Sometimes…Jellyfish Live on Land

Summary

The video script delves into the peculiar world of myxozoans, microscopic parasitic jellyfish relatives. These tiny creatures, with a complex genetic makeup despite their simple structure, have evolved to infect a variety of hosts, including fish and terrestrial animals like pygmy shrews. Myxozoans exhibit fascinating cellular processes like endogenous budding and have a reduced genome, highlighting mosaic evolution. Their ability to adapt to new hosts and environments, including land, challenges traditional views on parasite evolution and showcases their unique success in the evolutionary timeline.

Takeaways

• 🌊 Myxozoans are microscopic, jellyfish-like parasites that can also be found on land.
• 🔬 They are multicellular but barely, with some cells forming within other cells in a process called endogenous budding.
• 🐟 Myxozoans parasitize a variety of hosts including fish, worms, amphibians, aquatic birds, and pygmy shrews.
• 🕰️ They evolved around 651 million years ago, which is over a hundred million years before their known hosts appeared.
• 🔄 Myxozoans have a two-stage life cycle involving actinospores and myxospores, with fish as intermediate hosts and annelid worms as primary hosts.
• 🧬 Scientists classified myxozoans as cnidarians, related to jellyfish, after analyzing their genomes.
• 🏞️ Myxozoans have adapted to life on land, infecting terrestrial animals like pygmy shrews, although the exact mechanism is unclear.
• 🧬 They have a reduced genome, shedding unnecessary genes while enhancing those needed for survival and infection.
• 🌐 Mosaic evolution in myxozoans challenges traditional views of parasite evolution as a process of genetic downsizing.
• 🔍 These tiny organisms are helping scientists understand that complexity and success in evolution aren't tied to the number of cells an organism has.

Q & A

What are myxozoans and why are they considered strange relatives of jellyfish?

Myxozoans are microscopic, jellyfish-like parasites that are multicellular but barely. They are considered strange because they are very small, have a simple structure, and have evolved to live on land as well as in water, which is unusual for jellyfish relatives.

How small are myxozoans and what is unique about their cell formation process?

Myxozoans are incredibly tiny, having only a handful of cells at any point during their life cycle. Their cell formation process is unique through a method called endogenous budding, where cells form within other cells, leading to a nested structure of cells within cells.

What is the significance of the polar capsules in myxozoans?

Polar capsules in myxozoans are stinging structures similar to a jellyfish's nematocysts. While jellyfish use nematocysts to catch food, myxozoans use polar capsules to attach to their hosts, indicating a shared ancestry within the phylum Cnidaria.

How did scientists initially classify myxozoans and why was this classification problematic?

Initially, 19th-century scientists classified myxozoans as protists, which was a mostly dead taxonomic category that included amoebas. This was problematic because protists were considered single-celled organisms, and myxozoans, although simple, have more than one cell.

What evidence led scientists to conclude that myxozoans are indeed relatives of jellyfish?

In 2015, scientists analyzed the genomes of two distantly related species of myxozoan and concluded that they were sister species of Polypodium hydriforme, another cnidarian parasite. This shared ancestry and the presence of stinging cells indicated that myxozoans belong within the cnidarians, next to jellyfish.

When did myxozoans evolve and what is puzzling about this timing?

Myxozoans evolved around 651 million years ago during the late Cryogenian period, also known as the snowball Earth era. What's puzzling is that their known hosts, such as fish and annelid worms, did not appear until much later, around 530 million years ago.

What are the two main life stages of myxozoans and how do they relate to their hosts?

The two main life stages of myxozoans are the actinospore and the myxospore. Fish serve as an intermediate host where myxospores develop. When released, they infect annelid worms, where actinospores develop and then go on to infect fish again, continuing the cycle.

How have myxozoans adapted to infect terrestrial animals like pygmy shrews?

Myxozoans have adapted to infect terrestrial animals by possibly infecting earthworms that are then eaten by pygmy shrews. However, this theory is not well-established, as pygmy shrews do not often consume earthworms, and the exact mechanism of transmission to land is still unclear.

What is the concept of mosaic evolution and how does it apply to myxozoans?

Mosaic evolution refers to different parts of an organism's genome evolving at different rates. In myxozoans, this is seen as they have reduced their genome by eliminating unnecessary genes and enhancing others needed for stress coping, energy metabolism, and host invasion, which is a departure from the traditional view of parasites as genetically downsizing.

How do myxozoans' reduced genomes benefit them as parasites?

