Electric eels produce the most powerful shocks of any fish. They can zap prey with up to 600 volts of electricity, enough to hurt even a human. But the serpentlike fish have an even more amazing trick up their sleeve, new research reveals. The eels can shock their prey from meters away, making them twitch to reveal their hiding spot and providing the eel with an easy snack.
“It’s really a beautiful piece of work,” says biologist Jason Gallant of Michigan State University in East Lansing, who studies the evolution of electric fish but was not involved in the new research. “These findings were a total surprise to me.”
To conduct the new study, Kenneth Catania, a biologist at Vanderbilt University in Nashville, recorded electric eel (Electrophorus electricus) attacks on smaller fish, such as koi, in a large tank with a high-speed video camera, capturing 1000 frames per second. At the same time, he recorded the pulses emitted by the eel and the muscle contractions of the fish. When an eel senses the movements of a nearby fish, he showed, the eel releases a high-voltage volley of electric pulses that not only shock the fish as it’s trying to swim by, but also cause a massive, involuntary contraction of the animal’s muscles, freezing it in place. If Catania injected the fish with a drug that blocks communication between nerves and muscles, however, its muscles weren’t frozen. That experiment showed that the eel’s shock immobilizes its prey’s muscles by stimulating the fish’s motor neurons. It’s the first time a fish’s electricity has been shown to have such a specific biological effect in prey. “The eel is a swimming Taser,” Catania says. “The mechanism is the same.”
But that wasn’t all Catania found. He also noticed that when a hunting eel is pursuing a fish that suddenly stops moving, the eel produces a different pattern of discharges: two or three isolated pulses of high-voltage electricity. Catania recreated that situation in the lab by placing a paralyzed fish below a barrier in the eel’s tank. He showed that these electrical doublets and triplets make a fish twitch, giving away its location to the eel, which can then go into attack mode a mere 20 milliseconds later. In the study, published online today in Science, fish were only about 15 centimeters away from the eel, but it’s likely that the eel could induce movement in potential prey from farther away. “Each pulse of the eel caused a single muscle contraction in the fish in a 1-to-1 ratio,” he says. “It’s working like a remote control.”
“It’s a fascinating example of evolution in action,” Gallant says. “The eel isn’t just applying a voltage to the water and hoping everything dies. It’s a very specific behavior that’s obviously been acted on by selection to be refined.”
The ability to produce electricity may have first been selected for in electric eels, in fact, because it allowed the animals to reveal the location of prey, Catania hypothesizes. “Now that we know what the mechanism of the eel’s attack is, it’s definitely easier to ponder how it evolved.”