We wanted to let you know here  at the top of today’s episode   that Journey to the Microcosmos is now on TikTok.

You can think of our TikToks like the  fun size candy bar of Microcosmos. You will still find your full length,  full size Microcosmos here on YouTube,   but if you’re looking for a  quick little Microcosmos snack,   you can check us out on TikTok, where  we are @journeytothemicrocosmos.

Given the many fascinating   single-celled organisms that we have seen  on our journey through the microcosmos,   it would be easy to mistake this for yet another  fascinating, spiky ciliate—perhaps something   related to a suctorian, but with an unusually  giant organelle pulsating within its boundaries. But this is not a single-celled organism. This… is an egg, which is  also a thrilling sight for us.

Eggs are incredible. Whether they are kept within  the confines of their mother,   or—as with this one—released into the  world, the egg is all about what comes next. Within its confines are cells that divide  and differentiate, gathering in numbers while   creating their own individual identities that will  become organs and tissue in the animal to come.

And you can see the animal moving around in its  egg, restless and perhaps ready to hatch. And when it does, it will take on the  cumulative identity of all those cells,   in this case, an animal that we call the Gastrotrich. The Gastrotrich has long been a personal favorite   microbe of several members of the  Journey to the Microcosmos crew.

They were one of the first microbes  we dedicated an entire episode to,   and many of us came away charmed by  their hairy, dragon-shaped bodies. But while we were able to see a lot  with the microscopes we had at the time,   James—our master of microscopes—has made some   significant upgrades since then (thanks  in part to the support of many of you). And this means that we are now able to see  gastrotrichs in a whole new light.

So while before, we could see the hair  traveling down the length of their bodies,   and even make out the organs within them,  now it’s like you can touch them. Like if you run your finger along them,   you will feel the prickle of scales and the  feathery brush of hairs against your skin. And to be honest, that was our main  reason for wanting to revisit them today.

But while we are here, perhaps we should revisit  the gastrotrich more generally. If you haven’t watched it before  (or just want a refresher),   feel free to take a moment now  to rewatch it if you’d like. We will still be here when you’re done.

But if you’d rather stick around, the main  thing you need to know is that we titled that   episode “Four Day Old Grandmothers,”  because for some gastrotrich species,   reproduction operates on a very rushed timeline. Those species are members of the order Chaetonida,   which sets them in contrast to the other  order of Gastrotrichs, the Macrodasyida. While almost all of gastrotrichs across  both orders can reproduce sexually,   the Chaetonids have an additional  approach: parthenogenesis.

This means that they are able to form  an embryo from an unfertilized egg. The reason we’re revisiting the specifics of  gastrotrich reproduction is because it might be   related to another fundamental difference between  Chaetonids and Macrodasyoids: their habitats. Chaetonids are largely found in freshwater, while  Macrodasyoids are mostly found in marine waters.

Scientists have long wondered  just how Gastrotrichs found themselves in these different environments. It’s generally accepted that the order  Macrodasyida is older than the order Chaetonida. That suggests that Gastrotrichs began  as an order of marine animals, which   then went through some evolutionary pathway that  transitioned some of them into freshwater life.

There are many questions that  we could ask about that transition. Where did the gastrotrichs’ journey take them? Did this evolution happen  several times, or just once?

Did changes on a geologic  scale—like changes in sea   level—shape these long-lasting  changes within their bodies? Is the journey of Chaetonids  into freshwater complete,   or did any ever venture their  way back to marine environments? Well when scientists used the tools of molecular  phylogeny, they found evidence that the shift   from marine to freshwater life happened just  once, and that it happened fairly quickly.

And it may be that parthenogenesis  made that change possible. If some of those gastrotrichs in transition  had been able to reproduce via parthenogenesis,   they may have had a significant advantage  as they settled in a new environment. Without having to go through  the trouble of fertilization,   they could have quickly expanded their population,  gaining a foothold that allowed them to survive.

Now we don’t know if parthenogenesis  did directly drive the chaetonids’   ability to transition into freshwater, but  there is evolutionary precedent for it. Mud snails shifted to parthenogenetic reproduction  in their own marine-to-freshwater transition. But parthenogenesis is just one of the traits that   might have allowed these species  to spread into less salty waters.

They would have required other adaptations,  changes that would allow them to tolerate   the shifting landscape in the  ecology and geology around them. Scientists have also found that there are   some marine gastrotrichs among the  otherwise freshwater chaetonids. It might seem like this relationship  means those marine gastrotrichs are   remnants of the ancient gastrotrichs  who set the stage for freshwater life.

But that’s not what the genetics say.  Instead, it appears that these marine   gastrotrichs are the result of chaetonids who  decided that freshwater life wasn’t for them,   who returned to the saltier  environments of their ancestors. What drove those gastrotrichs back? Well aside from the fact that there was an evolutionary space for them.

We don’t know. Animals have been navigating new environments  and finding ways to adapt for eons,   each organism always laying the foundation for  new ways of living, building a biology so complex   that even the tiniest bits of it are worth of  the sort of intense, unrelenting investigation   that humans are so very, very good at. Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.

The people on the screen right  now, those are our Patreon patrons. They're the people who make this channel  possible, and who allow us to just really   unleash our unrelenting investigation into  this tiny, tiny world that so much deserves it. We're really excited to continue  that investigation into 2023, and the reason we can do  it, is our Patreon patrons.

So if you want to add your name onto this list,  you can go to Patreon.com/JournytoMicro. If you want to see more from our  master of microscopes, James Weiss,   you can check out Jam & Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.