbor.
So it turns out that where your anus sits on your body
is a pretty important part of being alive.
And apparently, some organisms get to choose for themselves, to an extent.
This is the kind of evolutionary discovery that you miss out on by studying
preserved samples rather than live organisms.
So just imagine all the things we haven’t discovered about how dinosaurs pooped!
It just goes to show there’s always more to learn about our world.
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