The high voltage strike of an electric eel has long been regarded as one of the most powerful attacks in nature. But now one scientist has discovered that the nocturnal animals can also use their shock waves to track prey in the dark.
Scientists have been trying to unlock the secrets of electric eels for centuries, but the creatures, actually a kind of knifefish, are notoriously difficult to study. Electric eels live in the remote, murky waters of the Amazon River Basin in South America where it’s tricky to observe them in their natural habitat. And there are many challenges in capturing an animal capable of growing as long as 8 feet (2.4 meters) and generating an electrical charge of up to 600 volts—a punch strong enough to knock down a horse.
Plus, the eel’s attack is really, really fast, says Ken Catania, a neurobiologist at Vanderbilt University and author of the study published today in Nature Communications. Just milliseconds after the animal unleashes its electric barrage, its head is already moving to gobble up the paralyzed prey.
By tricking electric eels into using their secret weapon in the lab, and recording these attacks with high-speed video, Catania has found a previously hidden aspect to their hunt. “The eel can use its electric attack simultaneously as a weapon and a sensory system,” he says. “It’s sort of a science-fiction-like ability.”
Lightning in the Dark
Electric eels have thousands of special cells, called electrocytes, that can store energy like a battery and then discharge it. When an eel hunts, it uses this high-voltage charge to disable the muscles of a fish, similar to the effects of a Taser gun, before sucking the fish into its mouth. The eels can also use the charge to protect themselves against predators, such as caiman.
In an earlier study, Catania showed that electric eels can use their high-voltage attacks to coax prey out of hiding by causing the fish’s muscles to twitch. The eel detects this movement and hoovers up the prey.
But what was unclear until now was how an eel knew where its prey was once disabled. Like a pheasant shot in mid-flight, a fast-swimming fish, driven by momentum, might keep careening through the water even after its muscles have been deactivated by the eel’s shocks.
To understand what was happening, Catania brought electric eels into the lab and presented them with anesthetized fish that were insulated from the eel’s electroreceptors by plastic bags. With an electrode, Catania made the fish flinch, and the eel discharged its high-voltage attack. But then it didn’t seem to know what to do next—the eel lunged in the direction of movement in the water but didn’t attempt to suck the fish into its mouth.
Catania then put an electrically conductive carbon rod into the tank along with the fish. He made the fish flinch, and the eel attacked with a shock. Sometimes the eel started to move in the direction of the fish, but then it changed course to lunge at the rod wherever it had been placed in the tank. To the eel, the fish seemed to be in two places at once.
The eel experienced even more of this “sensory conflict” when Catania placed the rod on a rotating wheel, with no fish present. The eels would twist and turn after an electric volley in an attempt to suck up what it sensed was a fish but was, in fact, the moving rod.
“The eel turns on its high-voltage as a way to deactivate the fish,” says Catania, “but at the same time, it’s also using that high-voltage as a way to track where the fish is.”
Bats, Whales, And… Electric Eels?
The eel’s strike may be unique, but aspects of its superpowers can be found in other animals. Sharks, rays, and a few other species can sense electrical fields around them, for example. And bats and some toothed whales produce sonar, which helps them hunt by listening to reflected sound.
A bat’s echolocation and an eel’s electrolocation are similar in some ways—both animals produce a high-pulse rate emission just before prey capture, notes Aaron Corcoran, a post-doctoral researcher at Wake Forest University and an expert in sonar. “Of course, bats don’t stun their prey,” he says. “That electric eels are electrolocating prey as they stun them is at once astonishing and intriguing.”
The findings could help illuminate how the eel’s electrical wallop evolved—an enigma Charles Darwin pondered in On the Origin of Species. Catania says it now seems likely that the eel’s high-voltage charge evolved first as a way of sensing the environment around them. The weapon part came later.
Before he began studying these animals, Catania used to think electric eels were fairly primitive. “Now that I’ve gotten into the details,” he says, he considers the eel “one of the most sophisticated predators out there.”