April 2019, ScientificAmerican.com 65KENNETH C. CATANIA
time that passed before the fish were seen to stop moving in the
slow-motion movies. Apparently eels invented the Taser long be-
fore humans. But the experiments showed much more. Eels do
not activate fish muscles directly. Instead their zaps activate the
nerves that lead to the fish muscles. Each high-voltage pulse
from an eel generates an action potential, or nerve impulse, in
the fish’s motor nerves.
This finding is remarkable when you consider that the eel’s
electric organ is a modified muscle activated by the animal’s
own motor nerves. The motor nerves are, in turn, activated by
neurons in its brain. For each high-voltage pulse, the flow of
command signals starts in the eel’s brain and travels to its motor
neurons, which then activate the electric organ. From there the
signal passes through the water to trigger the motor neurons,
and then muscles, in nearby fish. In other words, the eel immo-
bilizes its prey using a form of high-fidelity remote control.
Intriguingly, this insight suggests the eel’s electric output may
have been shaped in part by what happens to the muscles of its
prey. With this finding in mind, I began considering the eel’s high-
voltage volley with a new perspective. I was especially intrigued
by reports from a previous investigator, Richard Bauer, who in
1979 showed that hunting electric eels often pause to give off pairs
of high-voltage pulses, each separated by two milliseconds. These
paired pulses are called doublets, and all the eels in my lab exhib-
ited the same behavior. What, I wondered, are doublets for?A little research into muscle physiology revealed that dou-
blets—which can also be described as pairs of action poten-
tials—sent from motor neurons to muscles are the best way to
generate maximal muscle tension. Accordingly, my experiments
showed that eel doublets cause a brief, massive, whole-body
twitch in nearby prey, in contrast to the volleys, which cause
sustained paralysis. The twitch, in turn, produces a strong wa-
ter displacement—essentially an underwater sound. Given the
eel’s exquisite sensitivity to the slightest water movement, an
interesting possibility comes to mind. Could doublets be the
eel’s way of asking, “Are you alive?” After all, wild eels hunt at
night in the Amazon, surrounded by a vast diversity of hidden
prey—things that are far harder to find than worms and gold-
fish dropped into a tank.
Supporting this idea: when eels in my lab hunted novel prey,
such as crayfish, or prey hidden among plants in the tank, they
often gave off doublets while searching and attacked after the
prey twitched, as if the prey’s movement had tipped them off.
These were telling observations, but to provide more direct evi-
dence, I attached the dead fish to an electric stimulator that
could be triggered by either me or the eel’s doublets. I then
placed the wired fish in a ziplock bag so the eel’s own doublets
would have no effect on it. This setup allowed me to control
when the fish’s muscles twitched. Sure enough, the eels never
followed a doublet with an attack unless the fish twitched. TheTRACKING SYSTEM: The eel can track prey and other conductors using high-voltage electroreception. In experiments with a spinning disk
bearing one conductive insert and multiple nonconductive inserts, the eel singled out the conductive insert with remarkable accuracy.Conductive
carbon
insertPlastic
insert