A stunner: For eels, zaps have dual roles

Study: Electric fish shock, track prey

It turns out that the zaps of an electric eel are more than just a powerful weapon.

After using electric pulses to stun their prey, the electric eels use the same electric field as a sort of radar, allowing them to track down their meal without sight or touch.

The intriguing dual nature of the shocks was revealed by Vanderbilt University biologist Kenneth Catania. His results were published Tuesday in Nature Communications.

In a way, Catania said, this finding brings the story of the electric eel full circle.

Lots of little South American fish generate low-level electric fields. But the related electric eel, which is a fish, gives off pulses with hundreds of volts of electricity.

“Even Darwin puzzled over how this evolved,” Catania said.

Weaponized electricity made sense to biologists, but for a long time they couldn’t figure out why the low-level pulses — which had presumably evolved first — existed in the first place.

What was the point of generating a tiny electric field, too weak to fend off predators or stun prey?

In the 1950s, scientists realized that the smaller fish were using their weak pulses to feel out their surroundings.

“Electric fish in general have the ability to sort of probe their environment with electricity,” Catania said.

But Catania has shown for the first time that electric eels have retained this ability, even as they’ve evolved to use their electrical pulses for more aggressive means, as well.

When an electric eel stuns its prey, according to the new study, it also reads the way that prey interacts with the resulting electric field, allowing the eel to follow its prey without using any other senses.

To be sure that the tracking ability really existed, Catania replaced the prey with something artificial: a piece of carbon rod that would, like the fish, be more conductive than water itself. When Catania made the “fish” twitch, the electric eel would initially go after the movement in the water. Then it would send off a high-voltage pulse and then follow the conductor no matter how many other senses Catania took away.

“Because I was trying to show something kind of extraordinary, I made every control possible,” Catania explained. He made a spinning disk that featured several decoys along with one conductive carbon rod, and he covered it with agar — which conducts electricity — to keep the electric eel from touching the rod itself. He used only infrared light, which electric eels can’t see in, to illuminate the spinning disk.

Still, the eel would race after the conductive rod.

“The electricity is simultaneously a weapon and part of a radar system,” Catania said. “People have studied this animal now for centuries, and no one had entertained this possibility.”