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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 callwormsare 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 themicroscopic and innocuousmodel.

  • 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 havefur".

  • 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 wayon 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 wholefilm of nematodesthing 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 ]

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あなたが見たことがない最も重要な動物との出会い (Meet the Most Important Animal You’ve Never Seen)

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    林宜悉 に公開 2021 年 01 月 14 日