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In 1874, a naturalist by the name of Joseph Leidy took out his microscope and examined some water from a local Pennsylvania spring.

On the other side of his lenses, he found a creature that looked a bit like this one. Leidy documented the creature in his microscopy magnum opus “Fresh-water Rhizopods of North America,” which includes beautiful images produced through chromolithography, an early method of color printing, based on what he’d seen.

And just like you might, he had some assumptions about what he thought this thing was. To Leidy, the first glimpses seemed to suggest a giant amoeba dragging around mycelia, those threads that fungi use to spread and gather nutrients. But the more he found other creatures like this, the more he considered that those threads were a part of the amoeba itself—that perhaps they were part of a strange new structure, and this was a strange new amoeba.

While observations were rare, Leidy was able to find enough of his own samples, as well as descriptions from another scientist who had found a similar species. And so Leidy named a new species Ouramoeba vorax. But he still couldn’t quite let go of how much those threads looked like mycelium.

In Fresh-water Rhizopods, he wrote I have repeatedly recognized, among the food of various Amoeba, different kinds of fungus-spores, and it is not unlikely that these lowly creatures may be infested with fungus-parasites, just as we frequently find to be the case with insects. But he was not quite convinced of his fungal hypothesis either, favoring the idea that this threaded amoeba was its own species. And so the Ouramoeba vorax was documented, illustrated, and examined.

We’re narrowing in, right now, on one very small part of Joseph Leidy’s life, and we’re about to talk about why he was wrong about Ouramoeba vorax and should have followed this mycelial thread a bit further. But first, a brief moment to note that Joseph Leidy did incredible work in both paleontology and microscopy. There are even reports that he was the first person to use a microscope for forensic purposes.

But intelligence is not the absence of mistakes, and as Leidy wrote in the introduction to Fresh-water Rhizopods of North America: While I have endeavored to describe things as they appear to be, I am conscious of having been unable to avoid the usual proportion of errors, for which I beg indulgence, and which I leave for others who shall pursue the same path of investigation to correct.” Good call man! In the case of the Ouramoeba vorax, it would take several decades for someone to correct Leidy’s mistake. In 1902, the Swiss microbiologist Eugène Penard published his own extensive work on protozoans: “The Rhizopods of the Léman Basin”.

And when it came to this particular creature, he quibbled with Leidy’s quibbles with a fungal explanation. As far as Penard could see, those filaments had to be the result of some kind of parasite. And the observations he and later scientists would make have continued to support that what we are watching is a fungus in action.

More specifically, we are watching a fungus called Amoebophilus simplex infect an amoeba. The infection might have started with a fungal spore getting attached to an amoeba, or perhaps the spore was part of a meal. Well, once inside the amoeba, the spores migrate to one end and begin to form those chains that Leidy observed.

They’re called haustoria, and they flower through the amoeba’s membrane to create a thread of new spores that will eventually be released into the environment to find a new host. We here are really lucky to be able to see Amoebophilus simplex in action, even if it’s at the amoeba’s expense. This fungus is quite rare.

And it seems to infect only a few amoeba species, which made it difficult for researchers to study how the fungus works and how it compares to other amoeba-infecting fungi. And those other fungi are also mysterious and weird in their own ways! James, our master of microscopes, found this giant amoeba on the same slide where the Amoebophilus simplex fungus was infecting other smaller amoeba.

So James set the slide in a humidity chamber. And if you’re like the rest of us on the Microcosmos team, maybe you’ve wondered what a humidity chamber even is because it sounds kind of fancy. Well, it turns out it’s a Tupperware container with wet cotton balls lining the bottom.

Because sometimes the simplest tools are the most effective. And when James put his slide in the humidity chamber, he didn’t expect anything very dramatic to happen. And for a week, nothing did.

The Amoebophilus simplex continued to infect the smaller amoebas crawling on the slide, but the giant amoebas were still healthy. Until a week later, when he was casually checking the slide to see the infected ones and found that the giant amoebas were stuffed with a bunch of tiny spores that looked similar to the oomycetes we’ve seen before. Now, we don’t know what this fungus is, just that after James found one giant amoeba filled up with spores cases, he found another one, and then another one!

And even more oddly, the amoeba began to melt away and lay the spherical spore cases like they were eggs. And after several minutes, those spore cases melted away to release more spores. Now, the spores didn’t seem to go anywhere.

After several days, they were still just waiting on the slide. Maybe they’re like Amoebophilus simplex, just waiting for the right host to come along and eat them. Or maybe there’s something else going on that we don’t know about and haven’t accounted for.

After all, to steal a wise microscopist’s words: “While [we] have endeavored to describe things as they appear to be, [we are] conscious of having been unable to avoid the usual proportion of errors, for which [we] beg indulgence, and [we] leave to others who shall pursue the same path of investigation to correct” Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thank you, especially, to all of the people on the screen right now. These are our Patreon patrons.

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