A reduced genome can be advantageous for parasites as they do not need genes for functions they can obtain from their hosts. Myxozoans have streamlined their genetic makeup to focus on genes that help them cope with stress, metabolize energy, and invade hosts more effectively.

What can we learn from myxozoans about the complexity of organisms and their success?

Myxozoans demonstrate that an organism does not need a large number of cells to be genetically complex or successful. Their sophisticated adaptations and evolutionary strategies show that even simple organisms can be highly successful in various environments.

Outlines

00:00

🐙 The Bizarre World of Myxozoans

This paragraph introduces the myxozoans, tiny parasitic relatives of jellyfish that can also be found on land. Despite their microscopic size, they have a complex life cycle and can infect a variety of hosts, including fish, worms, amphibians, and even terrestrial animals like pygmy shrews. The paragraph delves into their unique cellular formation process, endogenous budding, and their evolutionary connection to cnidarians, as revealed by genome analysis. It also touches on the mystery of how these marine parasites ended up on land and the peculiarities of their life cycle, which involves two main stages: the actinospore and the myxospore, with fish as intermediate hosts and annelid worms as primary hosts.

05:01

🧬 Mosaic Evolution and the Simplicity of Myxozoans

The second paragraph explores the evolutionary strategies of myxozoans, which have streamlined their genetic makeup by discarding unnecessary genes while enhancing those vital for survival as parasites. This has led to a reduced genome, which paradoxically allows for complex adaptations, such as the ability to thrive without oxygen. The concept of mosaic evolution is introduced, highlighting how different parts of the genome evolve at varying rates, a phenomenon more commonly observed in larger organisms. The paragraph concludes by emphasizing the sophistication of these seemingly simple creatures and their ability to adapt and infect new species across diverse environments.

Keywords

💡Myxozoans

Myxozoans are microscopic, jellyfish-like parasites that are the focus of the video. They are multicellular but have a very small number of cells. The video discusses their unusual cellular formation process and their evolutionary history, emphasizing their strangeness and the mystery surrounding their life cycle. The term 'myxozoans' is central to the video's theme of exploring the bizarre and complex world of these tiny organisms.

💡Endogenous budding

Endogenous budding is a unique cellular formation process mentioned in the script where cells form within other cells instead of dividing. This process is a key characteristic of myxozoans, contributing to their peculiar life cycle and challenging traditional understandings of cell division. It exemplifies the video's theme of showcasing the strange and complex biological processes of these creatures.

💡Cnidarians

Cnidarians are a phylum of animals that includes sea anemones, corals, and jellyfish. The video explains that myxozoans have stinging structures called polar capsules, similar to cnidarians' nematocysts, which suggests a close evolutionary relationship. The term 'cnidarians' is used in the script to establish myxozoans as a relative of jellyfish and to discuss their shared evolutionary traits.

💡Polar capsules

Polar capsules are stinging structures found in myxozoans that are similar to the nematocysts of jellyfish. They are used by myxozoans to attach to their hosts, which is a key aspect of their parasitic nature. The script uses the term 'polar capsules' to illustrate the connection between myxozoans and cnidarians and to highlight the unique adaptations of these parasites.

💡Polypodium hydriforme

Polypodium hydriforme is another cnidarian parasite that shares a common ancestor with myxozoans, as revealed by a 2015 genome analysis mentioned in the video. The term is used to further establish the evolutionary link between myxozoans and other cnidarians, emphasizing the video's theme of exploring the complex relationships within the animal kingdom.

💡Actinospore and Myxospore

The actinospore and myxospore are the two main life stages of myxozoans. The video describes the life cycle of myxozoans, where fish serve as intermediate hosts and annelid worms are the primary hosts. Understanding these life stages is crucial to grasping the video's narrative about the complex life cycle and parasitic nature of myxozoans.

💡Mosaic evolution

Mosaic evolution is a type of evolution where different parts of an organism's genome evolve at different rates. The video uses this term to describe how myxozoans have adapted to their parasitic lifestyle by enhancing certain genes while discarding others. This concept is central to the video's message about the sophistication of evolution in seemingly simple organisms.

💡Reduced genome

A reduced genome refers to a smaller set of genetic material, which is common in parasites. The video explains that myxozoans have a reduced genome, which allows them to survive without certain genes by 'stealing' what they need from their hosts. This concept is used to illustrate the video's theme of the complex and efficient adaptations of these parasites.

