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Jellyfish can be found all over the world,

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from deep oceans to shallow coastal areas.

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They've been around for hundreds of millions of years

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boneless, brainless and bloodless.

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Jellyfish are some of the most diverse and fascinating creatures in the sea.

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Jellyfish aren't actually fish.

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With thousands of different species in two different biological phyla,

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jellyfish is more of a broad term than anything else.

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Unlike fish, which have backbones,

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jellyfish are invertebrates.

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Jellyfish got their common name from the jelly-like material they're made out of

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called mesoglea.

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In recent years, scientists have started using the umbrella term sea jellies

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to clear up the confusion.

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There is an immortal jellyfish.

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Jellyfish can reproduce sexually by releasing sperm and eggs into the water

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and reproduce asexually by splitting into two or cloning.

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But at least one jellyfish can actually reverse the aging process.

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The turritopsis dohrnii has earned the moniker the immortal jellyfish

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for being able to undergo a process called transdifferentiation.

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An adult or juvenile under stress instead of dying

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can revert back to a polyp and begin the life cycle all over again.

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It is the only animal in the world

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that is known to be able to reverse its life cycle.

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Box jellyfish are the most venomous marine animals in the world.

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Jellyfish stings are a common fear of beachgoers worldwide

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and with good reason.

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There are an estimated 150 million jellyfish stings reported annually.

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And while not all jellyfish have stingers,

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the Australian box jellyfish or chironex fleckeri

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is considered the most venomous marine animal in the world.

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Its deadly tentacles can grow up to 10 feet long.

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Humans and other animals that are unlucky enough to get stung

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may experience paralysis, cardiac arrest

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and even death within just a few minutes.

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Jellyfish are 95 percent water.

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To put this in perspective,

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the average adult human male is about 60 percent water

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but jellyfish are much simpler than humans.

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They don't have hearts, blood, brains or bones.

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Jellyfish do have very basic sensory organs in their bells and tentacles

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and a simple digestive cavity

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which serves as a stomach intestine and esophagus.

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If a jellyfish washes up on the beach,

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it will mostly evaporate due to the high water content.

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Groups of jellyfish are called blooms, swarms or smacks.

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Jellyfish blooms can form quickly

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and scientists think that the jellies congregate for mating purposes.

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In extreme cases, the bloom can be so dense

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there are more jellyfish than water in a given area.

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They can be as small as a cubic meter

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or cover hundreds of square miles.

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While these blooms are natural,

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they've caused problems all over the world

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such as clogging fishing equipment in Mexico,

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destroying Chinese ships

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and closing beaches in Australia and Europe.

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There is still so much to learn about these floating mysterious creatures.

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Scientists believe that there may be as many as 300,000 species of jellyfish

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that we haven't yet discovered.