The deadly jolt of the electric eel
Long before the first battery, the electric eel was an ambassador from the unknown world of electricity. So Kenneth Catania, a professor of biology at Vanderbilt University who has just published a study on the hunting behavior of the eel, was a bit surprised that nobody had studied exactly how the eel attacks its prey.
While teaching about eels, he says, “I got fascinated by their behavior. Under high-speed video — no-one had done that as far as I can tell — the prey were completely immobilized about 3 milliseconds after the eel began its electric attack.”
When the eel’s specialized muscle cells created an electric field in the water, “I imagined the prey would twitch,” Catania says, “but it completely froze up, which suggests this was caused by involuntary muscle contraction, which is pretty much how a Taser works.”
With its prey immobilized, the eel mounts “an incredibly fast, explosive strike,” says Catania. And even though the fish muscle cells must devote energy to generating current, “There was no indication in its behavior that suggests they had lost a substantial amount of movement. I thought, and others thought, the electric eel could lazily swim up, shock and consume the prey.”
All things considered, Catania says the eels “are quite a formidable predator.” In its native Amazon Basin, “Fishermen really don’t like them, and there are historic accounts of people who study eels who have slipped, been shocked and immobilized.”
A three-step “dance of doom”
But the shocks are only the final step in a sophisticated, three-prong attack. Electrically speaking, the eels use:
A form of low-voltage (think electric sonar) to detect prey in its muddy environment.
A different, low-voltage signal that elicits a twitch from nearby prey that makes waves that the eel’s pressure sensors can detect.
The immobilizing jolt at a frequency of 400 hertz. At 600 volts, it’s more than four times the voltage of household current. The attack causes tetanus — a widespread explosion of muscular activity that turns the fish into a rigid, helpless dinner.
“It’s a very sophisticated animal,” says Catania. “It’s got eyes, and even though a lot of people say they are mostly blind, the ones I study clearly are paying attention visually. They have passive electrical receptors and active electrical receptors; they can probe with electricity, and have that high-voltage immobilizing discharge. And they are incredibly sensitive to slight water movement, which will cause them to attack almost immediately.”
“First, you become my slave. Then you die!”
Perhaps the most astonishing part of the story is how the eel seizes control of the prey’s nervous system. In part of the experiment, Catania removed the brain and spinal cord from the prey fish, and saw that the electric attack nonetheless caused its muscles to fire by activating nerves linking the spinal cord to the muscles. “There is a remarkably tight correspondence between eel’s output and the muscles,” Catania says.
Other predators enslave their prey. For example, the emerald cockroach wasp injects its eggs and disables the cockroach’s nervous system, forcing the roach, zombie-like, to carry the wasp’s larvae until the larvae eats it alive. The rabies virus turns dogs aggressive to help the virus spread before its host dies. “But nothing I know of works as fast” as the electric eel, says Catania.
If it’s so powerful, why doesn’t the electric eel shock itself? “I’ve not found any obvious explanation,” says Catania. “Presumably there is some way of projecting the electricity through a path of least resistance that does not include its own brain.”
Creating all that electric energy does tire the fish, Catania adds. “You can see, over time, that the frequency of the pulses does slow down.”
The electric eel Electrophorus electricus is one of hundreds of fish that create electric current for defense, predation, navigation and communication. A new study by University of Wisconsin-Madison researchers shows that all six electric fish lineages use essentially the same genes and cellular pathways to make the electric organ. “What is amazing is that the electric organ arose independently six times in the course of evolutionary history,” says Lindsay Traeger, a UW-Madison graduate student and a co-lead author of the report.
Some scientific advances rely on new instrumentation or theories, but not the current (heh-heh!) study. Instead, the hunting behavior of the electric eel was hiding in plain sight. “Maybe it sounded open and shut,” Catania says, when asked why the eel’s electric secret was not unraveled before now. “What was there to know if eel shocked the heck out of its prey? But it turned out to be far more interesting than you could have imagined.”
– David J. Tenenbaum