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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.
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