💡Opportunistic

Opportunistic describes organisms that can adapt to new hosts or environments. The video portrays myxozoans as opportunistic, having evolved to infect various hosts over their long evolutionary history, even before the appearance of fish and annelid worms. This term is used to emphasize the adaptability and evolutionary success of myxozoans.

💡Land-dwelling

The term 'land-dwelling' is used in the video to describe the surprising ability of myxozoans to inhabit terrestrial environments, such as within pygmy shrews. This challenges the traditional view of these creatures as strictly aquatic and adds to the video's theme of the unexpected and complex nature of myxozoans.

💡Parthenogenesis

Although not explicitly mentioned in the transcript, the concept of parthenogenesis (asexual reproduction) is implied in the discussion of myxozoans' life cycle and their ability to form cells within other cells. This concept is relevant to understanding the unique reproductive strategies of these organisms and contributes to the video's exploration of the peculiarities of myxozoan biology.

Highlights

Myxozoans are microscopic, jellyfish-like parasites that can be found on land.

Myxozoans are multicellular but barely, with a handful of cells at any point in their life cycle.

They form cells through endogenous budding, a process where cells form within other cells.

Myxozoans have a complex cellular structure with cells within cells, likened to a molecular turducken.

19th-century scientists initially classified myxozoans as protists due to their simple structure.

Myxozoans possess stinging structures called polar capsules, similar to jellyfish nematocysts.

Polar capsules are used by myxozoans to attach to their hosts, unlike jellyfish which use them for catching food.

Genome analysis in 2015 revealed myxozoans as sister species of Polypodium hydriforme, confirming their relation to cnidarians.

Myxozoans evolved around 651 million years ago, a hundred million years before the appearance of fish.

Myxozoans have two main life stages: actinospore and myxospore, with fish as intermediate hosts.

The primary hosts of myxozoans are annelid worms, indicating an early adaptation to different invertebrates.

Myxozoans have been found in land-dwelling animals like pygmy shrews, suggesting an adaptation to terrestrial life.

The method of myxozoans infecting land animals remains a mystery, with theories involving earthworms and shrews.

Myxozoans have a reduced genome, shedding unnecessary genes while enhancing others for survival.

One species of myxozoan has evolved to survive without the need for oxygen.

Myxozoans exhibit mosaic evolution, where different parts of the genome evolve at different rates.

Myxozoans demonstrate that an organism does not need many cells to be genetically complex or successful.

Transcripts

You are familiar with jellyfish,

those ethereal creatures

often seen drifting peacefully

through bright blue water at your local aquarium.

You may be less familiar

with some of their stranger relatives.

Relatives I’d like to submit

for weirdest animal of all time.

I’m talking about the myxozoans,

which are microscopic, jellyfish-like parasites.

And when I say microscopic,

I really mean microscopic.

They are multicellular,

but barely.

And some of them, apparently,

can also be found on land.

Not because they washed up on the beach,

but because they like it here.

So come with me and we’ll talk about the bizarre,

land-dwelling parasitic jellyfish…

and what we can learn from them.

[intro]

Myxozoan parasites are well known

for infecting the skin, muscles, internal organs

and whatnot.

Of fish. Don’t freak out.

But also worms.

And sometimes amphibians, aquatic birds,

and pygmy shrews.

But let’s get back to that in a minute.

Like I said, myxozoans are tiny.

They only really have a  handful of cells at any point

during their life cycle.

Some researchers have proposed

they even have a very  unusual way of forming cells,

through a process called endogenous budding.

Which basically means instead of dividing,

their cells form within other cells.

In fact, at a certain point in their life cycle,

one cell is thought to contain another cell

that contains another cell

in the most bizarre molecular  turducken you can imagine.

That alone is pretty strange,

but myxozoans are many  other kinds of strange, too.

In fact they’re so strange that for a long time,

scientists did not know what to think of them.

Because they have a very simple,

microscopic structure,

19th century scientists

thought they were protists –

that’s a mostly dead taxonomic category,

but it would lump them in with amoebas.

But that doesn’t really make sense

because protists were considered

to be single-celled organisms

and these guys definitely have more than one cell…

just not that many more.

They also have stinging structures

called polar capsules ,

which are pretty similar

to a jellyfish’s stinging nematocysts.

While jellyfish use their  nematocysts to catch food,

myxozoans use their polar capsules

to attach to their host.

