Thanks to KiwiCo for supporting this  episode of Journey to the Microcosmos!   Click the link in the description to  learn more and for a special offer.

The paramecium is the consummate model organism.  It’s a protozoan that is both easy to grow   and easy to study, which means that  microbiologists have been able to learn   all sorts of secrets about eukaryotic life  by watching them. You might even call the   paramecium a hero in our pursuit of knowledge.

But just as heroes in stories have enemies,   the paramecium has its own foe. And it  is this strange critter. No, not the big   eggplant-shaped looking thing.

The smaller one,  the one that looks a bit like a swimming okra.  This is didinium, though it wasn’t always called  that. When Otto Friedrich Müller first described   it in 1786, he thought he was observing a  Vorticella, and so he named it Vorticella nasuta.   Almost a century later, Samuel Friedrich  Stein would change its name to Didinium.  Like its paramecium foil, the didinium is a  ciliate. But at first glance, it doesn’t seem   like it should be much of a problem.

For  one thing, paramecium can just get so much   larger than a didinium. Plus, the didinium is   just weird-looking. In fact, between the  paramecium and the didinium, the didinium is   the one that actually looks like food.

We compared  it to an okra earlier, but at other times it looks   more like a stretched out acorn. And the pointier  bit looks like a jalapeño stem. Of course,   it is not a stem, it is the didinium’s proboscis,  which means that is the front of the organism.  And there’s still plenty of weird to go.  The didinium has a long macronucleus that   curves into a sort of figure eight shape.  And circling the organism’s body are two   rows of cilia called pectinelles  that seem almost randomly placed.   One row sits at the border of the  proboscis and the rest of the body,   while the other clinches the middle like a  loose belt.

It’s a little bit tough to see   the pectinelles in their full fringy glory with  our microscope—they look more like 4 bundles of   hair on the sides of the didinium. But you can see  what it looks like better in this illustration.  And those cilia give didinium both its  very fast and very strange movement.   When the didinium moves, it really moves—but  not necessarily in a way that seems obvious   to us at first glance. Just trying to follow  it with a camera feels a little stressful.   The first time James found one, it slipped  so quickly in and out of view that he barely   caught a glimpse.

He had to switch to a lower  magnification so he could zoom out and see it.  But for the didinium, this seemingly  out-of-control movement is very deliberate. It’s   constantly rotating in a clockwise fashion, but  also leaning towards one side so that it’s moving   in a spiral path. In the early 20th century,  scientists added ink to jelly just so they   could slow the didinium down and follow this path.

The microbe’s seemingly aimless trajectory helps   the didinium cover a wide range space in a  short amount of time. Think of it compared   to another skilled hunter we watched recently, the  lacrymaria olor, which sends its neck out into all   directions to maximize the odds of encountering  something. The didinium is doing something similar   but instead of extending its neck, it’s sending  its whole body into different directions.  And that speed is a big reason why didinium is  so effective.

As it zips around the microcosmos,   it will bump into things that don’t  happen to be food. And when it does,   it politely backs up and moves on. But if its proboscis makes contact with something   edible, then the didinium goes to work seizing  its prey with its proboscis, paralyzing it, and   then eating it.

And while it is known to consume  other organisms, what didinium really really   likes to eat are paramecium. It doesn’t matter  that a paramecium might be six times its size,   the didinium is here for a meal and it will gladly  expand itself around the paramecium to swallow it.  But as you can see, this is not a smooth process.  The didinium is still moving and bumping around as   it consumes the paramecium, and for a while, it’s  hard to even tell where predator ends and the prey   begins. The only real didinium-like thing about  it is the spiral path it still takes as it eats   and eats and eats.

But eventually it settles  back into its more awkward-looking self.  So incredible is the didinium’s appetite  that scientists have observed it eating   a pair of dividing paramecium. They’ve  also occasionally been observed   exploding from eating too much. So  there are hazards to this lifestyle.  But while the didinium is extremely talented at  hunting paramecium, and also capable of hunting   other organisms, it’s actually pretty bad in some  of its other conquests.

When didinium try to eat   rotifers, they’re stymied by the relatively tough  exterior. And other ciliates like stentors are   too large and too tough and too active  for the didinium to do much damage to.  And paired with its favorite prey, the didinium is  its own model system. In the early 20th century,   mathematicians came up with equations to describe  how populations of predators and prey oscillate   in response to each other.

More predators means  less prey, which then means fewer predators and   more prey. Lather, rinse, and repeat, and you  get a cycle that shapes relationships between   species in many different environments. And in attempting to see if they could   re-create these equations that were for the  macroscopic world in a microscopic environment,   scientists in labs turned to paramecium and  didinium to act as a model predator-prey system.  It turned out to be not so straightforward  to mimic nature in the lab.   When scientists first started watching didinium  and paramecium in a flask, the didinium would   inevitably consume the paramecium to extinction,  in turn driving their own deaths.

It was only by   building on these experiments and finding ways to  adjust the interactions between predator and prey   that they could reconstruct the  oscillations we expect in nature.  Of course, that doesn’t mean they directly  copied what nature does. How could they   with an experimental design meant to strip  many of the confounding elements of ecology   from their observations? But their work still  provided insights into how organisms interact.  It’s like a story: incapable of holding the vast  entirety of our existence, and yet still very   useful in its narrowness.

That the consummate  model organism has such a consummate model   predator then is quite fortunate for us, even  if for the microcosmos, it’s just another story.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you again to KiwiCo  for supporting this episode.  KiwiCo creates super cool hands-on projects  for kids that make learning fun! With a KiwiCo   subscription, each month the kid in your life  will receive a fun, engaging new project which   will help develop their creativity and confidence!  They have eight subscription lines, each catering   to different age groups and topics, and each  box is designed by experts and tested with kids.  In their Eureka Crate, intended for ages 14  and up, you can get this 2-in-1 lantern kit   that lets you construct a camping lantern  that also transforms into a flashlight.  So if you want to give a gift that keeps  the fun and learning going all year long,   a KiwiCo subscription delivers STEAM discovery  long after the holiday decorations are put away. So head on over to kiwico.com/journey50  and use the code “journey50” or click   the link in the description for 50%  off your first month of ANY crate!

And thank you, as always, to our patrons, all of  the people on the screen right now. You’re the   people that keep this show happening. If you want  to become one of those people and support this   show, you can go to Patreon.com/journeytomicro If you want to see more from our Master   of Microscopes James, check  out Jam & Germs on Instagram.  And if you want to see more from us, there’s  always a subscribe button somewhere nearby.