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In our Season 2 debut, we're diving back into the world of our favorite little water bear friends!

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SOURCES:
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https://academic.oup.com/zoolinnean/article/178/4/856/2691448
Welcome to season two of Journey to the Microcosmos, and we are starting things off with a pun very much intended--a bang.

Or more accurately, a multitude of bangs. And we should warn you, it gets weird: there's poop!

There's molted skin! There's the inexhaustible spirit of the tardigrade in flagrante! So, how did we get here?

Well, sex, obviously, if we're talking in the grand biological scheme of things. It is for some, an evolutionarily advantageous way of making offspring. But we're talking about tardigrade sex right now in part because of a pregnant water bear that we started tracking in an episode last season.

Even though the mother-to-be was completely motionless, we were able to see the heads of baby tardigrades forming in the eggs, and we were excited to record and share the moment of all those moss piglets hatching. Unfortunately, nature had different plans, and we'll talk about that more in our next episode. But James, our master of microbes, has been hard at work documenting the lives of the tardigrades in his care.

And that means, not only have we been able to find more tardigrades hatching, we've seen some particularly vivid demonstrations of the steps that came before. These hardy animals can reproduce asexually via parthenogenesis, but our recent samples have been full of frisky water bears, and that is what we are going to focus on today. Sure, tardigrades contain an immense capacity for survival that astounds and boggles the mind, and it is a joy and a privilege to share the biology and visual wonders of their lives alongside all of the other tiny organisms we talk about.

But sometimes, what the job asks for, maybe even demands, is that we film nature as it occurs. This is not what James expected when he ventured out to a river to collect some mud, even when it turned out that he had hit a microbial jackpot and counted at least 15 tardigrades on the first slide he prepared from the sample. But as he continued to watch, he saw these two smaller tardigrades, both mysteriously drawn to the underbelly of a larger one.

They seemed to be trying to reach into the belly with their piercing mouths, but the why's and the how's and the who's were all unclear. Their goal was mysterious. And besides, tardigrades rarely demonstrate such a focused interest on much of anything.

But twenty minutes later, those two jabbers were still at it. Their determination became much more understandable when the eggs inside the bigger tardigrade become visible, marking her as a female holding a number of unfertilized eggs, and marking the smaller tardigrades as males vying to do the fertilization in a microscopic love triangle (or as we call it at the Microcosmos headquarters, a "ménage à trois-digrade"). Was that too much?

Who knows? So just how focused are these male tardigrades? Well, remember that time we watched a solitary tardigrade poop?

What a nice, surprisingly cute and relaxing clip that was. Well, now, let's watch a large female tardigrade defecate with apparently no care at all for her suitors, and watch those two continue poking the water bear anyway, undaunted by the cloud of poop around them. But even tardigrades surrender eventually, and after thirty minutes of this poking and prodding, one of the males made a graceful exit, perhaps having lost out on the mating battle.

With the path clear, the remaining male got a good hold onto the female's cuticle, sticking with her for a whole hour in their joint endeavor to fertilize her eggs. This couple was not the only one we observed in action. In fact, this sample was full of tardigrades mating.

And the behavior we observed was consistent, which is important scientifically. Given the collective fascination with tardigrades, it might surprise you to learn that their sex lives have not been well-documented. Perhaps the most thorough study we were able to find is a 2016 paper that was conducted by scientists at the Senckenberg Museum of Natural History in Görlitz, Germany.

It was published in the Zoological Journal of the Linnean Society, and their work chronicles the mating behavior of Isohypsibius dastychi, a different species from the one we are observing here. But their work was still a helpful basis from which to understand the various oddities and necessities of water bear sex. The adulthood of tardigrades is often defined by the number of molts it takes for them to reach sexual maturity.

For example, the female I. dastychi develops eggs during her third molt. But at this stage, the female doesn't shed off their molted outside, called the exuvia. They molt their cuticle...but then they stay in it...like wearing your shed skin like a coat, and why not?

They stay in there, along with their eggs. When male tardigrades were added to the mix, they made a beeline for these egg-bearing females. In the exact technical terms used by the scientists in the paper, the process starts with the male "bringing his cloaca close the mouth opening in the exuvia." And thus begins a process of "mutual stimulation," the female uses her stylets and sucking pharynx to prod at the male's abdomen as part of a mating process that will take, similar to our tardigrades, about an hour.

It all ends, of course, with ejaculation, but how that fertilization occurs can vary. In some species, sperm might enter through an opening in the exuvia. In others, the male might use their stylets to poke holes in the shed cuticle that they then deposit their sperm into.

One species has been seen using both of those methods. The observations that have been documented are that challenging combination of rare and different, where it's not clear how everything fits together. And in cases like the species we're looking at right now, that means the mechanisms actually remain unclear.

The sexual habits of this species are undocumented, this is science, right here, you’re watching it. As for when and how the sperm is deposited, either we weren’t able to film the exact right moment, or it happened so subtly that we missed it. In short, it’s different for different species, and it is not well studied and even though we had a slide full of frisky tardigrades, it was still a challenge to observe.

Sometimes, here at Journey to the Microcosmos, we end our episodes talking about how observing the microcosmos gives us new and better perspectives on the whole rest of the world. But sometimes, we’re just excited to have the opportunity to see something so few people have seen, and that’s just interesting… because it is. And hopefully this video will be part of a hopefully ongoing tradition to record tardigrade mating.

And next week, we’re going to be back with the fruits of their labor, to show you the highs and the lows of tardigrade birth. Thank you for coming on this journey with us as explore the unseen world that surrounds us. Journey to the Microcosmos is produced by Complexly.

If you want to keep imagining the world complexly with us, check out the Crash Course Business:. Entrepreneurship Learning Playlist hosted by Anna Akana. Over 17 episodes, she’ll explore how to take an idea and grow it into a thriving business.

The first video can help you figure out if you want to be (or already are) an entrepreneur. The link for the playlist is in the description. Also thank you so much to all of our patrons on Patreon for helping us be able to spend so much time researching and filming and observing these little beauties.

We could not do it without you. If you want to see more from our Master of Microscopes James, check out Jam and Germs on Instagram, and if you want to see more from us, you can go ahead and use your stylets and sucking pharynx to prod at that subscribe button.