Stinging structures like these

are only present in cnidarians,

the phylum that includes non-fishy aquatic species

like sea anemones, corals, and jellyfish.

These stinging cells are what  makes cnidarians cnidarians,

so this means scientists were kind of obliged

to conclude that the diminutive myxozoans

were in fact a jellyfish relative.

In 2015, scientists analyzed the genomes

of two distantly related species of myxozoan

and concluded that they were sister species

of Polypodium hydriforme,

another cnidarian parasite.

Meaning myxozoans and Polypodium

share a common jellyfish-like ancestor,

and definitely belong within the cnidarians

next to jellyfish.

It gets even stranger, though.

You could argue that

the tiny, primitive-seeming myxozoans

are ahead of their time…

like by around a hundred million years.

This episode is brought you by

This month’s presidents of science

McLaren Stanley and Charlie Stanley

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Democracy

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Leave your suggestions in the comments

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For being so awesome

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Want to learn more about

Becoming president of science

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Wonderful patron tiers,

Go to patreon.com/scishow

Like I said, myxozoans parasitize fish.

But here’s the thing…

they evolved around 651 million years ago

during the late Cryogenian,

otherwise known as the snowball Earth era.

That snowball was distinctly lacking in fish,

which didn’t start to appear

until around 530 million years ago.

So myxozoans basically evolved  a hundred million years

before the hosts we now know them for.

Now to be fair, myxozoans actually have

two main life stages:

the actinospore and the myxospore.

Fish are an intermediate host for myxozoans,

meaning they act as an in-between host w

here the parasite develops into a form

capable of infecting its primary host.

In this case, the primary  hosts are annelid worms –

the group that includes earthworms.

So their life cycle works like this:

The actinospore infects fish,

and the myxospores

develop inside the fish.

When the myxospores are released,

they go on to infect annelid worms.

The actinospores develop inside the worm,

and then they go on to infect fish.

And so on and so forth.

Wanna know the really fun bit?

Annelid worms also hadn’t evolved

when myxozoans first turned up.

So it seems like the myxozoans

were only infecting some  other kind of invertebrate

during that hundred million or so years

before they had worms,

and fish to also infect.

Which makes them them

pretty opportunistic little critters,

able to adapt to new hosts

and whatever else life throws at them.

Including, evidently, adapting to life on land.

Inside land dwelling animals.

And not just animals that  live on both land and water,

but entirely terrestrial animals

like pygmy shrews.

Scientists aren’t exactly sure

how they’re doing this.

After all, you don’t often see pygmy shrews

frolicking in the waves alongside jellyfish.

In fact, one of the places these infected shrews

have been found is in Hungary,

a nation I feel a need to point out

is landlocked.

There are no beaches.

How are you here, myxozoans.

One theory is that the myxozoans

are infecting earthworms,

which in turn get eaten by the shrews.

Um, except pygmy shrews don’t  eat earthworms very often,

so it’s not even a very good theory.

And it’s not exactly clear how myxozoans

might have made the trip from the ocean

to the underground homes  of landlubbing earthworms,

unless they’ve somehow also learned to fly.

Andor burrow.

And though they are pretty weird …

there’s no evidence of that. Yet.

Kind of the opposite, actually.

Rather than evolving brand new abilities,

myxozoans have dumped the genes

they don’t need

while improving the ones they do need.

Like a lot of parasites,

they have a reduced genome.

This may seem a little counterintuitive,

but having a reduced genome

can be a helpful survival tool.

See, parasites don’t necessarily need genes

for things they can just steal from their host.

At least one species of  myxozoan has even rid itself

of the need to breathe oxygen.

Meanwhile, they’ve also enhanced

some of their genes,

such as the ones they need to cope with stress,

metabolize energy,

and get inside their hosts.

Scientists call this mosaic evolution,

where different parts of the genome

evolve at different rates.

It’s a type of evolution

more often seen in larger animals,

like humans and other mammals.

Mosaic evolution in myxozoans

represents a departure

from the oversimplified traditional view

of parasite evolution

as a process of genetic downsizing. So these seemingly simple animals

are actually helping scientists understand

that an organism doesn’t  need to have a lot of cells

to be genetically complicated  or uniquely successful.

And though you may never admire

one drifting peacefully

through the bright blue  water at a public aquarium…

chasing after a pygmy shrew…

It’s enough to just know they’re out there,

invisibly evolving in sophisticated ways,

finding new species

to infect on land and sea.

Isn’t it?

[ OUTRO ]