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They predate the dinosaurs and they outnumber us by trillions, yet you might not ever have the pleasure of seeing one. So let us introduce to you, the nematode.

Hosted by: Hank Green

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[intro ].

In 1913, a scientist named Nathan Cobb wrote the following: “If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable … we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes.” He wasn’t writing a horror story, and he wasn’t exaggerating. The world really is covered by tiny, unnoticed roundworms.

And it’s worth paying them some notice. Because Cobb was right: we're pretty much waist deep in them everywhere at all times. And they’re doing a lot more than you might think.

Some nematodes can be deadly. Others might save the world. Here are seven reasons nematodes are worth paying attention to.

First of all, what is a nematode? Well, it’s a worm -- but a lot of animals are worms. Earthworms belong to a different group entirely: the segmented worms.

Then there’s flatworms, the group that includes tapeworms. And depending on who you ask, there’s a lot of other worm-like groups out there too. Only some of the things we call “worms” are nematodes, which scientists place in the phylum Nematoda.

Not that that narrows it down much. That phylum still represents a lot of worms. And by a lot, I mean nematodes basically run this show.

It’s their world. You’re just living in it. Let’s put it into horrible, horrible perspective.

Right now there’s a little less than eight billion people on planet Earth. By contrast, there are 57 billion nematodes. Not total, though. 57 billion... for every human on Earth.

That’s 438.9]million trillion nematodes. That estimate is for soil nematodes, by the way. There could be even more that don’t live in soil.

Put differently, according to one estimate, four out of every five animals that live on our planet are nematodes. So do me a favor go out into the woods and collect all the nematodes from one square meter of habitat, you’d probably have several million. Yet these things are barely noticeable.

You don’t see a horror movie-worthy mass of squirming invertebrates every time you open the front door. That’s because most nematodes are tiny -- often microscopic. Still, the combined weight of all the nematodes on Earth is around 300 million metric tons, which equals around 80% of the combined weight of all the world’s humans.

We’re not sure how many species of nematodes there are, but estimates range from a few tens of thousands into the millions, of species! A lot of these species are undescribed because there are more nematodes than nematode scientists, by a lot. But maybe we should be devoting more study to our nematode overlords.

Depending on who you ask, a single pair of rats may produce 15,000 descendants in a single year. But it’s nothing compared to what nematodes can do. The large intestinal roundworm, for example, can lay as many as200,000]eggs… in a single day.

These particular nematodes can also store as many as 27,000 eggs in their bodies at one time. Like, imagine if chickens could do that, we’d all eat nothing but omelettes. And that wasn’t enough for them, not all nematodes stick to a scheme of male and female for reproducing.

The well-studied nematode C. elegans has males and hermaphrodites capable of fertilizing themselves -- but no females. A related species referred to as Rhabditis SB347 -. This is how we have to name nematode species - has three sexes: males, females, and hermaphrodites.

Having more mating partners available -- including yourself, for the hermaphrodites -- creates a ton of flexibility. They can find partners -- or a single hermaphrodite pioneer can go forth and multiply. Because there aren’t enough nematodes already.

Whatever their sex, nematodes are built for breeding. An adult C. elegans only has about a thousand somatic or non-reproductive cells in its body. But it may have a roughly equal number of germ cells devoted to reproduction.

So if you’re a nematode, a pretty big percentage of your body is just dedicated to making more nematodes. Still, they can’t be everywhere, right? There are lots of places where life is sparse.

Maybe you could escape the worms by moving to the Arctic or something. But no, you can not. There are at least two species of nematode that are specifically adapted to living in Arctic ice -- at least one of which eats other nematodes, by the way.

So if you really dislike creepy-crawlies, you can’t escape to the far north. Nor the far south. The most abundant land animal in Antarctica’s polar desert is… wait for it… a nematode.

And they’re everywhere in between. Some nematodes also thrive in hot, dry conditions. Some, in fact, can live in places that are totally inhospitable to humans and most other animals.

Like Mono Lake in the eastern Sierra Nevada Mountains, which is the saltiest lake in California. It also contains enough arsenic to make it dangerous for humans and fish. In a 2019 study published in the journal Current Biology, researchers identified eight species of arsenic-resistant nematode.

All eight species found in the lake can tolerate about 500 times the amount of arsenic that would kill a human being. So they don’t mind the cold, they don’t mind the heat, and they don’t even seem to care too much about being poisoned. Nematodes aren’t especially shy, either.

They enjoy the company of other animals. Some of them like to share a meal. Because they’re intestinal parasites.

So they literally will share your meal, you know, after you’ve eaten it. Others prefer to live in close company with other species. Like the hookworm, which also thrives in innards, but doesn’t bother with intestinal contents — instead, juveniles live off of the blood and tissue of their host.

