Application Note
2 Fluke Corporation On the rivers of Suriname, electric eels reveal their secrets
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 find-
ing 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 electri-
cal 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, Cramp-
ton 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 undu-
lating 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 integ-
rity 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 sur-
face.” 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 manipu-
late 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 electro-
location, 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 self-
defense. 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.”