In the rivers of the Amazon rainforest is a fish like no other.
A fish that you would not want to find yourself wading in the water with.
A specimen of this fish was brought to Philadelphia from South America in 1773.
It was observed that:
It had the extraordinary power of communicating a painful sensation, like that of an electrical
shock, to people who touched it, and of killing its prey at a distance.”
It was a fish so unusual, that it would go on to change the course of science - and in fact,
the course of modern society - forever.
Electric eels are a genus of freshwater fish from South America,
famous for their ability to stun their prey by generating electricity, “Eel” is a misnomer, as
they are actually part of the knife fish family, and are more closely related to catfish than eels.
And though they aren’t the only electric fish in the world, they are by far, the most powerful.
Some individuals are able to deliver shocks at up to 860 volts and 1 amp.
The average shock from an electric eel lasts about two thousandths of a second.
A single shock likely wouldn’t kill a person, but it could
seriously stun one. If shocked, your muscles would briefly contract,
and then you’d feel numb. If you were in the water and got shocked, this could lead to drowning.
It's one of only a few animals that can kill its prey at a distance. And not
only can the eel use electricity to stun or kill its prey - it can also use electricity
to control its target remotely - the electricity coursing through the prey’s body,
making it act in a way detrimental to itself, and beneficial for the eel.
And as formidable a hunter as an electric eel is on its own, recent studies have discovered that
they are not the solitary predators researchers always believed them to be. They are in fact,
proficient and effective pack hunters, ambushing schools of fish in terrifying unison.
How is it possible for a fish to generate
so much electricity? And what has modern science learned from this improbable creature?
Electric eels live in streams, floodplains, and swamps of the Amazon and Orinoco river basins
in South America. Such murky habitats can be dangerously seasonal for a fish. During the dry
season, water levels drop, and oxygen availability can plummet. But luckily for the electric eels,
they can breathe air, thanks to a special mucous membrane in their mouth that can absorb oxygen.
Electric eels are snake-like in appearance, and they are pretty huge.
They can grow up to 8 feet, or 2.5 meters in length,
and weigh as much as 44 pounds, or 20 kg. They don’t have scales, and instead, have slimy,
dark gray or brown skin on the top of their body and a yellow-orange color on the bottom side.
And 80% of the eel's body, by length and by weight,
is dedicated to one thing: generating ridiculous amounts of electricity.
The electric eel has three pairs of electric organs that run through
the back portion of the its body: the Sach’s organ, the Hunter’s organ and the Main organ.
The Main and Hunters' organs are the high voltage producers, used for protection and stunning prey.
The Sachs' organ is capable only of producing
low voltage pulses - its purpose is mainly electro communication and navigation.
These organs contain hundreds of thousands of modified muscle cells
called electrocytes. These are flattened disk-like cells that are stacked in about
70 columns on each side of the fish’s body. Each column contains 5000–10 000 electrocytes.
In their resting state, these electrocytes maintain a negative charge
as the cells constantly work to pump positively charged sodium and potassium ions out.
And each electrocyte is connected to the nervous system - but on only one side.
When the eel senses prey or a threat, it sends a signal through the nerve endings causing an ion
channel on the electrocyte to open. Extracellular positive ions can then flow back into the cell.
The side without the nerve connection continues to pump the positive ions out of the cell.
This gives the left side of the cell a relatively positive charge,
and the right side a relatively negative charge.
Voila, a dipole has been created, and the eel can start zappin’. The electric charge flows
towards the eel’s head, which is the more positively charged side of its body, and
is then discharged through the water towards prey.
And the more cells that are stacked and lined up, the more electricity they can generate.
Because incredibly, the eel is able to open all of its membrane gates at exactly the same time,
creating a huge jolt of electricity.
This seems like an extreme ability, but it's an ability based on what typical muscle and
nerve cells already do. All muscle and nerve cells have electrical potential
and a simple contraction of a muscle will release a small amount of voltage.
The cells of an electric eel have simply amplified that potential.
An eel’s ability to make electricity is often compared to a battery - and this comparison is
no coincidence. The very first battery, invented in 1800, was inspired by the electric eel.
