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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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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
Patreon.com/journeytomicro. 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.