This episode is brought to you by our  patrons over at Patreon.com/journeytomicro.

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Its body   is long with many threads, and it seems more like  a crack on the screen than a living organism. Things may seem quiet and calm, but along  its body, the killer wears its victims. It would be understandable  though if you miss that part.

Those victims have been so engulfed from  within by their enemy, they look more like   unusual leaves or bits of debris. Not an  animal, swimming around the microcosmos. And certainly not a rotifer, caught  in the strands of a predaceous fungus.

If you have been following Journey  to the Microcosmos for some time,   this might sound like a familiar story. A few years ago, we featured  a horror story of sorts,   starring a fungus that spins a web to trap worms. So consider this a proper slasher  movie sequel.

You don’t need to   have seen the original to know what’s happening. But we hope that, when you’re done watching  rotifers suffer untimely ends under strange   circumstances, you’ll want to go back and  watch nematodes suffer a similar fate. Sequels are of course tricky things,   and you run the challenge of  treading a lot of the same ground.

Of course, we’ll have plenty of  violence and death, but fortunately,   this rotifer-packed sequel brings something  else to the table that the nematodes didn’t: sex. But first, the violence and death. There are at least 60 species of fungi that  we know of that like to attack rotifers.

Much of what we know about these species  comes from scientists using rotifers as bait. They take a sample of rotifers and then add a  bit of soil to see if the fungus is in there. And then they watch.

It sounds a little dark when we phrase it that  way, but of course here we are, watching this   Zoophagus fungus that was growing inside of a  humidity chamber as it eats away at a rotifer. From a distance, the fungus seems so dull  and uninteresting. It’s not even moving.

Except, of course, that it is. Inside the  fungus is a flurry of activity, as nutrients   and molecules stream through the cytoplasm and  across the vast expanses of this predator’s body. This rotifer has been caught by a fungus,   flopping from side to side while  the fungus seems to do nothing.

An animal that can swim up  against an organism that can’t… it shouldn’t be a fight. But the rotifer is stuck, caught on  adhesive pegs that line the fungus   and make it impossible for the rotifer to escape. And then there are the hyphae, the threads  of the fungus that grow and lie in wait.

Soon the rotifer will be more like this,  immobilized and ensnared in a cyst. The fungus’ digestive hyphae are embedded within  the rotifer’s body and are eating it from within. It takes only about a day for a fungus  to trap and consume its prey like this,   but it’s still not done.

Within another day, the rotifer will  essentially be filled with hyphae,   some even beginning to poke back  out through the body of its prey. The fungus has just been feeding and feeding. But still, it isn’t done.

Within another day, the hyphae will produce  spores, planting the seed for the fungi’s future— all at the expense of the rotifer. In 1954, one scientist documenting the  various fungal attacks on rotifers within   his lab noticed that they seemed  to have an unusual preference. They seemed to only target rotifers that  have a lorica, which is like a shell.

The soft-bodied rotifers within  their samples seemed to be able   to wander all around the fungi with no problem. That sounds wrong…right? Shells are protective, they keep you safe  from things that are trying to eat you.

But the shells aren’t a deterrent to the fungi. Quite the opposite, in fact. The fungi seem to use the shell as  a place to hook their hyphae into,   using the rotifer’s shield  instead as a tool for invasion.

These fungi also go after another type of rotifer: the bdelloid rotifers, a class of rotifers that  is fairly large, encompassing hundreds of species. These rotifers are known  for one very specific thing: no one has ever observed them having sex. It’s not like they’re shy or anything.

They just don’t seem to do it… (maybe). There’s been some very recent  work sifting through their   genomes that suggests that maybe sexual  reproduction has shown up in their past. But that is a discussion for another day.

For now, the fact remains that no one has   seen bdelloid rotifers actually  engaging in sexual reproduction. Even more curiously, no one has ever seen what  is confirmed to be a male bdelloid rotifer. There are, as far as we know,  only female bdelloid rotifers.

They reproduce through parthenogenesis,   able to make their own egg that will hatch into  a daughter who has only her mother’s genes. Scientists have wondered about  this process for some time,   and how bdelloid rotifers have been able to  survive for millions and millions of years   without the advantages that sexual  reproduction confers on a species— like the ability to introduce variation that  could help them adapt to the world around them,   a world that includes predatory fungi. But that’s also led scientists to wonder  if perhaps these fungi are the reason why   bdelloid rotifers are so singularly  reliant on asexual reproduction.

Because asexual reproduction has at least  one clear advantage on sexual reproduction: it’s faster. You don’t have to spend time finding  a mate or expend energy with all the   various processes involved with courting  and copulating that nature has engineered. That’s potentially useful to an animal  that finds itself on the run from a fungus.

And bdelloid rotifers have another secret weapon. They are extremely hardy, able to turn into a  dormant tun that can survive extreme conditions. That means, they could survive something like  a strong wind, which could blow them away from   their predators and drop them off in a new  home that is hopefully a little less fungal.

So this ability to outrun their predators and   quickly reproduce when they reach a  new, less threatening environment… that’s a potentially powerful combination,  one that could serve to explain why bdelloid   rotifers focus on asexual reproduction  at the expense of anything else. But it is, for now, still just one story for  why bdelloid rotifers live the way they do. The reality is probably even more complex  than what we know now, filled with more twists   than the fungus that hunts this rotifer  and more sequels than we’ll ever know.

Thank you for coming on this journey with us as  we explore the unseen world that surrounds us. The people on the screen right now, that's  right, they are our $8 patron patrons. So thank you so much to everybody who's  ever signed up to be a patron of Journey   to the Microcosmos, especially the people  who are currently signed up right now.

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