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These little moving domes are a ciliate called kerona pediculus. That’s “kerona,” with a k-e,
not “corona” like the beer…or the virus. These are much cuter, especially as they
climb up and down these stalks like it’s their own personal jungle gym1.
But it’s not exactly a jungle gym open for the public. Just ask this nauplius. Oh
wait, you can’t. Because those stalks are the tentacles of a hydra, and getting close to them
was the last mistake this nauplius ever made.
This isn’t our first time diving into the world of hydras. But there were a lot of details about
them that we could only talk about before, we couldn’t actually see them. But now, with the
upgrades we’ve made to our microscope, there’s a whole new world inside of hydras for us to see.
Like these nuclei, lying inside what we’re pretty sure are stem cells, which are the
source of the hydra’s incredible regenerative capabilities. And not only are they constantly
giving birth to new cells inside the hydra, they may have also given birth to the field
of experimental zoology in the 18th century. In 1744, the scientist Abraham Trembley
immortalized hydra in our collective scientific knowledge when he published his book:
“Mémoires pour servir à l'histoire d'un genre de polypes d'eau douce, à bras en forme de cornes.”
If you, like me, have not studied French in more than a decade, that translates to
“Memoirs concerning the natural history of a type of freshwater polyp with arms shaped like horns.”2
Yes, what we call tentacles today were once called horn-shaped arms.
But in French, so it sounded elegant.
Trembley was not the first person to describe hydra. For you frequent viewers,
you probably won’t be surprised to hear that Leeuwenhoek got there before him.
But when Trembley first saw his so-called polyps, he didn’t know what they were. He
thought they were a plant because they were just sitting there, their horn-shaped arms waving
like branches attached to a stationary body3. And what seemed to further support his argument
was the fact that different polyps had different numbers of arms. And that might not seem striking
to us, but think of the animal species you know. Barring any sort of injury or disease,
we think of our cats all having four legs, ants all having six legs, and so on. But
plants can have different numbers of leaves and branches, even when they’re the same species.
Trembley decided to see if there were any other plant-like qualities to these polyps.
And so he cut one in half. He figured that if those cut halves grew back out,
that was one more check in the “plant” column4. And so it didn’t surprise him when he saw the
halves regenerate. It was kind of like using plant cuttings to propagate new plants.
It’s very cool, but also not out of the ordinary in the world of plants.
So it was weird when he saw his supposed plant catching some prey,
including a millipede that got tangled up in the hydra’s arms. And not only were the
polyps capturing prey, they actually seemed to move around on occasion. Which meant that this
polyp wasn’t a plant after all. It was an animal. And so that meant that the regeneration Trembley
had observed was actually quite out of the ordinary. In fact, it ran contrary
to the initial assumptions that had driven Trembley to bisect his polyps to begin with,
the assumption that only plants could regenerate. If the hydra was an animal,
and the hydra could regenerate, then yeah…it looked like some animals could regenerate too.
And going beyond the spectacle of regeneration, Trembley had found an animal that could make
more of itself without having to mate. We know that hydras aren’t the only animals
that can reproduce asexually. They’re not even the only animals that can regenerate.
But Trembley’s observations were novel at the time he made them,
and our subsequent knowledge is built on his observations and techniques. And that’s
why some scientists argue that Trembley’s work marked the foundation of experimental zoology.
If you think about it, at every turn of Trembley’s discovery lies the hydra’s tentacles. When he
decided to cut the hydra in half, it was because of the discrepancies he’d found when counting
their tentacles. And when he later rejected his own hypothesis that the hydras were plants, it was
because he saw their tentacles capturing prey. When you look at hydras, their tentacles are
probably the main thing that stand out because otherwise, for an animal, they’re quite simple.
They’re basically a bag with streamers attached. And yet despite that simplicity, hydra are very
good at catching prey, and also very good at not becoming prey. And that is all due to the
power of their tentacles and the special cells inside of them called nematocytes.
Again, thanks to our microscope upgrades, we can see those nematocytes more clearly than we have
before, and we can even make out the special vesicles inside of them called nematocysts,
which discharge at incredible acceleration to punch through targets when activated5.
The nematocysts aren’t just for punching though. They’re also loaded with a
paralytic venom that can be fatal, as this nauplius here is experiencing.
If this all sounds and looks kind of brutal,
it is. But it’s also kind of elegant. If you take a closer look,
you can see that some of the nematocysts start to look a bit different from each other.
That’s because hydra have four different types of nematocysts, each with their own job: stenoteles
for paralyzing prey, desmonemes for ensnaring it, holotrichous isorhiza for protecting against
predators, and atrichous isorhiza for helping the hydra cartwheel around the microcosmos6.
Nematocysts are multifunctional, powerful, and also very rare —so rare that they’re restricted
to the phylum Cnidaria, which in addition to hydra, also includes jellyfish and sea anemone.
That doesn’t stop other animals from trying to get some nematocysts of their own though.
One species of flatworm is known to not only get past the hydra’s nematocyst defenses,
they’ll kill the hydra and steal those nematocysts for themselves, decking themselves out in the
molecular artillery of their deceased prey. So if the nematocysts are so deadly,
and the hydra tentacles are so full of them, how do ciliates like kerona pediculus survive in this
chemical and physical minefield? Well, we don’t know.
Kerona pediculus aren’t immune to the toxins in the nematocysts,
so they must somehow be sidestepping the issue altogether. But we don’t know how they do that.
Now, it’s possible that the Hydra are simply allowing the Kerona to live on their bodies.
You see, they’re not exactly trigger-happy with their nematocysts.
They do have some level of control here to make sure the nematocysts are deployed correctly.
Some of those controls make hydra more effective hunters.
For example, while nematocytes respond to physical stimulation,
if there’s a chemical hint of prey nearby, the amount of physical stimulus needed to
activate the stenoteles and desmonemes goes down, getting hydra on the alert and ready to attack.
And Hydra must also have some way to distinguish between predator and prey
because they’re able to turn off their defensive nematocysts while capturing their next meal.
Plus, if the hydra has caught something, some of the extract from their food will actually
turn off the locomotive nematocysts to keep them from moving away during their successful,
stationary hunt. So clearly the hydra has
ways to regulate its nematocytes, and maybe the kerona is taking advantage of one of those
mechanisms. Maybe the hydra is too. We don’t know, but for the kerona,
I guess it doesn’t matter. Instead of being targeted by the nematocysts, the ciliates
are protected by them too as they scavenge food from the surface of their gracious host.
And so the kerona pediculus climbs and climbs, not knowing what a deadly horn it ascends.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
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