Shocking Experience
Electric Eels More Talented Than We Imagined, Catania Suggests
Photo: Andrew Propp
What would make you put your bare hand in a tank with an electric eel? Research. That’s the answer you might give if you’re “Vanderbilt’s most shocking dude.” A biologist and neuroscientist, Dr. Kenneth Catania studies evolutionary interpretations of animal behavior and how animals process sensory information. He gave a lecture recently as part of the NIH Neuroscience Seminar Series in the Porter Neuroscience Research Center.
Stevenson professor of biological sciences at Vanderbilt University and winner of a 2006 MacArthur “genius” grant, the University of Maryland graduate has studied several animals that are not typically examined in neurobiology—the masked shrew, tentacle snakes, the water shrew, even crocodiles.
NICHD’s Dr. Harry Burgess, who invited Catania to give the seminar, quipped that apparently an undergrad internship at the National Zoo “kicked off what looks like [Catania’s] lifelong passion for the star-nosed mole.”
In 2011, Catania and colleagues patented the cortical representation of somatosensory input from the nasal rays of the star-nosed mole.
Exploring the Unexpected
“We all know the tremendous advances in neuroscience that have been made by studying everything from the giant axon of the squid to the Aplysia,” Burgess noted. “Here, we tend to work on more traditional model systems. It’s important to keep in mind that there’s this cornucopia of other models out there that can really provide unique insights…[Catania’s] work is inspiring in the sense that it makes people—not just scientists, but also the general public—really excited about science.”
Catania explained, “There’s kind of a theme for a lot of the research we’ve done in my laboratory—that the animals have been so much more interesting than I could possibly have imagined. Each and every one of them has been full of unexpected surprises.”
His NIH talk, “The Shocking Ability of Electric Eels,” focused on such an unexpected observance, which had prompted him to put his hand in an aquarium with a…stunner. But that’s getting ahead of the story.
Not Your Average Eel
First, Catania offered a primer: Despite its name, the electric eel, or Electrophorus electricus, actually is not an eel but a fish, close kin to the small South American electric fish, which modified its muscles to produce a weak electric field that became a low-voltage sensory system.
Photo: opencage.info
“The electric eel, I like to say, went down the weapons of mass destruction pathway,” Catania joked. Roughly four-fifths, or 80 percent, of the eel’s anatomy is a 600-volt electric organ that generates a powerful energy field—a dipole—in the water around its body. For comparison, a standard household wall socket contains about 120 volts.
While not powerful enough to kill an average human, the eel’s energy output can produce a temporary, painful numbing sensation similar to the effect of a Taser. The eel/fish developed this high-voltage “weapon” and also kept its fish-ancestor’s low-voltage sensory system.
Scientists have been studying electric eel energy output for more than two centuries, Catania pointed out, but what has not been understood completely is how, why and to what extent the animals deploy their power.
Analysis of Paralysis
An electric eel can stop all voluntary movement in a prey fish in about 3 milliseconds—without really touching it. Catania was curious: How, exactly, do electric eels paralyze the fish? Turns out, the eel charge acts as a remote control on the neurons of the prey’s muscles, he explained. Catania set up a fish tank experiment that not only measured the eel strike, but also recorded the action—sight and sound.
Catania said motor neuron action potentials travel to electrocytes in the eel—those modified muscles that create the high-voltage organ—which then generates an action potential that moves through the water to depolarize the prey’s motor neuron action potentials, causing muscle contraction.
Acrobatic Eels
Over the course of about 3 years, Catania documented other fascinating behavior: Electric eels strategically use their high-voltage power simultaneously for offense, defense and to probe their environment. Those dipole fields they create around their bodies are essentially a motion detector that informs the eels of potential food, enemies and other carbon-conducting objects nearby.
Photo: Andrew Propp
“This is not only active electroreception with high voltage,” Catania said, “but also it’s about the best example of active electroreception I’ve ever seen…It’s one of the most dichotomous traits I can think of—they’re using it as a weapon and as a high-resolution tracking system.”
In addition, the curling/coiling behavior the eels perform serves a purpose: they can double the effect of their energy output—in milliseconds—by bending into a loop thereby concentrating the voltage field on a target. Eels can use their strike force to fatigue their prey or a predator, immobilizing larger or otherwise more difficult opponents.
“I think that’s very analogous to the way that neurotoxins work,” Catania said, “so it’s kind of amazing to think that there’s a different way to inactivate muscles.”
Horse vs. Eel: Who Wins?
Around 1800, Italian physicist/chemist Alessandro Volta was inspired by eel anatomy to invent an “artificial electric organ,” or what we now call a battery. However, it was reading about another scientist of the same era—German biogeographer Alexander Humboldt—that inspired Catania to conduct these experiments.
Photo: Andrew Propp
When Humboldt needed electric eels for his research in 1807, he traveled to South America where he engaged fishermen to collect the animals along the Amazon River. The anglers embarked on the task, telling Humboldt they had to first round up the bait: horses.
That story, famous for decades among scientists who widely disregarded its truth, made Catania wonder: “Why would an electric eel attack a horse? Why not just swim in the other direction?”
An eel wouldn’t really go after an animal so much bigger than itself, right?
Don’t Try This at Home
Photo: Andrew Propp
In 1838, Catania noted, British electrochemist Michael Faraday reported about putting his hand in water with an electric eel. He described the shock as somewhat mild, felt only in the hand in the water [as opposed to his whole body].
“I trust Faraday,” Catania said, showing video of his own hand submerged to the wrist in the electric eel tank. “He was right. It hurts a little bit…but it’s not a big deal.”
However, the horse thing still nagged at Catania, so he then put a whole arm—not his, a prop—in the tank. The eel immediately attacked the carbon-containing prop, jumping up and out of the tank onto the faux arm.
“When you approach the eel with a large conductor, they do leap out of the water,” Catania confirmed, to gasps and chuckles from the audience. “It is a shocking experience, literally.”
Truth Stranger Than Fiction
On larger prey, electric eels employ a “repeater strike” motion with their high voltage attack to tire out the opponent. But eventually, the motion will tire out the eel too, depleting its electric energy and rendering it temporarily defenseless.
Humboldt’s report was probably true, Catania determined. The fishermen probably used horses to bait the eels, knowing the eel electricity would peter out battling such huge targets. The anglers then could collect the eels without getting themselves shocked in the process.
“[Electric eels are] really sophisticated animals,” Catania concluded. “The list of things that they can do that I never would have imagined gets harder to fit on my summary slide.”
See his full lecture at https://videocast.nih.gov/summary.asp?Live=19864&bhcp=1.