On the rivers of
Suriname, electric eels
reveal their secrets
It wasn’t his quarry’s good looks that drew
Will Crampton to the wilds of Suriname.
It was the animal’s technical skills.
With his Fluke ScopeMeter® portable oscilloscope in hand,
Professor Crampton and a National Geographic video crew
flew into Suriname’s 1.3-million-acre Raleigh Vallen nature
preserve in July 2011. Their goal: capture the world’s most
powerful electric fish—Electrophorus electricus, the electric
eel—and measure its punch.
Lanky and mud-colored, the beady-eyed electric eel
can grow up to six feet in length and 45 pounds. It’s not a
true eel, but one of the neo-tropical knifefishes in the order
Gymnotiformes, more closely related to catfish.
Like other knifefishes, the electric eel can generate lowlevel electrical fields that it uses for navigating through its
sometimes turbid environment and for identifying others of
its species. But Electrophorus electricus can also crank out
a dc current powerful enough to stun its prey and to shock
potential predators into choosing something else for dinner.
It was this unique capability that attracted producers
for the National Geographic’s
Nat Geo WILD channel as they
planned a series on “Animal
Superpowers.” The eel, which
carries on shockingly well in
water nearly devoid of oxygen,
would fit perfectly in the “Survivors” segment.
Application Note
Into the wild
So Nat Geo contacted Will
Crampton, assistant professor
in the biology department at
the University of Central Florida
in Orlando. Crampton is one of
the world’s experts on electric
fishes. In the 1990s he earned
his PhD through a four-year
project studying Gymnotiformes
in the Brazilian city of Tefé, high
up on the Amazon in the state
of Amazonas. Crampton knows
the tropics and the fish that
swim those waters. In 2009, he
worked with Nat Geo on an earlier project to investigate electric
eels.
Professor Will Crampton (with ScopeMeter Test Tool)
and the Suriname expedition crew. Left to right:
Sonny, who caught the eel; videographer Roeland
Doust of Windfall Films; field station manager;
Crampton and Benito, aka “Doctor Five.” Crew member
at right not identified. Courtesy of Windfall Films, Ltd.
From the Fluke Digital Library @ www.fluke.com/library
So in July 2011, Crampton
and Roeland Doust, producer/
director for London’s Windfall
Films, flew into the Suriname
capital of Paramaribo. There
they chartered a light plane for
the 50-minute flight southwest
to Raleigh Vallen and the dirt
strip on Foengoe Island, in the
midst of the Coppename River
not far from Suriname’s highest
waterfall.
“They wanted a relatively
short sequence that just
described what the electric eel
is, how it generates its electric
field, and what it uses it for,”
Crampton said. Sounds easy, but
it wasn’t.
“It was challenging—we had
great difficulty in actually finding anything,” he said. “The
water levels were very high.
We’d come in the aftermath of a
giant series of rainstorms, and
the whole river had come up.
Everything was flooded. We had
to change tactics.”
Crampton uses a fish finder he
designed to detect the electrical signals that knifefish and
electric eels produce. The finder
creates an audio signal that
warns when the fish are near.
“You can hear the electric eel,
which has a very distinctive
low-frequency clicking sound,”
Crampton said, “and you can
hear any of the weakly electric
knifefishes, which either sound
like pulsed clicks or like a hum
or whistle or tone. The electric
eel is distinctive because of its
low pulse rate, and because you
can detect it at a great distance.”
Using the fish finder, Crampton found a spot where he could
hear several eels. By that time
the water was receding quite
rapidly. He set a trap and also
arranged with several local
fishermen to fish the channel
with hook and line. With just
one day to go before leaving, an
eel about a half-meter long was
captured.
Designed to be electric
In its very shape, the eel is
optimized to be electric. It has
lost all fins except the pectorals
(which look like ears) and the
anal fin, which extends nearly
the full length of the fish along
the bottom. It moves not by
wriggling its body, but by undulating this elongated fin, and
travels almost as well backward
as forward.
“This has evolved because
they need to keep their bodies
straight to maintain the integrity of the electrostatic field
they generate,” Crampton said,
“and to make it more efficient to
generate an image of the world
around them.”
The eel’s body cavity is very
compact and close to the head.
“That allows them to dedicate
most of their bodies to electric
organ tissue,” he said. “All of the
electric fishes in South America
are essentially a giant electric
battery.”
The eel’s electric power helps
make it possible to survive in
waters where oxygen levels can
approach zero. The eel is also
an air breather, gathering 80
percent of its oxygen by taking
in mouthfuls of air. Its mouth is
lined with delicate blood vessels
to absorb the oxygen. “It has to
breathe air,” Crampton said. “It’ll
drown if it can’t reach the surface.” In the low-oxygen waters
where the eel lives, grabbing
a breath of fresh air means
survival.
