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There are plenty of horror stories that begin innocuously enough.

A new home, a camping trip with friends, a doll purchased at an estate sale…. This one starts with some ponds, the same set of ponds that James, our master of microscopes, has been sampling every week for the past three years.

Which means that he’s collected so many microbes from these waters that you might think they’d get a bit boring or redundant. But you should never underestimate nature’s capacity for surprise. Recently, James came home with some samples from these ponds.

And as usual, he prepared some slides and checked on the organisms within, finding some nematodes like this one slithering about on the slide. And all seemed well, so he stored the slides and his new friends in a humidity chamber and waited to observe them after a few more days. But two days later, all would not be well.

This is where we build our suspense. In a movie, this would be the moment where we assess the unsettling basement or the dark woods, and then consider retreating to safety. This is the creepy doll, only there hasn’t been any thumps in the middle of the night, so everything seems okay, right?

We’re looking at the spores of a fungus, one belonging to the group Arthrobotrys. And when it’s just floating around like this, it seems quite harmless—especially when compared to the nematodes we showed earlier, which are part of a whole family of worms that are notorious for their parasitic lifestyle. And if you were to write off Arthrobotrys as a potential threat, you would be correct…most of the time.

It does spend much of its life aligned with the dead, but only to sustain itself on the remains of decayed life and organic matter. Arthrobotrys species are found all around the world, occupying everything from soil to animal feces in the many varied climates that make up our planet. And wherever it is, the fungus ensures that nutrients like nitrogen from dead organisms and other waste cycle through ecosystems.

But when nitrogen is scarce, these fungi will resort to hunting it down from living sources. And what better prey than the nematode, a fellow dweller of the soil and one of the most abundant animals on earth? When James put his slides into the humidity chamber, he had no notion of what these nematodes would be facing, and so no expectation of what he would find.

But when the slides came back out, what he observed was something he’d only seen once before, in a drawing done two years ago by one of his close friends, Katelyn Solbakk. In it, you can see a nematode whose body has been clinched into segments by some kind of bulbous...thing. What you’re seeing is the fungus’ most brutal design.

But to get there, it must morph from decomposer to predator—no longer consuming what has already been dead, but actively killing. It begins by weaving a trap out of itself. It threads the hyphae of its mycelium out and then back in, forming a living loop that repeats to form a net.

But a net is only one part of a trap—the other part is the lure. The fungi can find nematodes by following traces of their pheromones like they’re breadcrumbs. And more nefariously, they can mimic the smell of certain food cues to draw the worm in, like a siren working through scent instead of song.

The nematode has no reason to suspect anything, even as it swims closer and closer and eventually through the fungal rings. But as it does, the movement of worm and water triggers the rings to constrict. The worm is trapped, but the worst is still yet to come.

The fungus’ hyphae begin to grow off from the loop, puncturing the worm’s cuticle and paralyzing it. The threads swell up into a bulb that produces more hyphae to spread through the rest of the nematode. And then the fungus feeds and feeds, quickly digesting the rest of the nematode’s body from within.

It is a gruesome death. Here is one nematode, just recently trapped. And here is the worm again, four days later.

You can see the infection bulb where the fungus first punctured and expanded. And the whole body of the worm seems taken over, no longer a clear tube, but instead a corpse that has become home to its cause of death. The Arthrobotrys fungi are not the only ones capable of trapping and feeding upon nematodes.

There is a whole range of nematode-trapping fungi with their own methods, though the species. Arthrobotrys oligospora is perhaps the most plentiful of these fungi and also the best studied. Maybe it’s just us, but it’s somewhat unsettling to realize that this insidiousness is all the work of a fungus, a thing that can seem so inert compared to the wiggling, active worm that it targets.

But fungi do have a kinship with horror stories. Their frequent role as decomposers naturally connects them with the dead. Plus, they come equipped with their own creeping sense of dread with images of mycelia weaving through bodies.

And authors have drawn inspiration from the notion of fungal horror. There are many works--like the famous Gothic tale We Have Always Lived in the Castle, or the short story “The Voice in the Night,” or recent novels like Mexican Gothic and Wanderers— that draw on everything from poisonous mushrooms to colonizing fungi to create their terror. But whatever we seek to scare ourselves with in fiction, horror has its purpose in nature.

As we’ve pointed out, nematodes are one of the most abundant animals on earth. They play an important role in decomposition...but they’re also the source of many diseases—both in animal bodies and in plants. So having them be slightly less abundant is important to our ecosystem as well.

In fact, scientists have been studying these fungi to develop better nematode-fighting strategies for agriculture. So as is the case with many good horror villains, there is a version of this story where the nematode-trapping fungus is the hero. Unless, of course, you’re the nematode.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And of course, thank you to all of the people on this screen right now. If you like this show, these are the folks you should be thanking.

They are our patrons on patreon and you can become one of them by going to Patreon.com/journeytomicro. Thank you so much to everyone who’s been able to do that. 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 always a subscribe button somewhere nearby.