Don't mess with an electric eel. Turns out these animals are neural puppetmasters, using electricity as the strings to manipulate their prey. While the serpentine creatures regularly use their zapping powers to temporarily paralyze unlucky fish, they can also send out a shock that will cause hidden fish to twitch, betraying their positions and making them easy targets.
The findings, described in the journal Science, reveal a hidden power lurking inside an already shocking animal.
"What the animals are doing, what they've evolved to do, is much more finely tuned than we ever realized," said Harold Zakon, an evolutionary neurobiologist at the University of Texas at Austin who was not involved in the study.
Electric eels are famous for their ability to wield a power that humans have only really harnessed in the last couple of centuries. And Alessandro Volta, who invented the forerunner of the modern battery in 1800, actually took some inspiration from the structure of the electric eel's organs.
Scientists have long known that eels use electricity to both attack and defend, but exactly what they were doing to their prey's bodies has remained something of a mystery, said study author Kenneth Catania, a neurobiologist at Vanderbilt University in Nashville.
To get a glimpse at the eels' technique, Catania first used a high-speed camera to track the eels' lightning-fast attacks on unwary fish, discovering that the eel could immobilize its prey in just 3 milliseconds.
"That's just so incredibly fast that I wanted to know how that could possibly happen," Catania said.
So Catania put an eel and a brain-dead fish in separate compartments divided by a thin agar barrier (which is translucent and electrically conductive). He got the eels to zap some tasty earthworms, and watched how those electric pulses affect the fish on the other side of the agar wall.
The eels wielded electricity in a number of different ways. They would send out low-level pulses that had no effect on nearby fish but gave them a radar-like ability to sense their surroundings -- an ability that has evolved multiple times in different species of electric fish.
But when the eels attacked, Catania found, they released high-frequency pulses that quickly immobilized the prey. These attacks can be shocking -- a large eel can deliver a stinging 600 volts.
Was this shock directly causing the fish's muscles to seize, or zapping the neurons in the brain? With a methodical series of tests, Catania whittled the possibilities down to one: The eels' electric pulses were targeting the motor neurons between spine and muscle that told the muscles when to fire. In some ways, that's a more sophisticated approach than directly targeting the muscle. It's as if, rather than turning your TV off by unplugging it from the wall, you turned it off by using the button on your remote control.
But Catania also noticed that the eels were occasionally firing off doublets of electric pulses and, when they detected the brain-dead fish spasm, trying to attack it through the translucent agar barrier. It seemed the eels were using another clever electric trick -- they were forcing hidden prey to reveal themselves by sending out electric pulses that made the hapless animals twitch uncontrollably. The eel, sensing this movement, could then attack with its full-fledged electric volley and paralyze the fish, rendering it easy pickings.
Other studies have shown that such doublets of electric pulses delivered to motor neurons can trigger high rates of muscle tension. And watching the pattern of the eel's electric zaps, it's almost as if the eel understands the way these neurons work, Catania said.
"It looks like the eel's been out there reading the literature," said Catania, who has read through much of the research himself. "It's been studying up on motor neuron physiology."
Using electricity, the eels were able to make their prey move on command, and also able to make it freeze. And all of this happens within a tiny fraction of a second, Catania said.
"I was completely amazed," Catania said. "I was really amazed."
Zakon, who studies other electric fish, praised the results.
"People knew that eels made shocks -- they just assumed that it paralyzed the fish by causing muscle contractions or possibly acting on the central nervous system," Zakon said. "Nobody really looked carefully to say, 'How does this work?'"
The discovery that the eel doesn't directly stimulate its prey's muscles, but affects the neurons connected to them, opened up a new set of questions, Zakon added.
"How does this not act on the muscles, how does it not act on the brain?" he said. "Somehow, the eels have evolved a system that works perfectly to incapacitate in a very simple way."