The electric eel—the scaleless Amazonian fish that can deliver a jolt strong enough to knock down a horse—possesses an electroshock system uncannily similar to a Taser. That is the conclusion of a study by Stevenson Professor of Biological Sciences Kenneth Catania described in the article “The Shocking Predatory Strike of the Electric Eel,” published in the Dec. 5, 2014, issue of Science.
People have known about electric fish for a long time. The ancient Egyptians used an electric marine ray to treat epilepsy. Michael Faraday used eels to investigate the nature of electricity, and eel anatomy helped inspire Volta to create the first battery. Biologists have determined that a 6-foot electric eel can generate about 600 volts of electricity.
To figure out how the eel’s electroshock system works, Catania equipped a large aquarium with a system that can detect the eel’s electric signals and obtained several eels up to 4 feet in length.
The biologist discovered that the eels’ movements are incredibly fast—they can strike and swallow a worm or small fish in about a tenth of a second. So he rigged up a high-speed video system to study the eels’ actions in slow motion.
Catania recorded three different kinds of electrical discharges from the eels: low-voltage pulses for sensing their environment; short sequences of two or three high-voltage millisecond pulses (called doublets and triplets) given off while hunting; and volleys of high-voltage, high-frequency pulses when capturing prey or defending themselves from attack.
He found that the eel begins its attack on free-swimming prey with a high-frequency volley of high-voltage pulses about 10 to 15 milliseconds before it strikes. In the high-speed video, it became apparent that the fish were completely immobilized within three to four milliseconds after the volley hit them. If the eel didn’t immediately capture them, the fish normally regained their mobility after a short period and swam away.
“I have some friends in law enforcement, so I was familiar with how a Taser works,” says Catania. “And I was struck by the similarity between the eel’s volley and a Taser discharge. A Taser delivers 19 high-voltage pulses per second while the electric eel produces 400 pulses per second.”
“If you take a step back and think about it, what the eel can do is extremely remarkable. It can use its electrical system to take remote control of its prey’s body.”
The Taser works by overwhelming nerves that control the muscles in the target’s body, causing the muscles to involuntarily contract. To determine if the eel’s electrical discharges had the same effect, Catania walled off part of the aquarium with an electrically permeable barrier. He placed a pithed fish on the other side of the barrier from the eel and then fed the eel some earthworms, which triggered its electrical volleys. The volleys passed through the barrier and struck the fish, producing strong muscle contractions.
To determine whether the discharges were acting on the prey’s motor neurons or on the muscles, he placed two pithed fish behind the barrier—one injected with saline solution and the other injected with curare, a paralytic agent that targets the nervous system. The muscles of the fish with the saline continued to contract in response to the eel’s electrical discharges, but the muscle contractions in the fish given the curare disappeared as the drug took effect. This demonstrated that the eel’s electrical discharges were acting through the motor neurons just like Taser discharges.
Next Catania turned his attention to how the eel uses electrical signals for hunting. The eel is nocturnal and doesn’t have very good eyesight, so it needs other ways to detect prey.
The biologist determined that the closely spaced doublets and triplets the eel emits correspond to the electric signal that motor neurons send to muscles to produce an extremely rapid contraction.
“Normally, you or I or any other animal can’t cause all the muscles in our body to contract at the same time,” Catania says. “However, that is just what the eel can cause with this signal.”
Putting together the fact that the eels are extremely sensitive to water movements with the fact that the whole-body muscle contraction causes the prey’s body to twitch, creating water movements that the eel can sense, Catania concluded that the eel is likely using these signals to locate hidden prey.
“If you take a step back and think about it, what the eel can do is extremely remarkable,” says Catania. “It can use its electrical system to take remote control of its prey’s body. If a fish is hiding nearby, the eel can force it to twitch, giving away its location, and if the eel is ready to capture a fish, it can paralyze its muscles so it can’t escape.”
The research was funded by a Pradel Award from the National Academy of Sciences, a Guggenheim fellowship, and a National Science Foundation grant.
Watch a video of the electric eel in action: