Electric Eels Sense with Electricity

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

An electric eel. Credit: chrisbrenschmidt/Flickr, CC BY 2.0

An electric eel. Credit: chrisbrenschmidt/Flickr, CC BY 2.0

Electric eels discovered electricity long before Benjamin Franklin did.

If they had to use their poor eyesight to hunt fish in the murky waters of the Amazon and the Orinoco, they’d starve. Although eel-like, they are really knifefish, more closely related to catfish than eels. These two-metre-long predators produce power and store it in stacks of flat disk-like special cells that cover most of their bodies. These cells are the inspiration for the batteries we use in our daily lives.

On a hunt, electric eels fire up to 600 volts of power at their fast-moving fish prey. That’s nearly three times the power of your home’s wall socket. Within milliseconds of electrocuting their victims, knifefish gobble them up.

Kenneth Catania, a neuroscientist at Vanderbilt University, U.S.A., discovered electric eels have many uses for their self-generated thunderbolts of current. After shocking them, the knifefish use electric feedback from their prey to track them, much as bats use high-pitched sounds to locate and track insects.

In an earlier study, Catania showed the stun gun-like prolonged charge stops fish in their tracks, by interfering with neurons that cause muscles to seize up. Electric eels also use electricity to reveal well-hidden prey in situations when even fishfinders are useless. Knifefish emit two quick bursts of high voltage current, and the muscles of any concealed fish twitch involuntarily. How do electric eels then locate the creature? Do they use sight, feel, or smell? Catania answers this question in his latest study.

He placed a lobotomised and anesthetised prey fish in a plastic bag, and insulated it with a thin agar layer. He rigged the inert fish with electrodes, so it could twitch only when he turned on the current, and not when the electric eel fired its double shots of current. Wearing rubber gloves like an electrician, Catania lowered the fish contraption into his live wire subject’s aquarium.

The knifefish couldn’t see or smell the fish, nor could it sense any electrical pulses. It could, however, feel the fish’s movements.

When Catania made the prey fish twitch, the electric eel sent a torrent of high voltage pulses, about 400 a second, to paralyse it. But it was unable to find its quarry. It headed towards the fish, grew bewildered, and gave up.

Catania inserted a carbon conductor in the aquarium and flipped the switch to make the dead fish move. Since the immobile conductor bounced the electric eel’s current, the predator attempted to swallow it. More than the dead fish’s movements, it seemed to require electrical cues to make the final lunge.

Catania lined up six equally spaced, identical plastic rods with the carbon rod. This time too, the electric eel focused unerringly on the conductor. He tried to confound the knifefish further. He inserted the plastic rods and conductor into a slowly revolving disk.

As soon as he made the fish in the plastic bag flinch, the electric eel fired a high voltage, high frequency torrent of power and tracked the moving conductor. Its reaction did not change even when Catania reduced the size of the conductor to a small thin disk. He added several plastic duds and spun the disk faster, with the same results. His video recording of the experiment shows the electric eel’s astonishing speed and accuracy in tracking with nothing more to guide it than its high voltage discharges.

When Catania removed the conductor and presented the eel with seven identical plastic rods, the electric eel seemed befuddled. It needed feedback from a conductor to direct its attack.

In the wild, an electrocuted fish would behave like a conductor, and a pursuing electric eel would have no difficulty in finding its limp prey, hidden or not, by sensing its electrical pulses.

Fish have no defence against this super-powered adversary. The predator’s electric discharge forces them to give themselves up, paralyses its victims’ muscles, and locates them even when they are hidden. The electric eel is every fish’s worst nightmare.

The study was published on October 20, 2015, in Nature Communications.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.