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Thank you to Truebill for  sponsoring today’s video.   Truebill helps you reach your financial  goals by canceling unwanted subscriptions,   negotiating bills on your behalf, and  budgeting.

Download Truebill for free   by heading to or by  clicking the link in the video description. Our master of microscopes, James, doesn’t  live particularly close to the sea.   But as we’ve seen in some previous episodes, that  distance has not stopped him from finding ways to   explore marine life.

If he can’t easily get to  the sea, well, the sea can be brought to him in   monthly deliveries of sand and sea water. So among  all the other microbe collections in his home,   James has fish tanks with no fish—just  archives of those marine deliveries.  And some of those tanks have been around  for a while, so you might think that he’d   found everything there was to find in  them. But the sea is always delivering   new surprises, even when it’s been firmly  encased in a fish tank for more than a year.  Recently, one of these old tanks turned  up some surprising tentacled creatures.   At first glance, James thought they might  be a Journey to the Microcosmos favorite,   the hydra…except hydra don’t particularly care  for saltwater.

They are freshwater creatures.  So naturally, James decided to take a closer look,   studying his miniature  creatures under the microscope.   He could see rows of tentacles attached to a pod.   So with this in mind, James began  digging through his books on Cnidaria—the   phylum that hydras belong to—and he narrowed  in on a possible species: the Moerisia.  That name may not mean anything to  you right now. But when James saw it,   it made him realize something else: somewhere  in his tank, there must be a jellyfish.  Now to understand why James had this realization,  you need to first hear a little bit more about how   the moerisia actually lives. An important part of  that is that the moerisia actually has two lives.  Moerisia are a type of hydrozoan, a group of  cnidarians that includes about 3700 species.   While they’re not as well-studied as other famous  hydrozoans like the hydra or the Portuguese   man-o-war, we do know that several Moerisia  species likely originated in the Black Sea   but have since spread widely—even found  in estuaries in the United States.   Their spread has probably been helped by  the fact that they’re able to withstand   a wide range of temperatures and salinities,  making it easy for them to navigate new waters.   They survive on zooplankton like our  favorite crustacean, the copepod.  And as we said earlier, the Moerisia has two  lives.

One, as we’ve been observing, is the   polyp form. And the other, is this…the medusa.  Or as James named it: Squishy the Jellyfish.  The double life is a common hydrozoan trait,  though we should note that Hydras are a notable   exception, living strictly in a polyp form. But for hydrozoans like the moerisia, things   are a little more complex.

When they hatch, they  emerge from an egg as a larva called a planula,   which settles onto the ground and develops  into a single polyp, radially symmetric and   usually attached to objects like plants, shells,  or rocks. And from there, the polyps begin to bud,   forming more and more polyps until there’s a  whole colony of them attached to each other.  Within this colony, polyps can take on different  forms to serve different functions. Some might   take on spiny tentacles that allow them  to defend the colony, while others have   mouths dedicated to eating.

Still others might  be dedicated strictly to reproductive function.  The polyp stage is clearly distinct from the  medusa stage. One looks more like a diffuse tree,   while the other looks more like a bag with  strings dangling off of it. But the real   crux of their difference is not in their  appearance, but in their reproduction.   Because hydrozoans can reproduce in both  stages, just very differently.

As a polyp,   it reproduces asexually, budding off to form  new polyps and eventually to form the medusa.  You can see the polyp actually budding off to  create this next stage of life, that little nub   eventually expanding and releasing  to create what will end up resembling   an inverted polyp…or an umbrella. And  unlike the sessile, anchored polyp,   medusas are free-swimming. Inside its opening  is an inner ring of tissue called the velum,   which helps the medusa move  around by tensing up and relaxing.  For this Moerisia species, the medusa are  phototaxic, meaning they like swimming toward   light.

That’s actually how James found them  in his tank. After identifying the possible   species and reading that they like to swim toward  light, he decided to check the side of the tank   that received all the morning sunshine.  When he did, he found his tiny jellies.  But the main function of this form is  less about light and more about sex.   In Moerisia that have been raised in the lab,  the medusa takes about 8 days to develop gonads.   And when they do, they will  proceed to their next step: sex.  The exact details of how hydrozoans  have sex varies with different species,   and we haven’t been able to find an answer to  exactly how the Moerisia do it in the literature.   But we want to briefly state that  one of the hydrozoan possibilities   is that the male releases sperm into  the water, and the female releases eggs   that chemically attract the sperm to them.  The fertilization is entirely external,   just happening in the water, and eventually that  fertilized egg will develop into a new organism.  Here’s an important question though, why the  double life? Why two different stages that can   each reproduce in completely different ways?

Well,  because asexual and sexual reproduction are both   pretty nifty talents to have on hand. Asexual  reproduction is fast, and it helps polyps build   their local populations quickly. The downside  is that an asexual polyp means limiting both the   scenery and the gene pool.

With that comes the  risk that the environment will shift suddenly   and both the polyp and the entire population  will be unable to adapt fast enough to survive.  But by leading a second sexually reproductive  life as a medusa, the hydrozoan gets to disperse   both physically and genetically. But there is a  cost—after all, sexual reproduction takes time   and special organs and the hope that sperm will  meet an egg. But that is why the hydrozoan doesn’t   strictly rely on sexual reproduction either.

This combined strategy is part of why the Moerisia   has been so successful at traveling beyond  its origins. It gets the best of both worlds,   as an organism that is at times static  and dividing, and at other times swimming   and mixing. Each generation produces a large local  population that can eventually expand and spread.  We should, here at the end however, admit one  thing.

Technically, our Squishy the Jellyfish   is not a “true” jellyfish. To be considered a  true jellyfish, an animal has to be a member   of the class Scyphozoa. And Scyphozoa—like the  hydrozoa—are part of the Cnidarian phylum.

So   while Moerisia and their fellow medusa hydrozoa  do bear a strong resemblance and relationship to   their true jelly relatives, they also have their  own unique structures that make them distinct   (and, alas, not true jellyfish). But in our tank of sea water,   Squishy the Jellyfish will keep his title. He  has worked and traveled a long way to earn it.  Thank you for coming on this journey with us as  we explore the unseen world that surrounds us.  And thank you to Truebill  for sponsoring this video.

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It can even negotiate your bills  for you, while also monitoring your credit   score and automating your savings. And you  can do all of this with just this one app. Download Truebill for free today by  heading to   or by clicking the link in the description There’s a bunch of names coming up on the  screen right now.

They are the people who   make this channel possible. They are our  patrons on Patreon. They are people who saw   chill, weird, interesting microscopy  videos on YouTube and they were like,   “I want to watch more of those.” And then  they watched a few more and decided to go to

So, if you’re looking  for someone to thank, those are the people. If you want to see more from our Master  of Microscopes, James Weiss, check out   Jam & Germs on Instagram or TikTok or pick up his  book, The Hidden Beauty of the Microscopic World. And if you want to see more from us, there’s  always a subscribe button somewhere nearby.