The electric eel鈥攖he scaleless Amazonian fish that can deliver a jolt strong enough to knock down a horse鈥攑ossesses an electroshock system uncannily similar to a Taser. That is the conclusion of a study by Stevenson Professor of Biological Sciences described in the article 鈥淭he Shocking Predatory Strike of the Electric Eel,鈥� 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鈥檚 electroshock system works, Catania equipped a large aquarium with a system that can detect the eel鈥檚 electric signals and obtained several eels up to 4 feet in length.
The biologist discovered that the eels鈥� movements are incredibly fast鈥攖hey 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鈥� 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鈥檛 immediately capture them, the fish normally regained their mobility after a short period and swam away.
鈥淚 have some friends in law enforcement, so I was familiar with how a Taser works,鈥� says Catania. 鈥淎nd I was struck by the similarity between the eel鈥檚 volley and a Taser discharge. A Taser delivers 19 high-voltage pulses per second while the electric eel produces 400 pulses per second.鈥�
鈥淚f 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鈥檚 body.鈥�
The Taser works by overwhelming nerves that control the muscles in the target鈥檚 body, causing the muscles to involuntarily contract. To determine if the eel鈥檚 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鈥檚 motor neurons or on the muscles, he placed two pithed fish behind the barrier鈥攐ne 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鈥檚 electrical discharges, but the muscle contractions in the fish given the curare disappeared as the drug took effect. This demonstrated that the eel鈥檚 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鈥檛 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.
鈥淣ormally, you or I or any other animal can鈥檛 cause all the muscles in our body to contract at the same time,鈥� Catania says. 鈥淗owever, that is just what the eel can cause with this signal.鈥�
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鈥檚 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.
鈥淚f you take a step back and think about it, what the eel can do is extremely remarkable,鈥� says Catania. 鈥淚t can use its electrical system to take remote control of its prey鈥檚 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鈥檛 escape.鈥�
The research was funded by a Pradel Award from the , a , and a grant.
Watch a video of the electric eel in action: