Electric eels get on their prey’s nerves

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A zap from an electric eel acts like a natural Taser. It gives the creature the power to stop prey in its tracks. This gives the eel the upper hand — er, fin — that allows it to prevent dinner from swimming away. And that zap acts directly on the prey’s nervous system, scientists now show.

But that’s not all. This species (Electrophorus electricus) can release different types of electric discharges, the new study finds. The eels, which hunt at night, sometimes send out just two or three short pulses. These cause nearby fish to twitch against their will. The eels detect those involuntary movements — which shows them where their dinner had been hiding.

That the eel may use electricity to scout for prey is “a remarkable finding,” Mark Nelson told Science News. A biologist and eel researcher at the University of Illinois at Urbana-Champaign, Nelson did not work on the new study. The new finding, he says, “elevates the eel’s strategy from a simple, reflexive stun-and-strike behavior to a more deliberative process.” (Deliberate, as a verb, means “to consider carefully.”)

Better understanding of those zappers

Researchers have long been fascinated by the electric eel. This species can release bursts of electricity measuring up to 600 volts. That’s enough to stop a fish cold in about three thousandths of a second. However, little research has been done on the details of how the eel’s body reacts to that electricity.

Biologist Kenneth Catania, works at Vanderbilt University in Nashville, Tenn. He became curious about the eels’ stun-gun action while studying high-speed videos of an attack, he told Science News.

For the new study, he put two unconscious fish in a tank with an electric eel. One prey fish had been treated with curare (ku-RAAR-ee). This substance prevented the fish’s nerves from communicating with its muscles. The other fish was left untreated. Catania watched as the eel approached. When it sent out electric shocks, the fish that had not been treated with curare began to twitch. The curare-treated fish, however, never flinched.

That experiment told Catania that the eel’s electricity must act on the nervous system, and not on the muscles themselves. (If the electricity affected the muscles directly, both fish would have twitched.)

Before a full shocking attack, these eels often emit very short bursts of electricity, just two or three quick zaps. The short zaps can make nearby fish twitch, too. Catania wondered if the eel used this process to hunt for hidden fish.

To find out, he conducted another experiment. He put an unconscious fish in a plastic bag in the tank. The plastic insulated the fish from the eel’s electricity. He also added an electric device he could control inside the bag. The device would made the fish inside twitch on his command.

Then he unleashed the eel. It soon sent out short bursts of electricity. When the bag kept the fish from responding to those small pulses, the eel didn’t follow up. But when Catania flipped the switch to make the fish twitch (and seemingly respond to the eel’s zaps), the eel unleashed its full electric force. That convinced the scientist that the eel uses those short bursts to hunt down hidden prey. He published his findings December 5 in Science.

Biologist Jason Gallant at Michigan State University, in East Lansing, did not work on the new study. Electric organs have evolved at least six times in different species of fish, he notes. Gallant says he’d like to know if other electric predators also send out zaps to scout for prey. 

Video of an electric eel hunting and catching a small fish (shown in speeds slower than life). In full-color sequences, knocklike sounds indicate electrical discharges from the eels. Black-and-white sequences have been colorized (red) to indicate when the eel is releasing electrical charges.

Credit: Images, video, and illustration: K. Catania/Vanderbilt University; Produced and narrated by Ashley Yeager

Power Words

eel  A fish with a snakelike body and no scales. Many migrate from freshwater to saltwater when it’s time to spawn.

electricity  A flow of charge, usually from the movement of negatively charged particles, called electrons.

involuntary  An action that is not done intentionally.

muscle  A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.

nerves  Long, delicate fibers that communicate across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold, and pain.

nervous system  The network of nerve cells and fibers that transmits signals between parts of the body.

organ  Various parts of an organism that perform one or more particular functions. For instance, an ovary makes eggs, the brain interprets nerve signals, and a plant’s roots take in nutrients and moisture.

predator  (adjective: predatory) A creature that preys on other animals for most or all of its food.

prey  Animal species eaten by others.

TASER  A brand-name version of a “conducted electrical weapon” used by police, the military, prisons, security guards, and others. It sends out electrodes that deliver a stunning discharge of electricity into a person to incapacitate them. Such devices are popularly referred to as stun guns.

unconscious  Not aware of and not responding to one’s surroundings; not awake.