This episode is sponsored by Manukora Honey.

This amoeba has a shell around it, which seems like a pretty good idea. The world is large and full of dangers, and shells seem like a straightforward strategy to ward off predators.

Creating a suit of armor that you can live inside of, emerging only when you need something from the outside world… sign me up. But what if this amoeba’s shell wasn’t just a form of protection? What if it's actually dangerous?

This idea is one that James, our master of microscopes, learned about while reading a recent paper titled “It’s time to consider the Arcellinida shell as a weapon.” Now when I hear the words “shell” and “weapon,” my mind is jumping to Super Mario brothers and all of the shells that get thrown around to achieve various tasks. But when it comes to amoeba, that is not what's happening. For them, it’s not the outside of the amoeba shell that’s doing the damage.

The weaponry is coming from inside— the shell is just one important piece of its machinery. To understand that though, let’s take a step back to a bigger question that some of you might have: amoeba have shells? Well, not all of them.

Plenty of amoeba are the familiar, naked blobs we know and love, defined by the pseudopodia— or false feet— that extend from their body and help them move around. But a number of amoeba have, over the course of evolution, discovered the protective wonders of shells. These amoeba are known as testate amoeba, and if you want to explore some vintage Journey to the Microcosmos, we have a video about them.

What will be really important to understand for today, though, is that testate amoeba are very crafty about how they make their shells. The Arcellinida are a particular group of testate amoebas, and as we’ll see throughout this video, their shells can take on some incredible forms. This Difflugia, for example, has a shell— also called a test— that looks like tree bark rendered in crystal.

And these tests are made of a number of different materials gathered from their surroundings, like diatom shells and bits of other minerals. Meanwhile, these Arcella amoeba are emerging from their broken shells. But the task of reconstructing their shells is a bit different from the Difflugia because Arcella can produce the material they need for their shells out of protein building blocks.

So as we look across testate amoeba, we can see that the materials used in their shells are different. The sizes and shapes are different. There’s just a lot of different shells going on in their world, shielding them from predators and even from drought.

There is at least one constant though. Every shell needs an opening, otherwise it’s just a trap. So in the test is an opening called the aperture from which the amoeba can poke out its pseudopodia and move around in search of food.

And for the amoebas, the size of the aperture puts a pretty clear limit on what food they can hunt down. If they can’t get the food past the opening in their test, how are they going to eat it? But the arcellinidian testate amoeba has been observed eating things that seemed a bit bigger for them— things like nematodes and algae that should be too large to get through their apertures.

And scientists haven’t really understood how they’re able to do it. But a few recent studies have shed some more light on how different testate amoebas are able to bite off more than they can supposedly chew. Cryptodifflugia oviformis has an ovoid shell and are known to eat fungi, similar to this Nebela.

How it eats those fungi depends on just what it’s hunting down. If it’s munching on tiny yeast, the Cryptodifflugia can just swallow it up. And if it’s dealing with the long strands of hyphae found in fungi, then the Cryptodifflugia has to take a few extra steps, either breaking it up or— if the hyphae is too thick for that— poking holes in it.

Now on the surface, that doesn’t sound so remarkable. What makes a testate amoeba different from say, just me, sitting down at a table and having to cut apart a chicken breast so that it fits in my mouth? Well, I have limbs.

And utensils. And teeth. Those are all things that help me in eating food that is too big to fit into my mouth.

And a testate amoeba has none of that. It’s got a formless body packed into a shell. But it does have something else: a molecule called actin, which is usually used as a part of the amoeba’s cytoskeleton to help it move around.

And scientists have found that when Cryptodifflugia oviformis is grazing on fungi, it can form cones out of that actin that embed deep into its body, anchoring to the inside of the organism’s shell and then extending outwards to the prey. This actin might actually be helping the amoeba exert force on the prey so that it can break the hyphae. Which means that the shell is able to help the organism hunt, providing an anchor from which it can break down its prey into a more consumable size.

And another species called Phryganella paradoxa has also been observed using its shell to break open diatoms, a prey that can be challenging to crack because of its own tough exterior. These observations have raised more questions about how the shells of the arcellinida have evolved, and how much of it is driven by the different strategies individual species rely on to gather their varied prey. After all, when it comes to surviving the elements or predators, you might not expect to see the kind of diversity we actually see in testate amoeba.

Presumably, a predator is a predator, and a shield is a shield. You could tweak certain elements of the design, but the basic premise would remain the same. But if a shield is also meant to serve an additional role as a means for hunting, then you might expect more variations on the theme, adapted to suit the different meals that different amoeba are hunting down.

At least, that’s the idea. But much of this work is quite recent, and scientists are still learning more about the role testate amoeba shells might play in helping them hunt down different prey. It’s just one more way that amoeba can surprise you.

They seem so simple and straightforward, like if someone wanted to design a microbe but got distracted halfway through and decided to let the microbe just be whatever shape it wants to be. But just when you think you have a handle on its form, it becomes something else entirely. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

Before you go, I want to take a moment to share something sweet with you – and I mean really sweet. I’m talking about today’s sponsor, Manukora Honey. Now, when I say “honey” you might be picturing those (water) bear-shaped bottles– No, bear shaped bottles.

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If you head to manukora.com/microcosms right now, that’s M-A-N-U-K-O-R-A dot com slash and listen closely here, microcosms, C-O-S-M-S, you can get $25 off your Starter Kit! It is the ultimate honey - so indulge and try some Honey With Superpowers™ from Manukora. The people on the screen right now, they are our Patreon patrons, allowing us to dive deeper into the literature and find the cutting edge of the weirdness of the microcosmos.

You are the true heroes today. And I hope that each one of you gets to have a delicious meal that you eat, however works for you. Whether you're an amoeba or a human being.

And look, you never know. If you would like to become one of our Patreon patrons, you can go to Patreon.com/JourneytoMicro. If you want to see more from our Master of Microscopes, James Weiss, you can check out Jam and Germs on Instagram.

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