Getting zapped for science

Researcher finds out why an electric eel leaps out of the water to give him huge shocks

P Rajendran

Kenneth Catania

Kenneth Catania, Stevenson professor in
Vanderbilt University’s Department of
Biological Sciences.
Photo: Steve Green/Vanderbilt University

Some people would do just about anything to sate their curiosity.

Take the case of Kenneth Catania, who plunged his hands repeatedly into a tank containing a rather irritated eel – the electric kind. He wanted to find out the shock was worse when he was zapped by the electric eel underwater or when it leaped out to deliver a jolt higher up on his arm.

“It was not that day one I’d stick my hand in the aquarium,” Catania, a professor at Vanderbilt University, told Truly Curious, disabusing concerns that he may be a closet masochist.

“I’ve been studying electric eels,” he said, and went on to describe the shocking behavior of so much interest to him. But then Catania loves studying all animals – the weirder the better.

As Catania puts it, “We’ve studied quite a number of species. I try to be a bit of detective for unusual animals.”

So he has discovered that star-nosed moles are the fastest-eating mammal forager. That water shrews blow bubbles into water and plunge into those same air pockets to check for new odors. That a tentacled snake will feint at a fish to set off a reflexive turn away from the threat only be waiting with its mouth right where it turns to… Naturally, electric eels are right up his alley.

The electric eel (Electrophorus electricus)

The electric eel (Electrophorus electricus), Photo: CC BY-SA 3.0

To get this out of the way, electric eels are actually freshwater fish, not traditional eels, only looking a bit like the latter for being finned, slender and sinuous. Unlike electric eels (to keep the name while noting the difference), real eels do not breathe air through their mouths, and do have teeth and a dorsal fin. These characteristics put it in the order Angulliformes. The electric eel, named Electrophorus electricus – just so there is no confusion about its main selling point – belongs to the order Gymnotiformes, a group of nocturnal freshwater fish found only in South America and whose close relatives are catfish and carp.

Now, many fish, such as lampreys, sharks and their relatives, have electroreceptors that can help them gauge their environment. With the advent of color vision and improving eyesight, many fish that lived closer to the water’s surface had no real use of the ability and lost it. There is good evidence, though, that our fishy ancestors who first lugged their way up onto land had electroreceptors, too.

One of the most repeated stories about the stunning electric eel came from an account of Alexander von Humboldt, a German biogeographer who collected five healthy specimens at Cano de Bera, a marsh in a small town called Rastro de Abaxo, in modern-day Venezuela. He had to thank the locals, who took the expeditious step of sending a troupe of horses into the shallow water to exhaust the eels’ electric charges.

The swarm of eels attacked en masse, laying their lengths along the horses’ bellies and letting loose a strong charge, nowadays put at about 600 volts. Though it takes more than one eel to do the job, the massed attack led to two horses collapsing in exhaustion into the water and drowning. But since there were about 30 horses, the eels used up their stored charge and, unable to replenish it in time, were easily picked up by the natives.

Humboldt had a merry time documenting the electric eels’ ability. He also dissected some of them find the electric organs, collecting a few shocks himself in the bargain, and regaled everyone with stories about their behavior – in person and print.

An article in The Atlantic in 1947 by Christopher Coates, a curator at the New York Zoological Society, disparagingly described the tale as “tommyrot.” When covering Catania’s work more recently, the magazine graciously apologized for that error of 69 years vintage.

Though Humboldt did not describe the electric eels actually leaving the water during their attack, Catania found out at first hand that the animal could indeed leap out of the water to deliver a shock.

In a paper last year, with masterful understatement Catania wrote that the ability was “serendipitously discovered during research into electric eel predatory behavior and sensory abilities.” That is, Catania, who does most of his own research, was moving the animal using a metal-rimmed net when it leaped up to the handle, apparently determined to pump him full of stored electrons. Fortunately for him – at least then – he was wearing rubber gloves.

As he put it in that paper, “This behavior was both literally and figuratively shocking.” Never before had eels been observed leaping out of the water, not even in the Humboldt-led team’s steed vs serpent fish gambit.

But just a month after that paper was out, came a viral video of a hapless Brazilian wading into a pool and have an electric eel leap on him, push him backward into the water.

Catania pointed out to TC that the electric eel is not all about offense. It also has a low-voltage setting for its sensory system, generating a a weak electric field it uses as a form of radar to ‘see’ in the murky water it lives in. It can also send out high voltage to cause prey to twitch and thus reveal itself while being also being stunned into quiescence.

Other researchers have shown electric eels curling over to deliver a double whammy to prey, but when it comes to bigger threats, they rely on maximum long-term contact.

