Ah, food, one of life’s great pleasures.

Whether you’re an organism whose body can create its own food, or you make do with the consumption of other organisms, food sustains and connects life both large and small. But we’re not going to talk about food today.

We’re going to talk about what happens…after. No great meal comes without consequences. Even photosynthetic microbes produce oxygen they don’t need, a byproduct that is quite useful to many other organisms, including us.

And for organisms that have to digest their food to extract nutrients, well, there’s all sorts of stuff left behind when the process is done. In the world of the microcosmos, there are not only many varied tiny organisms. There are also many varied ways to produce even tinier poop.

And when we say “tinier,” we mean it. If you’re worried about being grossed out by this video, or you really want to make sure you’re not missing anything, just know that some of this pooping will require you to pay very close attention. Some of this will probably look similar to macroscopic bowel movements, and some will seem less so.

So let’s start with the more poop-like poop that comes from some of the multicellular members of the microcosmos, particularly the ones with digestive systems complete with specialized regions for digesting food and releasing solid waste. Our dragon-like friend the gastrotrich is doing the old scrunch-and-poop, probably after having finished a nice meal made up of bacteria or protozoa. The food came in through its pharynx and was then sent through the intestine, a straight tube full of digestive cells.

After, the remains continued their way to the end of the tube, where lies the anus--a microscopic gateway out of the gastrotrich and into the world. That’s all well and good for the more complex organisms, but how does one poop when one does not have a butthole? Well, for some single-celled organisms, one becomes the butthole.

Such is the case for amoebas. Their formlessness allows them to easily shift their shape around to take in food via phagocytosis. The amoeba here happens to have found a nice meal made of a ciliate, which is now contained in a food vacuole filling with digestive enzymes.

Those enzymes will eventually break the ciliate down, allowing the amoeba to absorb nutrients into its cytoplasm. But then what happens with the remains? Well, as we see with this amoeba here, it gets released.

Amoebas and other similar organisms use a process called exocytosis to send waste back to the membrane where it is removed from their body. You can see it again here in this Mastigamoeba, slowly gliding along the microscope slide while a round bit seems to come off up top. It almost looks like it’s leaving behind a little piece of itself, which I mean, maybe it is.

Ciliates have a more refined approach to pooping, which is to say they have the unicellular equivalent of an anus called the cytoproct. Ciliates gather food into an oral groove, and then consume it in the digestive vacuoles that travel through the cell. Eventually, when the vacuole makes its way to the cytoproct, the contents get out, and there you have ciliate poop.

The way organisms poop—the way microbes poop—are even more varied and wonderful than we’ve been able to show you here. But you might be wondering how you even get from something relatively simple like this ciliate releasing a small bit of waste to something more complex, like this tardigrade with its more compartmentalized approach to defecation. Well the answer, of course, is evolution, but it’s an answer that leads to many more unanswered questions.

Even within animals, the evolution of the anus—or in some species, the cloaca—is not well understood, as discussed in a 2015 review paper with the spectacular title, “Getting to the bottom of anal evolution”. While the anus itself is linked to the evolution of a digestive system, not all metazoans have a designated anus. The hydra gut has only one opening, which means that the organism’s mouth has to pull digestive double duty and open back up to release the waste.

And how that system fits in with the evolution of digestion overall is unclear. Much like the evolution of sex, anuses are a trait that appears and disappears in evolutionary lineages, reflecting the fact that they might be useful for some animals and less so for others. The duality of the hydra’s mouth seems to be working out great for them after all.

But having a digestive system with a distinct entrance and exit is pretty handy too. For one, it’s just a little less disgusting. But it also keeps food flowing in one direction.

And that means animals like the tardigrade and us don’t have to wait to finish digesting to eat more food. That would be annoying. We just get to keep eating, all thanks to that evolutionary mystery that is the anus.

In the end though, anus or not, everybody poops. Food, after all, might be one of life’s great pleasures, but all good things must come to an end. And perhaps, with the microcosmos, we can find some pleasure in that as well.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. It’s been quite awhile since we did a video answering some of your most frequently asked questions, so we would like to let you know that we’re going to be doing another one of those soon. If you have any questions that you’d like us to answer in that video, please leave them in the comments, or you can tweet us your questions @journeytomicro or leave them for us on our Patreon.

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If you want to see more from our Master of Microscopes James Weiss, check out Jam & Germs on Instagram, and if you want to see more from us, there’s probably a subscribe button somewhere nearby.