Electric Eels Put Their Prey in a Killer Sleeper Hold

Why not deliver double the shock if you can do it with no extra effort?

Ken Catania

Kenneth Catania started studying electric eels a few years ago, and soon became obsessed. In quick order, he showed that they paralyze their prey with intense high-voltage shocks, which stimulate the neurons that feed into muscles, forcing the victim's body to involuntarily stiffen. He discovered that they can generate subtler shocks to make their prey twitch, and so give away their position. And, as I covered just last week, he found that they use electric fields to sense the position of their helpless targets and guide their final strikes.

Last year, he started noticing something odd. Although electric eels typically swallow their stunned victims with a quick strike, they sometimes curl their bodies around their prey, pinioning them between head and tail.

“I wrote a little note to myself on a Post-It, saying: Is there an effect?” he says.

As it happens: yes.

Most of the electric eel's organs are squished up near its head, and some 80 percent of its sinuous body consists of the modified muscles that produce its infamous electric discharges. It is, essentially, a long living-battery, with the positive pole at its head and the negative one at its tail. In a typical attack, a victim only experiences the electric field coming out of the head end. But Catania predicted that the eel should theoretically be able to double the strength of that field simply by curling its tail round, delivering twice the shock for no extra effort.

“From watching the videos, I got a big clue: When the eel curls, it's most likely because it's dealing with something struggling,” he says. “And I know it holds on tight because I was hand-feeding my big guy with rubber gloves on, and he got a bit too enthusiastic. There are no teeth per se, but it's a rasping hold.”

So, in lieu of his own hand, Catania created what he called an “eel chew-toy” by fitting a dead fish with electrodes. When the eel struck, he shook the electrode leads to simulate a struggling prey. Hence: curling.

The electrodes confirmed that the curled position more than doubles the voltage delivered to the eel's prey. “Then, on my Post-It, I wrote: HA!” says Catania, “because the effect was so huge. It's beautiful because it's straight out of introductory physics.”

These amplified discharges don't kill a target outright. Instead, they leave it weak and fatigued by forcing its muscles to continuously contract. In a fraction of a second, the eel can make you feel like you've been through the longest full-body workout of your life—and the last.

The venom glands of many snakes, spiders, and scorpions inflict a similar incapacitation. “There's a whole host of very familiar neurotoxic animals that have come up with mechanisms of inactivating muscles with chemicals,” says Catania. “And the eel has done the same thing with electricity. That just blows me away.”

But why does an electric eel even need to double its electric field? Surely its vanilla 600-volt shocks are enough to subdue any prey? Indeed, when I spoke to Catania last year, back when he had first started studying the eels, he suggested that their powers should be unbeatable. Since then, he has backpedaled a bit.

In the lab, “we want to make this animal's life as easy as possible, so they're basically fed goldfish,” he says. “It's like shocking fish in a barrel.” But the Amazon, where the eel actually lives, is full of well-defended fish, with armor and spines. Some of these may have adapted to the eel's attacks by evolving more resistant skin.

Young eels may not be able to muster enough of a voltage to stun these tougher targets. Even if they do, these predators swallow their prey whole, and if they don't get a good first bite, they have to let go to reposition themselves. That creates a brief window when prey can shake off the shock and escape. This probably explains why even large eels use the curling technique when tackling unwieldy targets. “I saw this very large eel curl around a crayfish and just shock the heck out of it for a minute,” says Catania. “You can't help but watch that and wonder what's going on in the Amazon.”

Here's the thing: We have no idea.

Despite the electric eel's fame, we still don't know really basic things about it, like what it eats. Catania tracked down every possible reference, and couldn't find a decent source that documented their prey. The only halfway-decent observation was indirect, from a scientist who was recording the outputs of weakly electric fish and saw a burst that potentially came from an eel. Does the eel hunt its own kin, or other weaker wielders of electricity? No one knows.

I also wondered, on talking to Catania, if the ability to concentrate its electric field could have contributed to the evolution of the eel's long body. After all, the electric eel isn't an eel. It's a knifefish. Most of its relatives look more like flattened blades—hence their name. They can't curl to the same extent. The electric eel is both the only dangerously electric member of the family, and the only one that... well, looks like an eel. Are those things connected?

“The scientist in me is always cautious about evolutionary stories,” says Catania. “But if you look at an eel, whose body is mostly the electric organ, it seems fair to say that this animal was selected over time to have a body that increases power output. And here's a behavior that doubles your power through your prey for almost nothing.”