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Go to CuriosityStream.com/microcosmos to start streaming thousands of documentaries and non fiction TV shows. This frontonia is swimming in a minefield.

We can’t see all the dangers it’s evading, but abruptly, the frontonia stops, and then very, very slowly, begins to expand outward. In this moment, the frontonia’s entire body seems to shift away from its placid demeanor. Gone is the swimming organism.

Now we have something more strained. It’s like when a cat is staring off in the distance with every muscle tensed and every bit of fur standing on end. You know just by looking that something is causing distress, but the source of the terror is invisible.

But still you keep looking, because you want to know— you have to know— what has frozen this creature in alarm? And why is it reacting this way? The homalozoon we’re watching right now has a lot in common with the frontonia we were just observing.

Like Frontonia and most of the organisms we’ll be talking about today, the homalozoon is a ciliate, a single-celled organism marked by the hair-like structures around its body. But this homalozoon isn’t dealing with invisible threats. It is the threat, on the hunt for food.

And when it comes across this other ciliate, a paramecium, the homalozoon has an invisible weapon to help bring down its prey. But instead, the homalozoon flinches from the paramecium as if it has been stung, because in a sense, it has been. What we’ve just watched unfold is a battle between extrusomes, a type of vesicle found within single-celled organisms.

Within the cell, extrusomes usually take on crystal-like structures or are housed in capsules. But as we’ll see in a bit, when the time comes to be used, extrusomes can be quite dramatic to watch in action. What I’ve just told you doesn’t reveal much about an extrusome, though.

Is it an attack? A defense? Something else entirely?

Yes. But more specifically, it depends. Think of extrusomes sort of like a knife.

It can be a weapon, sure, but it can also be used to feed or to craft. It all depends on the nature of the blade and the person wielding it. Which is to say that there are a lot of different types of knives in the world, and also there are many types of extrusomes.

And while we cannot provide an exhaustive display of all of the extrusomes known to scientists right now, we can at least observe a few of them. Let’s start with the extrusome wielded by our hungry homalozoon from earlier, which is called a toxicyst because, it is loaded with toxins. We have seen toxicysts in action in various ciliates throughout our journey through the microcosmos, and perhaps our best view has come from the dileptus.

All along that trunk-like structure are hundreds of toxicysts. And as the dileptus hunts for food, it sweeps its trunk from side to side so that any prey that comes in contact with the trunk will also come in contact with those toxicysts. For the ciliates that have them, the main task of toxicysts is to help catch prey.

When not in use, they are stored neatly within capsules, lying in wait. But should an unsuspecting meal come too close, predators like this lacrymaria will extend their toxicysts out from their body and towards their prey to paralyze it. But look, different predators have different needs, so their toxicysts may look different and be wielded differently.

Some organisms might rely more on the paralytic activity of the toxins, while others use the toxicysts to physically hold onto their prey. There are also extrusomes that are similar to toxicysts, but not quite similar enough to be grouped with them. Like rhabdocysts, rod-shaped extrusomes found on karyorelictid ciliates like this Trachelocercid, that shoot like arrows into their prey.

And then there are suctorians, which look very unassuming here. They’re the round bodies sticking out from a stalk in the middle of your screen right now. As we change our perspective though, we can start to make out the tentacles lining their bodies.

And among those tentacles are extrusomes called haptocysts, which have a round knob at the end. As unthreatening as suctorians appear, those haptocysts were able to grab ahold of some unsuspecting creature that got too close. Not only were the haptocysts able to puncture the prey’s body, they were able to act as a highway back to the predator, creating a connection that ultimately feeds the suctorian.

But as we saw earlier, the homalozoon wielded a toxicyst against the paramecium, but the paramecium also had the ability to fight back. The paramecium rely on a set of extrusomes called trichocysts. In this clip, James—our master of microscopes—has added some diluted ethyl alcohol onto a slide.

The paramecia are trying to huddle in the pockets of water where there’s less alcohol, but as they come in contact with more and more of the alcohol, they begin to release their trichocysts. It might be a little difficult to see, but up close, with some patience and a careful eye, you may be able to see it better. Ignore the bubbles that are emerging from the edge of the paramecium, apparently that's something that can just happen as a paramecium sits on the slide for a while.

But look around them, at the long threads emerging from the side of the paramecium, which seem to overwhelm the ciliate. Paramecium and their trichocysts are perhaps the best-studied extrusomes among all ciliates. Unlike toxicysts, which are usually localized to one part of the predator’s body, trichocysts are usually found throughout the length of the organism’s body.

But they do have special docking sites that keep the trichocysts situated right below the plasma membrane. When a stimulus like a predator (or alcohol) comes along, the membrane of the trichocyst will begin to fuse with the paramecium’s membrane, creating an opening from which the trichocyst can discharge. While paramecium have the best characterized trichocysts of all, other species do have trichocysts as well, including the frontonia we showed you in the beginning of this episode.

In fact, we saw them in action, set off by the cleaning chemicals that James had added to the water. He repeated this multiple times, adding drops of this invisible danger so we can observe the frontonia’s trichocysts. Again and again, you can see how the cell stops and seems instantaneously surrounded by a mass of thin threads, similar in structure to the paramecium’s trichocysts we saw earlier.

The extrusomes we’ve talked about so far have clear roles in the eternal battle between predator and prey. Some defend, some attack. But some extrusomes have a murkier purpose.

Like mucosysts. We’re showing them here in euglena, which are flagellates, but they’re found in ciliates as well like loxophyllum. They might be a way to get food, helping their organisms create a sticky cell surface that acts as a trap.

But they might also be a way to help the organism build a cyst, a tool for movement. Their exact purpose remains unclear, and who knows, maybe they have a range of uses. And with so many more types of extrusomes that exist in the ciliate world, there are so many opportunities for us to understand more how these organisms interact with the world around them.

These are all organelles perched along the borders, an expression of hunger or a demand for protection or something more mysterious, waiting for a signal that says it’s their time to eject out into the world. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. This episode has been brought to you by CuriosityStream, a subscription streaming service that offers thousands of documentaries and non­fiction TV shows from some of the world's best filmmakers, including award winning exclusives & originals.

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