But one thing it does not do is vary the amount of high voltage depending on the threat its surroundings. High voltage output was fixed for any given electric eel.

Catania wanted to know why electric eels would attack in the first place. He guessed it would be because there is not much place to flee to in their tiny ponds. Less clear was why the animal would expose all of itself in a leap rather than deliver a jolt beneath the surface before discreetly withdrawing into the murky shallows.

The researcher guessed that the animal was relying on what is called Ohm’s law, which says that voltage is a product of current and resistance.

To inform or to refresh you, electricity is based on the flow of electrons, each of which carry a unit negative charge.

Current – akin to water current – is the rate at which electrons flow, with one amp, the measure of current, being the movement of a little over 6 quintillion electrons going past a point in a second.

Voltage is just the difference in the charge at two positions in the circuit.

Resistance is whatever slows electron flow. So if resistance to current goes up, its flow will slow down. Higher resistance ensures higher voltage (since there is resistance to the effort to correct the difference in charge).

The plates experiment

Two plates, one in the water and the other above it were separated by an insulator. When the electric eel reached the maximum height in its leap to deliver a shock, the upper plate showed a voltage of 127 volts. Image: Kenneth Catania

Catania believed that water conducts away some of the charge. and that leaping out and having less contact with water ensured most of the charge traveled through the target.

To make sure, Catania set up two metal plates, one underwater and the other above, with insulating material in between. He measured the voltage in both plates as the electric eel rose out of the water to shock the upper plate. Sure enough, as the electric eel rose and reduced connection with the water, the voltage difference between the plates rose to 127 volts.

Conductive metal plates are one thing, human flesh is another. As the leaping animal attacked a person, would the current be more than one delivered in the water? There was but one way to find out…

And so it was that Catania rolled up his sleeve, stuck his hand in a water-filled plastic container covered with aluminum tape inside and one outside but not directly in contact with each other. An insulated wire did link them, first passing through an ammeter, a unit that measures current.

Catania guessed that if the circuit was complete, he would get a bigger shock as the electric eel wiggled its electrified chin up his arm. Of course, because it would happen so quickly, the attack would have to be recorded for posterity. For this, he used PowerLab, which also took timestamps so Catania could later confirm that the sore arm he was nursing got the biggest supply of current when the electric eel had least contact with the water.

Catania put his hand in a plastic chamber that had a low-resistance path back to the main body of water. Current was measured with an independent hall-effect ammeter. This was connected to a PowerLab unit, which also recorded timestamps from the high-speed video camera. Images: Kenneth Catania

While everyone harps on the voltage (600 V), Catania said, “Current is more important,” and points out that static electricity delivers thousands of volts but few die of it. Most electric shocks used to get a withdrawal reflex from humans rely on just 5-10 millamps (mA). The juvenile electric eel Catania used pumped in 40-50 mA into his forearm.

As Catania described in his paper, ”It is therefore not surprising that the subject reported that the eel’s shocking leaps were strongly aversive. The subjective report was that involuntary arm withdrawal occurred on every trial during which a circuit was made.” He then went on to mull over whether the withdrawal could be described with confidence as a reflexive response without using actually measuring muscular electrical activity and the time to react.

He conceded, “it would be unusual if the withdrawal reflex had not been elicited.”

Catania was being conservative when he used a juvenile eel, just 40 cm long. In another study where his forearm did not play a starring role, he had measured the voltage generated in four electric eels. The longest one he studied was 113 cm long and generated 382 V. A larger one, he argued, could generate 500 V.

The electric eel delivering the shocks

Above, A series of images showing the electric eel delivering the shocks up the human forearm. Below, the corresponding rise in measured current as the animal leaves the water. Images: Kenneth Catania

“They never bite. That tells us [touching with the chin] is not for hunting,” he said. “It’s pretty clear they’re trying to make electrical contact.”
Bruce Carlson, a professor at Washington University in St. Louis who studies electric fish, told NPR of his admiration for Catania’s work.
He said, “Really, I think the community was kind of naïve and just thought that, ‘Well, it’s really simple, the eel generates up to 600 or 700 volts of electricity and so it just shocks whatever is near it, and it’s as simple as that, and there’s really nothing to study here…’ Ken kind of has a knack for observing behavior carefully and seeing things that other people haven’t thought of before and then designing really elegant studies to dissect those behaviors in more detail.”

Told of that quote, Catania said, “I find that very flattering. I do try very hard to do definitive experiments, and hopefully let the animal speak for itself.”

Use this link to get to the full paper from Current Biology

Ken Catania describes his work in a Vanderbilt University video:

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