This episode is sponsored by Endel, an app that creates personalized soundscapes to help you focus, relax and sleep.

The first 100 people to click on the link in the description will get a free week of audio experiences. Every experiment has to start somewhere.

This one began in a container full of dying microbes, and the five cute, pink ciliates called blepharisma that James—our master of microscopes—carefully transferred to a new flask for rescue. Within that flask was clean water and a few grains of wheat. But those grains were not there to feed the blepharisma.

They were there to feed the other hidden residents of the flask, the bacteria, expanding their numbers to create a buffet for the blepharisma to eat. And in their new home with its plentiful fields of bacteria to graze on, the blepharisma thrived. They divided—and sometimes multiple times per day—so that five became 10, became 20, became 40, and so on.

After only 25 rounds of division, those 5 cells had become more than 150 million iterations of themselves. And in just two weeks, what was a fresh flask full of clear water had become tinged with flecks of red, pigmented with the thriving population of blepharisma descended from just a handful of survivors. If microbes like the blepharisma can make so many of themselves in such a short amount of time, why is our planet not just completely coated in any single species?

Why don’t we see this reddish water everywhere, chock full of blepharisma? Well for one, the flask these blepharisma reside in is an idealized world, with endless nutrients and countless bacteria—not to mention an intervening master of microscopes. But the world outside of this flask is far less simple.

There are so many organisms to compete with, all trying to make a life out of the space and food and oxygen available to them. And those resources are constantly in flux, heightening the stakes as organisms contend for the possibility of future shortages. The problem for blepharisma is not just the intricacies of ecology.

They face another threat. And it is not a threat that you or I or even a myriad of other organisms on this earth could relate to. We’re talking, of course, about giant cannibals.

The cannibals in this flask did not make their appearance known until James began to reduce the amount of food he added to this culture of blepharisma, which makes sense. Cannibalism doesn’t necessarily make for a good survival strategy. When your species is attempting to make more of itself, taking the time to chow down on each other just seems counter-productive.

And yet, there are various ciliates that have been observed eating their own kind. For example, our favorite blue trumpets, Stentor coeruleus, have been documented tearing chunks off other Stentor coeruleus-es, grabbing them by the tail and swallowing them1. While we haven’t been able to record a successful cannibalistic Stentor, we have seen them try —like this one, attempting to get its mouth around the other.

In this case, the targeted individual was able to get away, probably because it was too big for the other Stentor to actually fit into its mouth. There’s a lesson in there for ciliates pursuing the cannibalistic lifestyle: it’s not enough to be vicious, you also need to be big. Ciliates do not have forks and knives to carve up their plate of food.

They don’t even have limbs. What most ciliates do have is an oral groove, an opening lined with hair to stir up a vortex and draw food in. So whatever the food they’re eating, ciliates have only a few options.

They can jab away at their prey, tearing chunks off in the process. Or they can engulf it in one piece. And for most blepharisma, peacefully minding their own business with bacteria meals, that’s fine.

But cannibalism demands more. The blepharisma needs to be big enough to eat other blepharisma. And that’s why cannibalistic blepharisma also tend to be giant blepharisma.

Ciliate cannibalism is not very well understood, and so the exact relationship between size and cannibalism is murky. Are the blepharisma cannibals because they are giants, or are they giants because they’re cannibals? In nature, maybe it’s a bit of both.

But in the lab, well, scientists have realized that there’s a more intriguing possibility, and our own blepharisma have shown this. It’s the possibility that you can actually intentionally turn your jolly pink ciliates into cannibals. Like we said at the beginning, every experiment has to start somewhere, and in the 1930s, the scientist Arthur C.

Giese started with a group of blepharisma that had grown up on bacteria. These blepharisma were content with their smaller meals, and cannibalism was rare. But Giese hypothesized that this peace was a conditional one, that resulted simply from the fact that the blepharisma he was looking at were all too close in size for any individual to pursue cannibalism.

But what if one could make a larger blepharisma? That is what Giese decided to find out, and his method was pretty simple: to get a bigger blepharisma, feed them bigger food. The blepharisma he was working with weren’t large enough yet to eat each other, but they could eat a ciliate called Tetrahymena that was smaller than them but larger than bacteria.

After eating the Tetrahymena, the blepharisma grew. Their mouths were larger, and even their macronucleus had expanded to almost three times the size as their bacteria-fed counterparts. Then, starved of their Tetrahymena diets, the giant blepharisma were set upon a batch of blepharisma that were small enough to stuff in their microbial gullet.

The smaller blepharisma, but did not stand a chance. Within minutes, most of the giant blepharisma Giese observed had eaten at least one Blepharisma. Eight hours later, he found one giant had eaten 12!

And while they are quick to be captured, the ingested blepharisma’s death is slow. The cannibal might take around 30 seconds to engulf their prey, but Giese reported seeing the captured blepharismas swimming laps inside food vacuoles for up to an hour, after which the digestion began to kick in. The cannibalism Giese observed was not permanent though.

Eventually, after being starved, the giant cannibals he had cultivated divided several times, creating smaller versions of themselves in the process, bringing his experiment full circle. Small blepharisma sustaining larger blepharisma who will eventually return to their small size…until the call for cannibalism strikes again. The blepharisma in our flask grew up differently than Giese’s did.

They weren’t separated into two batches and fed different diets that would eventually drive the physical differences that would make one group cannibalistic. Yet there were still blepharisma that grew into giants capable of cannibalism. And we don’t know the mechanisms that spurred those changes, whether there was something in these organisms’ pasts that predisposed some of them towards gigantism.

Nor do we know what their futures look like, and whether it will be full of cannibalism. Every experiment has to start somewhere. But by the end, it is only a snapshot of a moment in time, one that captures the effects of many mysterious histories, and the beginning of many unknown futures.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you again to Endel for sponsoring this episode. Endel is an app that takes everything we know about sound, combines it with technology, and creates personalized soundscapes to help you focus, relax, and sleep.

Their app was named the Apple Watch App of the Year in 2020 and their Study soundscape is a nice way to stay calm and concentrated while learning. Sound has a direct impact on your physical and mental wellbeing, and by adapting in real-time to things like your location, weather, and heart rate, Endel creates simple, pleasant sounds that can help to calm your mind. So, whether you’re struggling to get a good, deep sleep, or you just need to focus while you study, Endel can help you out.

If you’re interested in trying out Endel, just be one of the first 100 people to download it using the link in the description and you will get a free week of audio experiences! Quick shout out to all the people whose names are on the screen right now. They are our Patreon Patrons, and we know that if we put them all in a tank together, none of them would grow extra giant and consume the rest.

Right? Right?! I need you all to agree that none of you would become giant cannibals.

Well, now I’m a little bit worried about that, maybe we should add a little form you have to sign when you sign up to be a patron. If you would like to be one of the patrons of Journey of the Microcosmos, because you think that stuff like this should exist. You can go to Patreon.com/JourneyToMicro.

If you want to see more from our master of microscopes, James Weiss, check out Jam and Germs on Instagram. And if you want to see more from us, there’s always a subscribe button somewhere nearby.