Shocked!

Electric eels are living batteries that taser their prey with 860-volt jolts. Sharks use electricity like an extra sense to see fish and sneak up on them. Spiders fly using the atmosphere’s electric charge, and bumblebees and flowers communicate through their personal electric fields. How else does the natural world use electricity?

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Giselle Clarkson

Imagine you are a South American freshwater fish, hiding among the weeds in a murky tributary of the Orinoco River. Out there somewhere a predator is lurking, but if you keep still, there’s no chance of being seen. You hope.

Suddenly, a powerful electric charge twangs your nerves like guitar strings, forcing every muscle in your body to momentarily contract. This convulsive twitch sends waves radiating through the muddy water, where they collide with the sensitive motion receptors lining the sides of an electric eel.

The eel doesn’t need to see you to know exactly where you are. It sends a high-voltage volley of electricity in your direction. The barrage of shocks freezes your body, preventing you from swimming away, and simultaneously enables the eel to zero in on your position in the water. There is no escape.

“It’s like the ultimate Taser,” says neurobiologist Kenneth Catania from Vanderbilt University in Nashville, Tennessee.

You are a small fish, so the eel swallows you right away. If you had been a bit larger, the eel would have curled around you to amplify its power and shocked you repeatedly, at 100 pulses a second, until your muscles slackened with exhaustion.

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Electric eels have fascinated scientists for centuries—their anatomy inspired Italian physicist Alessandro Volta to invent the battery. No one quite realised just how terrifying they were until Catania started filming them with high-speed cameras and studying them in the lab.

“They’re kind of inconvenient laboratory animals,” he says. “They’re big, and they’re a little bit scary.” But watching the eels hunt in slow motion led to discovery after discovery about how they use their electric superpower.

Technically, electric eels are not eels at all, but a kind of knifefish. The best-known electric eel, Electrophorus electricus, can reach two and half metres long and discharge shocks of up to 650 volts. (New Zealand’s mains voltage that powers your toaster is around 230 volts.) The newly discovered Electrophorus voltai—Volta’s electric eelis even more powerful, letting off shocks of up to 860 volts.

A living battery that plays your muscles via remote control, forces you to reveal your location, and then Tasers you before swallowing you up? It’s like something out of a horror movieand an awe-inspiring illustration of the wonders produced by evolution.

“The most surprising thing about an electric eel is that it exists at all,” writes Catania in his recent book, Great Adaptations. “We share the planet with a creature that seems like a science-fiction fantasy.”

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Although no other animal can rival the electric eel for sheer power, at least four different groups of fish have evolved shock-producing electric organs. There are electric eels in the Amazon, electric catfish in the Nile, and electric rays and electric stargazers in the oceans. Some of those stargazers are also venomous, leading one scientist to describe them as “the meanest things in creation”. (New Zealand has stargazers, but none of them are electric. We think.)

In a way, these creatures have simply harnessed and amplified a power source that exists within all of us. Electricity flows in the human body and inside all animals with a nervous system. Some of the elements in our bodies—sodium, potassium, calcium and magnesium—exist in the form of electrically charged ions, and carry a positive or negative charge. When our nerves allow these ions in and out of our cells, they generate electrical currents that carry information to our brain, make our hearts beat, and contract our muscles so we can move.

Life is pretty salty when you live in the sea, so to ensure fish don’t end up with too much sodium in their bodies, they’re always releasing sodium ions back into the water through their gills, creating a weak electric field. In the 1970s, scientists showed that sharks were using mucus-filled organs in their skin (called “ampullae of Lorenzini”) to detect fishes’ electric fields. We now know that most sharks and rays, and at least one kind of dolphin, have this “electrosense”—even if they can’t give you a shock. Platypuses, and to a lesser extent echidnas, have a related kind of electrosense that they use in moist soil.

What does electrosense feel like? For a shark, it may be something akin to seeing a glowing fish appear against a black background. It’s a short-distance sense—the shark needs to be within a metre or so of the fish to feel its electric field.

Scientists think that all marine fish once had this ability, but most of them lost it—a sensory mismatch that works out rather well for the sharks. A Californian study of carpet sharks, close cousins of New Zealand’s draughtsboard sharks, found that they hang out off reefs at night, waiting for fish to get washed towards them in the swell.

The sharks can feel the prey coming with their electrosense, so all they have to do is open their mouth at the right moment. “It’s the laziest way to make a living,” says John Montgomery, a marine biologist at the University of Auckland. “The shark’s got it made.”

Humans release excess ions through urination—but, says Montgomery, you don’t need to worry about attracting sharks when you wee in your wetsuit. A one-off widdle generates a tiny, short-lived amount of electricity, he says, though it may be best avoided “for olfactory reasons rather than electric reasons”.