Italian physicist Alessandro Volta observed that electric eels had stacks of cells that looked
like a roll of coins. He figured if he could mimic this arrangement, he too, might be able
to generate electricity. He cut coin-shaped disks from various “dissimilar metals” and stacked them,
trying to find any combination of them that would create electricity.
Eventually, he tried copper and zinc disks, and separated the stacked pairs with saltwater.
And lo and behold - electricity started flowing. This came to be known as the
voltaic stack - and is considered to be the first electric battery ever made.
Before this discovery, scientists barely understood
static electricity. And the electric eel was only beginning to reveal its mysteries.
Scientists soon realized that eels don’t have a
one-zap-fits-all policy. They instead are able to use different charges for
different things - some for sensing their environment, some for communicating,
some for stunning their prey, and even some for controlling the movements of their prey.
Electric eels can control their electricity with finesse.
In their dark and murky habitat, electric eels can’t rely on eyesight for navigation.
In fact, their eyesight is extremely poor.
They instead use weak electric discharges, around 10 volts, to electrolocate.
This weak charge emanates from their body, and the eel can detect distortions in the electric
field from objects either conducting or resisting their electricity. This weak
electric discharge is created when the eel simply activates only a subset of it’s electrocytes.
Electric eels can even communicate using low voltage electric pulses.
By varing the frequency that they produce these pulses, researchers believe that eels
can convey information about their sex and reproductive receptivity.
And, of course, eels use their electricity for hunting.
Electric eels are mostly nocturnal. They will wait for darkness, and then sneak up on their prey,
and unleash a series of high frequency, high voltage pulses.
In this video, the red frames indicate when the electric organs are discharging.
After just a few milliseconds the prey is completely immobilized.
Such a high frequency, high voltage attack induces massive whole-body muscle contractions in the prey
fish. The electric shock causes the prey fish’s motor neurons, and thus their muscles, to become
simultaneously activated. The higher the voltage, the more intense the muscle contraction - and at
600 volts or more, these attacks often cause the the maximum possible muscle contraction,
causing the animal to completely seize up. This is similar to how a TASER works.
Then, once it is immobilized, the eel can feast. Electric eels don’t have upper teeth,
so they rely so on suction to pull the prey into their mouth before swallowing them whole.
For a long time, scientists thought that eels implemented these two types of discharge - low
voltage for navigation and communication; high voltage for immobilizing prey. But, they soon
learned, there is more to the story. And that eels use their electricity in even more cunning ways.
When looking at the timing of the electric pulses, researchers noticed an interesting pattern.
‘‘Introduction of prey into the aquarium arouses the eel, causing it to swim around,
but often stopping in a particular corner of the aquarium.
During these stops, two high-voltage pulses with an interval of about 2 ms are emitted.’’
After these two high voltage pulses, the eel takes a pause,
and then proceeds with its usual high-voltage, high frequency attack volley.
These two high voltage pulses are known as doublets - and at first,
their purpose wasn’t fully understood. Why let out two quick pulses initially,
if the massive attack volley will be more than enough to immobilize prey?
Scientists carefully observed the eels’ behavior,
and noticed a few things. 1) a massive attack volley would almost always come after the doublet;
and 2) the doublets themselves resulted in a massive whole body twitch in nearby fish.
Perhaps the purpose of these doublets was not to stun the prey,
but to force the prey to reveal its location to the eel.
To test this, scientists set up a clever experiment.
On one side of an agar barrier they placed an electric eel. On the other side of the barrier,
they placed a dead fish, and connected it to a electrical stimulator.
The fish was then sealed in a bag, and electrically isolated from the eel. The scientists
could electrically jolt the fish with their stimulator, but the eel could not. The stimulator
was set up so that when the eel emitted a doublet, it would send a similar shock to the dead fish.
When the eels emitted its doublet, and the dead
fish received the simulated doublet - the eels immediately responded to the
twitch with a high-voltage volley and strike toward the dead fish.
But when the investigators turned off the stimulator,
and the eel emitted its doublet - no twitch was produced in the dead fish, and the eel did not
attack. The eel would only attack the fish if it first twitched in response to its doublet.
