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When you picture a jellyfish, you probably think of this.
The jellyfish that many people know and love is the scyphozoan medusa.
But that same animal can also look like this.
That’s what the scyphozoan polyp looks like.
And it’s another stage of this class of jellies’ life cycle
one that often comes before the medusa phase,
like how toddlers come before middle aged people.
And yeah, it looks nothing like the medusa you may be used to seeing.
They look so unrelated that originally,
researchers thought the different stages of the jellyfish life cycle
were completely separate animals.
.
And the more scientists learn about jellies,
the more they realize that these life cycles are even more nuanced than that.
Polyps don’t always turn into medusae and medusa reproduction doesn’t always lead
to little polyps.
The fact is, jellyfish can go for generations without appearing in the form
that you may think of as a jellyfish.
And researchers have some guesses as to why.
When a jellyfish life cycle progresses in a nice linear fashion,
it starts as a larva,
becomes a polyp that hangs out attached to pretty much any ocean surface,
then separates into multiple individuals,
each of which then floats off to eventually mature into a medusa.
The medusae produce offspring that then settle on the ocean floor
and mature into polyps.
But the thing is,
most jellyfish don’t go through this linear life cycle,
progressing from polyp to medusa and back to polyp in the next generation.
Sometimes, the polyps skip the medusa stage entirely
and make more polyps through asexual reproduction.
Other times, the medusae skip the polyp stage
and make more medusae through sexual reproduction.
Researchers think that whether the next generation of jellies
takes the form of medusae or polyps may have to do with resource availability.
See, polyps can sometimes be the more stable life stage compared to medusae,
so when resources are hard to come by,
or when the environment becomes unwelcoming,
jellyfish might err on the side of caution and make more polyps.
And that stability comes, in part, from how they reproduce.
One polyp doesn’t need the help of another polyp to reproduce.
They can release the juvenile form of medusae or grow new polyps on their own.
So they can replace themselves faster than medusae,
making the population more resilient to threats like being eaten into extinction.
It could also have something to do with how long they can stay in that state,
because immature polyps seem to last longer than other stages.
Now, stability is great, so if the medusa stage is less stable,
then why grow into them at all?
It’s still worth it for some jellies to become medusae
because they can reproduce sexually!
So the species is stronger when medusae get the chance to make little baby jellies.
And that’s because sexual reproduction
allows a species to increase its genetic diversity,
so they can adapt to new environments faster,
outcompete other animals, and avoid new predators.
Polyps that make polyps, as stable as they may be,
do it through asexual reproduction.
So they don’t bring those benefits to the sea floor.
Now, you might have heard that some jellyfish can live forever,
which would make all this talk about jellyfish generations a little unexpected.
But that's the hydrozoan jellyfish, a group
that includes at least one species that can actually reverse aging in its cells,
allowing an individual to transition back into a young polyp!
And as far as anyone knows,
that doesn’t happen in the scyphozoan jellies that you see at the beach.
Scyphozoan medusae usually die about a year after transitioning
into the medusa stage of life, because of disease, lack of food,
or many of the other reasons that most things die.
And some researchers thought that’s what happened to all scyphozoan medusae.
. . .
But it turns out some of them live longer!
They just spend the winter on the seafloor.
Researchers have reported medusae being caught in trawling nets in the winter,
which means they can withstand cold temperatures
and don’t need to be in the polyp stage to make it through those months.
It seems that both medusae and polyps can survive
less than optimal conditions as well as thrive under good ones.
So it’s not that the more linear life cycle of the jellyfish is wrong,
just that there are more options and more directions that the jellies
can go in than we once thought.
And some of those options lead to generations of jellyfish that look nothing like the ones
you might picture.
Now, these jellies may look different from how you imagined,
but watching this video might have changed your impression of them.
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