Jellyfish 101 | Nat Geo Wild

Jellyfish can be found all over the world,

from deep oceans to shallow coastal areas.

They've been around for hundreds of millions of years

boneless, brainless and bloodless.

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

Jellyfish aren't actually fish.

With thousands of different species in two different biological phyla,

jellyfish is more of a broad term than anything else.

Unlike fish, which have backbones,

jellyfish are invertebrates.

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

called mesoglea.

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

to clear up the confusion.

There is an immortal jellyfish.

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

and reproduce asexually by splitting into two or cloning.

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

The turritopsis dohrnii has earned the moniker the immortal jellyfish

for being able to undergo a process called transdifferentiation.

An adult or juvenile under stress instead of dying

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

It is the only animal in the world

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

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

Jellyfish stings are a common fear of beachgoers worldwide

and with good reason.

There are an estimated 150 million jellyfish stings reported annually.

And while not all jellyfish have stingers,

the Australian box jellyfish or chironex fleckeri

is considered the most venomous marine animal in the world.

Its deadly tentacles can grow up to 10 feet long.

Humans and other animals that are unlucky enough to get stung

may experience paralysis, cardiac arrest

and even death within just a few minutes.

Jellyfish are 95 percent water.

To put this in perspective,

the average adult human male is about 60 percent water

but jellyfish are much simpler than humans.

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

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

and a simple digestive cavity

which serves as a stomach intestine and esophagus.

If a jellyfish washes up on the beach,

it will mostly evaporate due to the high water content.

Groups of jellyfish are called blooms, swarms or smacks.

Jellyfish blooms can form quickly

and scientists think that the jellies congregate for mating purposes.

In extreme cases, the bloom can be so dense

there are more jellyfish than water in a given area.

They can be as small as a cubic meter

or cover hundreds of square miles.

While these blooms are natural,

they've caused problems all over the world

such as clogging fishing equipment in Mexico,

destroying Chinese ships

and closing beaches in Australia and Europe.

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

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

that we haven't yet discovered.