This study shows that electric eels can indeed remotely control their prey,
forcing them to reveal their location. The doublet allows the eel to ask the question:
any fish out there I can eat? The prey have no choice but to respond.
But what if an eel finds itself near an entire shoal of fish - a shoal that seems particularly
good at keeping its distance to avoid such shocks? To deal with this, some electric eels
display a remarkable behavior - a behavior unprecedented in the world of electric fish.
Social predation - where groups of animals coordinate to hunt and kill
prey - is a tactic commonly seen in mammals like wolves,
orcas, or lions. In fish, this behavior is almost unheard of.
So when scientists travelling through the Amazon river noticed groups of eels,
sometimes over 100 of them, seemingly rounding up and attacking prey together,
they almost could not believe their eyes. And these weren’t just any of the electric eels -
they were volta eels the most powerful of any of the electric eels, with shocks up to 860 volts.
At dawn and dusk, the scientists noticed an increase in eel activity,
with many individuals leaving the murky depths and swimming near the surface,
appearing to congregate. These groups of eels would then swim together towards a shallow hunting
area that contained thousands of small fish. These groups of sometimes 100s of eels would then start
to swim in circles, herding the fish into prey balls, and pushing them to shallower water.
Then, between 2 to 10 individual eels would launch a joint predatory strike. In this
video you can recognize the attacking eels by their synchronized sinusoidal body posture.
The fish hit with the electrical attack spasm so hard that they jump out of the
water - and then return to the surface stunned and motionless. The eels then quickly eat the fish.
The eels would carry out this attack sequence five to seven
times, and each time, a different subset of eels would produce the electrical attack.
Although the researchers weren’t able to measure the voltage of the coordinated electric attacks,
they estimate that 10 Volta’s electric eels working together could create 8600 volts.
This hunting technique allows the eels to subdue
huge quantities of prey that would normally be too evasive to capture.
And it’s not just that these eels were stuck together in a confined
area, forcing them to feed on the same groups of fish.
These eels were congregating from far and wide, to repeatedly forage together over time,
using communication in the form of low voltage electric pulses and body posturing.
It’s unknown if the eels have any familial relationship to each other,
like is common with many other pack hunters. But scientists are working to find that out.
Only 9 other species of fish have ever been observed to hunt together - and the electric
eel’s tactics may be the most formidable. This shows us that animals we think we understand
can have more than one survival strategy - that in isolated regions and specific groups of animals,
predator prey interactions may not be at all what we expect.
One lingering question that may have come to your mind is - how do the eels not shock each other?
It’s believed that the electric organ of an electric eel is padded with
adipose tissue that insulates the fish from its own electric shock.
But how do they prevent getting shocked from other eels?
The likely answer is, that they don’t.
Electric eels are probably pretty huge for exactly this reason. They are much larger than their
prey - such that an eel’s current will easily incapacitate their target, but won’t do much to
their larger bodies, similarly to how a single electric eel shock to us would hurt for a sec,
but not kill us. To an eel, getting shocked is likely just part of doing business.
An eel’s entire world revolves around its electricity. And largely thanks to the electric
eel, so does ours. Next time you charge your phone or stick some double As in your remote, you can
thank the electric eel for its contribution to our understanding of electricity. And we continue to
be inspired by them. Researchers are currently working on an entirely new type of battery
inspired by eels - a soft and flexible battery that can power medical implants and soft robots.
We are likely just beginning to understand these incredible creatures,
with many more of their mysteries still lurking in the muddy waters of the Amazon.
Understanding the electric eel has ignited a curiosity in me about the electricity
in the world all around us. Its nearly impossible to NOT take it for granted.
I know I certainly don’t often think about how the lights in my kitchen work,
how my computer stays charged, or how the grid in Texas is teetering on a knife edge
and is doomed to fail at any moment - ok maybe I do think about that last one. But in general,
how electricity works is rarely on my mind. But the more I learn, the more incredible it becomes.
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And if you’re looking for something else to watch right now,
you can watch our previous video about the mysteries of deep sea gigantism - or watch
Real Engineering’s latest video about self healing space suits.