Name: 【TED-Ed】How do fish make electricity? - Eleanor Nelsen
Uploaded: 2017-11-30T22:30:31.000Z
Duration: 5 min 15 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

In 1800, the explorer  Alexander von Humboldt

witnessed a swarm of electric eels leap out of the water

Reflexive pronouns

to defend themselves  against oncoming horses.

Most people thought the story so unusual that Humboldt made it up.

But fish using electricity is more common than you might think;

and yes, electric eels are a type of fish.

electrical signals offer ways to communicate,

and find—plus, in rare cases, stun—prey.

Nearly 350 species of fish have specialized anatomical structures

that generate  and detect electrical signals.

depending on how much  electricity they produce.

Scientists call the first group the weakly electric fish.

Structures near their tails  called electric organs

produce up to a volt of electricity, about two-thirds as much as a AA battery.

The fish's brain sends a signal through its nervous system to the electric organ,

or thousands of disc-shaped  cells called electrocytes.

Normally, electrocytes pump out sodium and potassium ions

to maintain a positive charge outside and negative charge inside.

But when the nerve signal arrives at the electrocyte,

Now, one face of the electrocyte is negatively charged outside

But the far side  has the opposite charge pattern.

These alternating charges  can drive a current,

turning the electrocyte  into a biological battery.

The key to these fish's powers  is that nerve signals are coordinated

to arrive at each cell  at exactly the same time.

That makes the stacks of electrocytes act like thousands of batteries in series.

The tiny charges from each one add up to an electrical field

Cells called electroreceptors buried in the skin

allow the fish to constantly sense this field

and the changes to it caused  by the surroundings or other fish.

The Peter’s elephantnose fish,  for example,

has an elongated chin called a schnauzenorgan

That allows it to intercept signals from other fish,

detect the shape and size of nearby objects,

and even determine whether a buried insect is dead or alive.

But the elephantnose and other weakly electric fish

don't produce enough electricity to attack their prey.

That ability belongs  to the strongly electric fish,

of which there are only  a handful of species.

The most powerful strongly electric fish is the electric knife fish,

Three electric organs span almost its entire two-meter body.

the electric eel uses its signals to navigate and communicate,

but it reserves its strongest  electric discharges for hunting

using a two-phased attack that susses out and then incapacitates its prey.

First, it emits two  or three strong pulses,

These stimulate the prey's muscles, sending it into spasms

and generating waves  that reveal its hiding place.

Then, a volley of fast,  high-voltage discharges

causes even more intense  muscle contractions.

The electric eel can also curl up so that the electric fields

generated at each end  of the electric organ overlap.

The electrical storm eventually exhausts and immobilizes the prey,

and the electric eel  can swallow its meal alive.

The other two strongly electric fish are the electric catfish,

with an electric organ  that occupies most of its torso,

and the electric ray, with kidney-shaped electric organs on either side of its head

There is one mystery in the world of electric fish:

It may be that the size  of strongly electric fish

allows them to withstand their own shocks,

or that the current passes out  of their bodies too quickly.

Some scientists think that special  proteins may shield the electric organs,

but the truth is, this is one mystery  science still hasn't illuminated.