In an unusual discovery, electric eels leap from the water to attack predators with a high-voltage punch, a new study says.
In recent experiments, a scientist found that the South American fish go after large, moving, and partially submerged objects, pressing their chins against the target to discharge shocks.
The finding lends support to a centuries-old account of eel fishing by the German naturalist Alexander von Humboldt.
In 1800, Humboldt observed native fishermen in Venezuela collecting electric eels by "fishing with horses." The men herded horses into a muddy pool containing electric eels, provoking the eels to repeatedly attack. After the eels had exhausted themselves—and caused a few horses to drown—the natives safely captured the 1.5-metre-long fish.
This famous story has been repeated and illustrated numerous times over the years, but many were skeptical about its veracity, including sole author Kenneth Catania, a biologist at Vanderbilt University.
That is, until he serendipitously witnessed a similar behavior in electric eels in his laboratory.
"Pretty Shocking Experience"
While conducting previous experiments, Catania transferred eels from a home aquarium to an experimental chamber with a net that had a metal rim and handle.
He noticed that as the net approached the eel would often turn around and pounce.
"It would press its chin against the handle and explode out of the water upwards along the handle towards my hand," says Catania, whose study appears June 6 in the journal Proceedings of the National Academy of Sciences.
“I was wearing gloves, so I wasn’t in any danger of being shocked, but it was a pretty shocking experience, anyway.”
But Catania noticed that the electric eels were coordinating their leaps with volleys of high-voltage pulses, indicating this might be more than just a simple escape response.
To measure the voltage and current of these pulses, Catania submerged a conductive rod and plate partway into the water of the eel's aquarium.
The eels leapt and electrocuted the approaching rod and plate, and both the voltage and the magnitude of current rose as the eels ascended higher.
Catania also connected sets of LED lights to strips of conductive tape and attached them to a fake predator, which showed that by leaping, the eels progressively electrify greater portions of the partially submerged target.
Finally, slow-motion video revealed the eels bent their necks to maintain contact between their electric organs and the threat.
This lunging strategy enables eels to deliver much of their electrical power, normally distributed throughout the water around them, directly to a threat.
Catania says these attacks seem to be most effective against terrestrial or semiaquatic predators.
Eels might resort to this aggressive behavior when they are trapped in dried-up pools during the Amazonian dry season—the time of year when Humboldt's encounter occurred.
“Many have been skeptical of Humboldt’s original claim, myself included, but this study shows that his account could be true,” says Bruce Carlson, a biologist at Washington University in St. Louis who was not involved in the research.
“More fundamentally for biology, this is a beautiful example of how the eel has evolved a fairly simple behavior that exploits the basic physics of electricity."
“Before Catania’s recent string of papers, the eel’s predatory behavior was largely viewed simplistically, as though the eel just turns on the voltage, stuns the prey, and then easily catches it," he adds.
Instead, eels have a "surprising degree of sophistication and nuance" in how they both detect and manipulate their prey and avoid predators.
Many questions remain, including whether eels actually perform this behavior in their natural environment and how potential predators respond to these attacks.