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Host: Deboki Chakravarti
Executive Producer: Hank Green
Writer: Deboki Chakravarti
Producer/Editor: Matthew Gaydos
Music: Andrew Huang
Footage: James Weiss

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Journey to the Microcosmos is a Complexly production.
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Thanks to KiwiCo for supporting this  episode of Journey to the Microcosmos!   Click the link in the description for  50% off your first month of ANY crate!  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.  And thank you again to KiwiCo  for supporting this episode.  KiwiCo creates super cool hands-on projects for  kids that make learning fun!

They have eight   subscription lines, each catering to different  age groups and topics, and each box is designed by   experts and tested by kids. And 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!  With their Tinker Crate, intended for kids  9 and up, your little one can put together   this really cool color-mixing LED crystal  that will teach them how to use red, blue,   and green LEDs to explore mixing colors of light  so they can make whatever color crystal they want.  So if you want to keep the fun  and learning going all year long,   head on over to or  click the link in the description for 50%   off your first month of ANY crate! The people on the screen right now,   they are our patrons.

If you like what we  do here, these are the people to thank and   if you would like to become one of them,  you can go to  If you want to see more from our Master  of Microscopes James Weiss, you can check   out Jam & Germs on Instagram or pick up his new  book, The Hidden Beauty of the Microscopic World.  And if you want to see more from us, there’s  probably a subscribe button somewhere nearby.