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Some are longer than a blue whale.

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Others are barely larger 
than a grain of sand.

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One species unleashes one of
the most deadly venoms on Earth.

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Another holds a secret that's behind some 
of the greatest breakthroughs in biology.

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They've inhabited the ocean for at least
half a billion years,

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and they're still flourishing as the sea
changes around them.

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Jellyfish are soft-bodied sea creatures
that aren't really fish.

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They're part of a diverse team
of gelatinous zooplankton,

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zooplankton being animals that drift
in the ocean.

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There are more than 1,000 species
of jellyfish,

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and many others 
that are often mistaken for them.

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A noted feature of jellyfish 
is a translucent bell

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made of a soft delicate material
called mesoglea.

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Sandwiched between two layers of skin,

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the mesoglea is more than 95% water
held together by protein fibers.

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The jellyfish can contract and 
relax their bells to propel themselves.

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They don't have a brain or a spinal cord,

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but a neural net around the bell's
inner margin

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forms a rudimentary nervous system

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that can sense the ocean's currents
and the touch of other animals.

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Jellyfish don't have typical 
digestive systems, either.

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These gelatinous carnivores consume 
plankton and other small sea creatures

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through a hole in the underside 
of their bells.

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The nutrients are absorbed by
an inner layer of cells

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with waste excreted back through
their mouths.

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But the jellyfish's relatively 
simple anatomy

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doesn't prevent it from having
some remarkable abilities.

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One kind of box jellyfish has 24 eyes.

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Scientists think it can see color and form
images within its simple nervous system.

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Four of its eyes are curved 
upward on stalks.

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This allows the jellyfish to peer
through the surface of the water,

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looking for the canopy 
of the mangrove trees where it feeds.

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In fact, this may be one 
of the only creatures

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with a 360-degree view of its environment.

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The jellyfish's sting, which helps it
capture prey and defend itself,

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is its most infamous calling card.

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In the jelly's epidermis,

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cells called nematocysts
lie coiled like poisonous harpoons.

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When they're triggered by contact,
they shoot with an explosive force.

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It exerts over 550 times the pressure
of Mike Tyson's strongest punch

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to inject venom into the victim.

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Some jellyfish stings barely tingle,

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but others cause severe skin damage.

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The venom of one box jellyfish
can kill a human in under five minutes,

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making it one of the most potent
poisons of any animal in the world.

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Other jellyfish superpowers 
are less lethal.

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One species of jellyfish glows green
when it's agitated,

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mostly thanks to a biofluorescent compound
called green fluorescent protein,

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or GFP.

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Scientists isolated the gene for GFP

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and figured out how to insert it
into the DNA of other cells.

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There, it acts like a biochemical beacon,

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marking genetic modifications,

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or revealing the path 
of critical molecules.

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Scientists have used the glow of GFP
to watch cancer cells proliferate,

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track the development of Alzheimer's,

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and illuminate countless other
biological processes.

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Developing the tools and techniques
from GFP

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has netted three scientists
a Nobel Prize in 2008,

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and another three in 2014.

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But it's jellyfish who may be the most
successful organisms on Earth.

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Ancient fossils prove that jellyfish
have inhabited the seas

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for at least 500 million years,

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and maybe go back over 700 million.

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That's longer than any 
other multiorgan animal.

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And as other marine animals 
are struggling to survive

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in warmer and more acidic oceans,

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the jellyfish are thriving,

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and perhaps getting even more numerous.

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It doesn't hurt that some can lay
as many as 45,000 eggs in a single night.

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And there's some jellyfish 
whose survival strategy

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almost sounds like science fiction.

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When the immortal jellyfish is sick,
aging, or under stress,

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its struggling cells can change
their identity.

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The tiny bell and tentacles deteriorate

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and turn into an immature polyp

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that spawns brand new clones
of the parent.

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As far as we know, these are the only
animals who found a loophole

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when facing mortality.

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That's pretty sophisticated for species
that are 95% water

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and predate the dinosaurs.