64 Scientific American, April 2019
I
t’s no secret that electric eels stun their prey—accounts of such occurrences date
back centuries. But unless you work security on the starship Enterprise, “stun” is a
vague term. What really happens when these creatures attack? Until recently, biolo-
gists knew surprisingly little about the electric eel’s superpower. I was not planning to
study this phenomenon, and I certainly never imagined I would offer an eel my arm in
the name of science, as I eventually did. But as a professor of biological sciences at
Vanderbilt University, I teach about electric fish, and when I brought some eels to my
laboratory so I could obtain new photographs and slow-motion movies to liven up my lecture,
I saw something so strange that I had to drop everything else to investigate.
When an eel attacked a prey fish with high voltage, all the
nearby fish in the tank became completely immobile in only
three milliseconds. It was as if they had been turned into little
statues; they just floated stock-still in the water. At first, I won-
dered if they had simply been killed. But if the eel missed its tar-
get and turned off the high voltage, the fish “unfroze” and took
off at full speed. The eel’s effect was temporary. I was hooked; I
had to know how the eel’s electric attack worked.
The most obvious analogy that came to mind was a law-en-
forcement Taser, which causes neuromuscular incapacitation by
interfering with the nervous system’s ability to control muscles.
Tasers deliver electricity along wires in short, high-voltage puls-
es at a rate of 19 pulses a second. Electric eels do not need wires,
because the water allows current to flow, as happens when a
hair dryer falls into a bathtub. But otherwise, the eel’s output is
reminiscent of a Taser’s: it comes in brief pulses, each lasting
only about two milliseconds. Eels can give off more than 400
pulses per second during an attack volley, however—a much
higher rate than the law-enforcement devices. Could electric
eels be souped-up, swimming Tasers?
With this question in mind, I set out on what would become
a three-year mission to unravel the mechanism of the eel’s attack
and the effects of its shocks on both prey and would-be preda-
tors. I was surprised at every turn by the eel’s sophisticated use
of electricity and reminded that humankind’s inventions don’t
hold a candle to nature’s.
SHOCK VALUE
you might be surprised to learn the electric eel is not a true eel
but rather belongs to a family of fish known as the Gymnotidae
that live in South America. The other members of this group
give off very weak electric discharges that they use to sense their
surroundings and to communicate. The electric eel has amped
up its power over the course of evolution. It can generate a
charge of up to 600 volts, thanks to the electric organ that spans
nearly the length of their body (the animals can reach eight feet
in length and weigh more than 40 pounds). The organ is com-
posed of thousands of special disk-shaped cells called electro-
cytes that work like batteries to discharge electricity.
To investigate the possibility that the electric eel operates like
a Taser to incapacitate its prey, I needed to observe the animal in
hunting mode. So I devised an experiment that took advantage
of the eel’s insatiable appetite for earthworms. First, I placed a
dead fish that still had working nerves and muscles in the water
with the eel (but separated by an electrically permeable barrier)
and attached it with a string to a device for measuring muscle
contractions. Then I fed the eel earthworms, which it happily
shocked and ate. This setup allowed me to conduct a series of
tests on the fish muscle responses to the high-voltage pulses em-
anating from the hunting eel.
The volleys of high-voltage pulses from the eel caused mas-
sive muscle contractions in the fish that started three millisec-
onds after the electric attack began—exactly the same amount of
Kenneth C. Catania is a professor of biological
sciences at Vanderbilt University. He studies
comparative neuro biology, with an emphasis
on animal sensory systems. This is his fourth article
for Scientific American.
IN BRIEF
The electric eel has long been known to stun its
prey. But the mechanism of the eel’s attack and how
the shocks affect prey were a mystery.
A series of laboratory experiments has revealed
how the creature uses electric fields to detect, track
and immobilize prey.
The eel also uses its electrical powers when threat-
ened, leaping from the water to intensify the cur-
rent it delivers to potential predators.
