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You're swimming in the ocean
when something brushes your leg.

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When the tingling sets in,

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you realize you've been stung 
by a jellyfish.

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How do these beautiful, gelatinous
creatures pack such a painful punch?

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Jellyfish are soft 
because they are 95% water

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and are mostly made of a translucent 
gel-like substance called mesoglea.

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With such delicate bodies,

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they rely on thousands of venom-containing
stinging cells called cnidocytes

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for protection and prey capture.

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Even baby jellyfish, 
the size of a pencil eraser,

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have the ability to sting.

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Larval jellyfish, ephyrae, look like
tiny flowers pulsating in the sea.

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As they grow, they become umbrella-shaped
with a bell at the top

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and descending tentacles 
around the margin.

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The largest species of jellyfish,
the lion's mane,

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has tentacles that can extend 
more than 100 feet,

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longer than a blue whale.

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These tentacles contain 
most of the stinging cells,

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although some species have them
on their bells, too.

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Venom is ejected via a nematocyst,

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a whip-like hollow tubule,

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which lies coiled under 
high osmotic pressure.

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When mechanical or chemical stimuli
activate an external trigger,

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the lid of the cell pops open
and sea water rushes in.

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This forces a microscopic barbed
harpoon to shoot out,

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penetrate and inject 
venom into its victim.

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Nematocyst discharge can occur
in less than a millionth of a second,

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making it one of nature's fastest
biomechanical processes.

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Nematocysts can continue
to fire even after a jellyfish has died,

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so it's important to remove
lingering tentacles stuck to the skin.

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Rinsing with vinegar will usually render
undischarged nematocysts inactive.

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Seawater can also help 
remove residual nematocysts.

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But don't use fresh water
because any change in salt balance

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alters the osmotic pressure 
outside of the cnidocyte

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and will trigger the nematocyst to fire.

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That's why urinating on the affected area,
a common folk remedy,

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may do more harm that good,
depending on the composition of the urine.

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Most jellyfish stings 
are a painful nuisance,

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but some can be deadly.

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An Indo-Pacific box jelly,
also called a sea wasp,

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releases venom which can cause
contraction of the heart muscles

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and rapid death in large doses.

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There's an anti-venom,
but the venom is fast-acting,

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so you'd need immediate 
medical intervention.

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Despite the impressive power
in their tentacles,

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jellies aren't invincible.

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Their stinging cells are no match
for the armor of thick-skin predators,

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like the leatherback turtle
and ocean sunfish.

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These predators both have adaptations
that prevents slippery jellyfish

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from escaping after they are engulfed:

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backwards pointing spines 
in the turtle's mouth and esophagus

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and recurved teeth 
behind the sunfish's cheeks.

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Even tiny lobster slipper larvae
can cling to the bell of a jellyfish

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and hitch a ride,

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snacking on the jelly while they preserve
their own energy for growth.

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Small agile fish use the jellies
as moving reefs for protection,

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darting between tentacles
without ever touching them.

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Nudibranchs, which are sea slugs
covered in protective slime,

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can actually steal the jelly's defenses
by eating the cnidocytes

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and transferring them 
to specialized sacks for later use,

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as weapons against their own predators.

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Even humans might benefit
from the sting of a jellyfish one day.

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Scientists are working on manipulating
cnidocytes to deliver medicine,

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with nematocysts rarely 3% of the size
of a typical syringe needle.

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So, the next time you're out 
in the ocean, be careful.

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But also, take a second 
to marvel at its wonders.