Not all nematodes are parasites, but some scientists think parasitic species may number around 25,000]— and those are just the ones that parasitize vertebrates. In fact, some researchers think that one out of every two animals has its own parasitic nematode, which cozies up to no other type of animal. How sweet.

Humans got lucky. We have around sixty nematode species that like to parasitize us, though we get to share at least some of those with other organisms. Capillaria philippinensis, for example, usually parasitizes birds, but humans can get it from eating certain kinds of fish.

That can happen when we eat the fish, instead of the normal bird predators that the nematode counts on for its life cycle. Humans can become infected with Trichinella, too -- but so can pigs and feral hogs, mountain lions, and bears. And a parasitic nematode infection isn’t just gross; some can be deadly.

Especially if left untreated. Move over, viruses, there’s a new friend in town. Nematodes’ planetary domination isn’t new.

At least, we don’t think it is. Nematodes have soft bodies and they decay rapidly, so they’re not commonly found in the fossil record. Even so, the oldest-known nematodes date to four hundred million years ago.

Some scientists think nematodes have been around a lot longer than that, though, for at least a billion years. If that’s true, it means they evolved just after bacteria, protozoa, and fungi, and way before pretty much everything else. The first parasitic nematodes probably evolved from free-living marine nematodes — they likely evolved to parasitize marine invertebrates.

So not only have they been around since the dawn of multicellular life, some of them have been getting a free ride off of other organisms the whole way. Now nematodes are really basic life forms — really, they’re just tubes that digest food with a few other rudimentary organs thrown in there. But they’re still animals, like us.

They’re simple, and yet there’s a seemingly endless variety of these things. And not all of them follow the “microscopic and innocuous” model. Some of them get weird.

The biggest nematode. Placentonema gigantissima can reach between eight and nine meters in length. It also lives in the placenta of a sperm whale, so there’s that.

Some of them even have “fur". It’s actually a thick layer of bacteria that oxidizes sulfur, which makes it possible for this particular type of nematode to survive in sulfur-rich habitats on the ocean floor. They do creepy things, too.

Nematodes pee through their skin, for example. Kind of. Humans and other mammals excrete nitrogen waste in urine.

Nematodes can’t be bothered to wait in line for the restroom, so they excrete nitrogen waste directly through their body wall. Also, some nematodes have amoeboid sperm -- which means it doesn’t swim, it crawls. So despite the fact that their basic body plan is just... a gut, they manage to be pretty weird.

Nematodes have also taught us a surprising amount of what we know about our own bodies. In particular, C. elegans, that well-studied species we mentioned earlier, is widely used for biological research. Scientists like this particular nematode because each adult has a fixed number of cells.

What’s more, those cells develop according to the same pattern every time, which makes it possible for scientists to follow the fate of each and every one as the organism develops from an embryo. Even though C. elegans is a very simple creature, many of its genes have functional counterparts in larger animals like humans. Nematodes also share some of the same biological characteristics as humans.

Like some of the same tissues: skin cells, neurons, muscles, and others all passed down to both humans and nematodes from a common ancestor. Research using C. elegans has led to a lot of really important breakthroughs, like discoveries about human kidney disease, and improving our understanding of cancer. C. elegans was also the first multi-cellular organism to have its entire genome sequenced.

And because C. elegans produce more than a thousand eggs a day, with a life cycle lasting only two weeks, they can provide scientists with a never-ending supply of themselves. We kind of have to love nematodes, or at least acknowledge their worth. Because not only are they medically important -- they can help us in other ways, too.

In fact, they can teach us a lot about the most important scientific challenge of our time — the climate. Nematodes are major players in the carbon cycle. They exhale roughly two percent of soil carbon emissions — emissions that come exclusively from organisms that live in soil.

That’s roughly equivalent to 15 percent of the carbon we emit through fossil fuels. They also respond to changes in temperature and precipitation in really important ways. For example, a 2019 study found that drought conditions in grasslands can harm populations of predatory nematodes.

That leads to an increase in their prey: nematodes that eat grass roots. And that can have a snowball effect — in a negative way — on grass growth. When root-eating nematodes over-eat, the grass weakens and dies.

Meanwhile, microbial respiration releases even more carbon into the atmosphere. Which means even subtle changes to the climate can be amplified… via the effects on nematodes. Just because the things being affected by climate change are microscopic, doesn’t mean the consequences can’t be felt.

So we have to keep nematodes in mind when building our understanding of climate change. Yeah, the whole “film of nematodes” thing is kinda gross. Things that writhe and squirm and live in your guts aren’t usually very high on most people’s lists of favorites.

Thankfully, most nematodes are so tiny that they’re functionally invisible. But that doesn’t mean you can ignore them, because they're also really, really huge. In more ways than one.

Thanks for watching this episode of SciShow, and thanks to our patrons for making it happen. We are pretty sure we have the coolest community of supporters ever, and they make it possible for us to provide free educational videos for everyone. If you want to get involved, head on over to patreon.com/scishow. [ outro ].