“Electric eels swallow fishes,
crustaceans, and frogs and
things—they swallow them
whole,” Crampton said. “It’s
possible that being able to
shock things and swallow them
without having to manipulate them in their mouth, and
without having to worry about
spines and things, has enabled
them to develop the mouth as a
respiratory organ that wouldn’t
normally be possible in a fish.”
From electric fish, two kinds of current
Electric fish, like knifefish, produce electrical signals used for sensing
where they and their schoolmates are. Electric eels produce, in addition,
more powerful currents for hunting and defense.
The weakly-electric fishes generate signals of just one or two volts,
which have an effective range of a few centimeters for navigation, and
one or two body lengths for communication.
The electric eel also produces low-voltage electricity for electrolocation, using what’s called the Sachs organ. Inside the organ are
many muscle-like cells, called electrocytes. Each can only produce
0.15 V, though together the organ transmits a signal of about 10 V at
around 25Hz. Electrosensors in the eel’s skin detect distortions in
the field caused by nearby objects, giving the eel another sense of its
environment.
Using two other organs, the Main organ and the Hunter’s organ, the
eel can also kick out a stunning charge that it uses in hunting and selfdefense. It generates its electrical pulse in a manner similar to a battery,
in which stacked plates produce an electrical charge. In the eel, some
5,000 to 6,000 stacked electroplaques are capable of producing a shock
at up to 500 volts and 1 ampere of current (500 watts). It’s enough
to deter just about any other animal, except possibly the alligator-like
caiman. “I have heard reports that a caiman will bite an electric eel in half,
and then devour it,” said eel expert Will Crampton.
Are these electric fish a danger to humans? “Theoretically it could be
enough to stop someone’s heart and kill them,” he said, “but I’ve never
heard of that happening. I don’t think there’s a single documented case of
anyone being killed or even seriously injured by an electric eel. Not one.”
2 Fluke Corporation On the rivers of Suriname, electric eels reveal their secrets
While the Electric Eel relaxes in its wading pool, Prof. Will Crampton prepares the Fluke ScopeMeter to test the most
powerful electric fish. Courtesy of Windfall Films, Ltd.
How the eel measures up
With the eel safely corralled in an inflatable holding
pool, Crampton and the crew
deployed their ScopeMeter
190-202 to measure the animal’s output. Thought it wasn’t
designed for biometrics in the
rain and humidity of the Suriname jungle, the instrument’s
durability and compact design
proved ideal. Its batteries provided ample power for days in
the field.
“You can knock it around a
bit and not be worried about it
malfunctioning,” Crampton said.
“I didn’t test whether it was
completely waterproof, but it
drizzled with rain at one point
and I didn’t bother to cover it up.
It was fine.
“The Fluke very clearly
showed the wave forms,”
Crampton said. Next came a
demonstration using an array of
LED lights and capacitors created
by Jeff Lambert, electrical engineer in the Crampton laboratory.
“We couldn’t test it—we don’t
have any electric eels in the
lab,” he said. “It’s illegal to have
electric eels in Florida, except
with a permit. They would do
very well here.” As if on cue, the
eel in Suriname lit up the LEDs.
“The finale was to measure
voltage,” he continued. “You
have to isolate the electric eel
from any load on its electric
circuit. That’s done by placing
it on a dry plastic sheet. We set
up the ScopeMeter so that we
had an electrode on the eel’s
head and a ground on its tail.
This was an eel that was a half
a meter long, and I believe the
voltage that came up on the
screen was 498 volts. The current was about one ampere.”
Crampton appreciated the
flexibility of the Fluke portable
oscilloscope. “With the Fluke
190-202 there’s a nice opportunity,” he said. “You can capture
signals at very high sample rates
and pretty good bit resolution
just by going straight in—you
don’t have to amplify either the
weak or the strong discharges. I
was able to get recordings of the
weak discharges when the electric eel was in the water, and
to get the strong discharges we
had it out on the plastic sheet,
and I would just tap it lightly on
the head to annoy it, and that
would be enough to generate
the strong discharge.”
3 Fluke Corporation On the rivers of Suriname, electric eels reveal their secrets
Something in the Water . . .
Unlike sound, which can travel tremendous distances in water, electrical signals
decay rapidly as distance increases,
Crampton said. The nature of water in
the Amazon basin also affects the signals. Much of the water is low in mineral
salts, and electrical conductivity can range
from three to four micro Siemens per
centimeter (µS/cm) to 20 or 30 µS/cm,
Crampton said—not far from the 10 µS/cm
of distilled water. That inhibits electrical
signals from traveling far. In the so-called
“white water” rivers infused with minerals eroded from the slopes of the Andes,
conductivity is higher: 100 to 300 µS/cm.
Sea water, by contrast, may measure
20,000 µS/cm.
What the Electric Eel lacks in looks it makes up for in talent. The eels can reach six feet in length, weigh 40 pounds and
produce a 500-volt DC shock. Here Professor Crampton shows off a midsize specimen. Courtesy of Windfall Films, Ltd.
4 Fluke Corporation On the rivers of Suriname, electric eels reveal their secrets
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