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https://link.springer.com/chapter/10.1007/978-1-4020-8239-9_5
https://www.semanticscholar.org/paper/The-karyorelictids-(Protozoa%3A-Ciliophora)%2C-a-unique-Foissner/da89b9cf926f28092dc3a02a5bf31015338d2bf8
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5831164/
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Some of the ciliates we find in the  microcosmos have a sort of stoutness to them,   like the declarative trumpet shape of the  stentor or the oblong of the paramecium.  But then you have this little weird fellow,  which looks more like a twisty, slithering worm   than a unicellular organism.  This is a trachelocercid, and no,   I do not know if I am pronouncing that entirely  correctly. It’s a member of a class of ciliates   called the Karyorelictea.

We’ll be showing you  a few different trachelocercids today, though   we can’t specify their genera any further without  using other experimental and microscopy techniques   to see the smaller structures inside them. What they all have in common though   is that they come from a place that we have  not really visited yet on our journey through   the microcosmos. They come from the beach.

James, our master of microscopes, hired two PhD   students to help him with a very important task:  every month, they bring him wet sand from the sea.   Yeah, maybe some of us might get a subscription  box filled with items related to our favorite   hobby. James gets a monthly subscription of sand. And not just a little bit of sand here or there.   He gets liters and liters of it that he then  sifts through to find the microbes living inside.  They’re tucked away in the space  between the grains of sand,   in pockets of water called interstitial water.

And while for many of us, a day at the beach might   sound like a nice, relaxing vacation, things are  very different for the microbes who live there.  Those little bits of interstitial water are a  chaotic habitat. Just imagine your life at that   scale, surrounded by sharp pieces of sand that to  you probably would look more like giant boulders.  But imagine that those rocks aren’t just gigantic,  they’re moving in a constantly shifting landscape   of sedimentary rubble, a cutting maze that’s  always changing as waves crash upon you.  To survive, you can’t just rely on  adapting in the moment. You have to rely   on countless generations of adaptation and  evolution that have made it possible for you   to respond and survive in the world around you.

And for the karyorelictids, evolution has   provided them with a number of tools to  handle their tumultuous surroundings.  Most karyorelictids live in the  sands of marine environments.   The notable exception are species of  loxodes, which live in freshwater.  You’ve probably noticed that the  trachelocercid, while very worm-like,   still has some of its own distinct shapes. If we  were asked to compare it to another ciliate, we   might look at the lacrymaria olor, with its neck  similarly extending and seeking and retracting.  And like the lacrymaria, the focus of the  karyorelicted’s pursuit is probably food. They   particularly enjoy a diet of other organisms  like bacteria, diatoms, and even rotifers.  But the karyorelictea doesn’t just get to focus  on hunting.

In a world that is always changing,   the ability for the karyorelictea to  twist and turn their elongated body   helps them navigate their uncertain terrain and  the narrow corridors that fill it. It also helps   that some species can grow as large as 5 mm long. Karyorelictids can also use their cilia to attach   themselves to grains of sand, giving themselves  something to anchor onto if needed.

But even then,   a little bit of damage seems inevitable. I  mean, this is sand and waves. That can hurt,   even when you’re not a microbe.

But the karyorelictids, they have   a solid back-up plan: regeneration. It’s a classic amongst microbes.   You cut one piece, and the organism grows  that piece back, stitching itself anew. In   the case of the karyorelictea, some species can  regrow parts of their body within 3 or 4 hours.  A lot of the traits we’re describing in the  karyorelictids are traits shared by other ciliates   that live in their own environments, but just  refined through generation upon generation of   interstitial life.

But there is something that’s  a bit more unusual, their nucleus doesn’t divide.  Like other ciliates, the karyorelictids  have macronuclei and micronuclei,   sometimes multiples of them. The macronuclei provide the   genetic template for the cell’s day-to-day  life, while the micronuclei provides the   genetic material that will be used and passed  on during sexual reproduction. (We talked about   this process more in our recent conjugation  episode if you’re curious how that works.)  And for most ciliates, when the organism  divides, the macronucleus divides with it,   ensuring that the new daughter cells each  have their own macronucleus. But this is   not the case for the karyorelictids.

When they divide, their macronuclei   don’t. So the daughter cells use their micronuclei  to make a whole new macronuclei. Karyorelictids   are usually observed with multiple macronuclei,  which is probably a way to ensure that   there’s at least some functional nucleus  around while the organism is making a new one.  It’s not clear what this strange macronuclei  behavior and its specificity to karyorelictids   says about these organisms and their evolution.  To some scientists, it means that maybe   karyorelictids mirror some kind of  ancestral ciliate state of nucleus   non-division, while others consider it  a potential evolutionary add-on that   helps offset mutational damage in the organism.

One of the challenges of being able to study their   nuclei is the fact that most karyorelictid species  have been difficult to cultivate in the lab. There   is something a little frustrating but also very  true to our attempts to understand biology,   that these organisms are so difficult to  house in the well-fed comfort of a flask,   and yet thrive in the harsh reality that  makes them so fascinating to begin with.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you to Brilliant  for sponsoring this episode.  Brilliant has courses about science, engineering,  computer science and math. And while we may not   be fighting to survive between giant grains of  sand as waves of water crash down around us,   we are constantly experiencing other  forms of waves in our everyday life.

Right now, as you’re listening to this  video, sound waves are coming out of your   speakers or headphones. And it may be hard  to think of those waves in the same way you   think about waves on a sandy shore, but with  courses like “Waves and Light” on Brilliant,   you can learn what sound and  water waves have in common.  You’ll learn the basics like, what  a wave actually is, how they travel,   and by the end you’ll have a greater understanding  of how waves work in relation to everything from   noise cancelling headphones to earthquakes.  It’s a wonderful way to develop a more robust   understanding of how our universe works. The courses are designed to be hands-on with   interactive quizzes and guided problems with  explanations.

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