The electric eel's two-meter long body is packed with specialized cells that can generate their own electricity, and deliver shocks of up to 600 volts. That's more than enough to stun the small fish and invertebrates that the eel typically eats. It's even enough to incapacitate a human or a horse (although contra The Amazing Spider-Man 2, it will not give you superpowers.)
Despite their intensity, these shocks are not blunt instruments. Kenneth Catania from Vanderbilt University has shown that the eel's high-voltage discharges are immensely subtle weapons with at least three distinct functions. They force prey to twitch and so reveal their presence. They then paralyze the victims by causing all their muscles to contract. And they monitor the movements of these stiffened, floating mouthfuls. The electric eel is a battery, Taser, remote control, and tracking device, all in one.
Despite its name, the electric eel is not an eel at all, but a knifefish—one of several South American fish that are closely related to carp and catfish, and that specialize in producing electric fields. Most members only produce weak fields, which they use to find their way through murky water, and to identify and communicate with their peers.
The electric eel shares these abilities, but supplements them with much stronger discharges produced by separate organs. These have made it a mainstay of urban legend, pulp fiction, and scientific studies. In the 19th century, Michael Faraday got his hands on electric eels to understand the role that electricity plays in the nervous system. I mean that literally: He would sometimes test the eel's ability by touching it with his bare hands. “The shock of this animal was very powerful when the hands were placed in a favourable position, i.e. one on the body near the head, and the other near the tail,” he wrote.
Two centuries on, and scientists are a little more circumspect. When Catania started studying the eels, he used thick rubber gloves.
Catania is a rarity: a neurobiologist who specializes in single-handedly uncovering the secrets of unusual and amazing animals. In a century dominated by big, collaborative science, Catania stands out for a string of single-author papers in top-tier scientific journals. He showed how star-nosed moles smell underwater by blowing bubbles. He discovered how tentacled snakes force fish to swim into their mouths. He showed how humans, those weirdest of all animals, mimic moles to harvest worms. And last year, he showed that the electric eel's lethal pulses are even more shocking (sorry) than anyone had thought.
When the eels approach their prey, they release around 400 high-voltage pulses every second. Catania discovered that the pulses act like a wireless Taser: They stimulate the neurons that feed into a fish's muscles, forcing those muscles to contract. This paralyzes the fish, allowing the eel to swallow it with a fast strike.
When they hunt, the eels release simple pairs of quick pulses, of a kind that trigger exceptionally strong muscle contractions. Even if fish are hiding, these doublet pulses compel them to reveal their location through uncontrollable twitches, which the eels sense. “This is one of the most amazing things I’ve encountered in studying animals, and I’ve seen a lot of unusual things,” Catania told me at the time.
Since then, Catania has discovered that the eel's high-voltage pulse isn't just a weapon—it's also a tracking system. Since animals naturally conduct electricity, the eels can use feedback from its pulses to sense the location of its immobilized prey.
Catania proved this by providing captive eels with lobotomized and anesthetized fish, whose movements he could control. Typically, the eels would release their doublet pulses, Catania would twitch his zombie fish, and the eels would strike. But when Catania insulated the fish in plastic bags, the eels became oddly ineffective and non-committal. They still struck in the direction of a twitch, but they usually aborted their attacks halfway through.
That changed when Catania put a conductive carbon rod into the water. Now, the twitching fish once again provoked a violent strike—but the eels would aim for the rod instead. The eels would even single out the conductive rod when it was surrounded by six plastic non-conductive ones. They could accurately track a small moving conductor that was spinning about on a rotating disk and surrounded by plastic ones. And they proved to be incredible fast and flexible: Even if the disk reversed its spin, the eels could change direction mid-strike.
This suggests that they use their high-voltage pulses to track the movements of their prey. This might seem absurd since those same pulses also paralyze small animals, but the immobilized victims can still drift through the water, especially if they had started escaping before the eel shocked them.
“To my knowledge, this is the first example of an animal generating a signal that simultaneously serves as a weapon and as a carrier for sensory imaging,” says Bruce Carlson from Washington University in St Louis. Until now, scientists had assumed that the eel uses its weak discharges for communication and navigation, and its strong ones for offense and defense—two sets of entirely separate functions. But this new study shows that the strong ones enable a form of navigation, too.