Sharks are so sensitive they can detect tiny electric fields of just five nanovolts per centimetre—a nanovolt being one billionth of a volt. So, how can the shark distinguish between all the noise generated by its own electric field and the signal coming from its prey?

Montgomery and a colleague, David Bodznick, anaesthetised draughtsboard sharks in their lab at Leigh, north of Auckland, and discovered that sharks’ brains learn to cancel out electric signals from their own bodies. “It’s like very sophisticated noise-cancelling headphones,” Montgomery says.

Inspired by this research, other scientists took a closer look at the human brain and discovered that we have a similar—though much less sophisticated—set of headphones dampening the noise of our own chewing and heartbeat.

Two families of freshwater fish—African elephantfishes and South American knifefish—evolved their own forms of electrosense and took it one step further than sharks. The elephantnose fish, which is native to the rivers of West and Central Africa, generates a weak electric field with an organ near its tail, then uses electroreceptive organs in its skin to locate objects based on the distortions they cause in the field.

“It’s electric instead of visual, but they get an image of their environment,” says Krista Perks, a neuroscientist at Wesleyan University in Connecticut.

Both families also appear to use electricity to communicate, Perks says, though they do it in different ways. The African fish turn their electric fields on and off like a torch, creating different patterns, while some of the South American knifefish send out electrical chirps. If they approach another fish that’s chirping at the same frequency, they’ll change their frequency to ensure they’re not talking over the top of each other. And in just one genus of knifefish, evolution modified the electric organ into a powerful weapon.

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In March 1800, the great European naturalist Alexander von Humboldt was travelling around northern South America, wide-eyed with wonder. One animal he was especially keen to study was the electric eel, so he enlisted a group of local fishers to help him catch one. The best way, they told Humboldt, was to go fishing with horses.

Picture the scene: a muddy pond. The men drive about 30 horses and mules into the water, then climb into the surrounding trees, waving branches and shouting so the animals don’t escape. The surface of the water begins to swarm with lead-coloured eels, which press their bodies against the horses’ bellies and discharge huge electric shocks. “Several horses sink under the violence of the invisible blows,” Humboldt wrote afterwards. “Others, panting, with erect mane, and haggard eyes expressive of anguish, raise themselves and endeavour to escape from the storm which overtakes them.”

After five frenzied minutes, the tempest is over. Two horses have drowned, and the eels are exhausted enough for the fishers to wade in and scoop five out of the mud for Humboldt to examine.

Humboldt’s account of this scene became famous, but also raised scepticism. Why would eels attack an animal too big to eat? When another naturalist visited the same village in 1877 and asked about hunting eels with horses, everyone laughed at him. Later scientists dismissed the tale as “poetically transfigured” or even “tommyrot”.

Then, one day in the lab, Catania tried to transfer a large eel from one tank to another using a net with a metallic rim and handle. Instead of backing away, the eel exploded out of the water and let off a burst of high-voltage electricity. “It was both literally and figuratively shocking,” says Catania. (Luckily, he was wearing rubber gloves.)

So Catania devised a series of experiments to test the eels’ aggressive behaviour, involving a Hallowe’en decoration: a severed zombie arm he fitted with LED lights and poked into their tank. The eels leapt out of the water and pressed their bellies against the arm, making the LEDs flicker like deranged strobes. The higher the eels jumped, the more electricity flowed through the arm instead of the water, increasing the power of the shock and enabling the eels to scare off intruders in one manoeuvre.

Finally, to model the electrical circuit created when an eel attacks in self-defence, Catania needed one last data point: the resistance through the target. So he built a special device out of a plastic box and some copper wire, and while holding it, stuck his bare arm into the tank with an electric eel—thereby joining a long line of electric eel biologists allowing themselves to be shocked for the sake of science. The eel—Catania affectionately called it Finless—was just a little one, about 40 centimetres long with 198 volts in its arsenal. When it reared up and shocked Catania, delivering a current of 40 to 50 milliamps, his arm jerked involuntarily away. (In terms of intensity, amperage—the volume of the electrical current—is more important than voltage. Anything over 10 milliamps will hurt you, while 100 milliamps will kill you.)

The sensation was similar to touching an electric fence, Catania says. It was painful, but he knew the eel was just jangling his nerve endings, and he wouldn’t be injured—a perspective unavailable to a horse in an eel-infested pond.

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In April 2022, vast spiderwebs carpeted the grass, fences and trees of Napier’s parks. The phenomenon is often observed after heavy rain or flooding, as spiders “balloon” to safety, flying through the air on delicate gossamer parachutes. Baby spiders also balloon to escape their cannibal siblings, says sensory biologist Erica Morley. “If you’re that small, and you don’t have wings, it’s a really good way of getting away from a bad situation and finding somewhere better,” she says.

But how do the spiders fly? Charles Darwin mused on this question one morning in November 1832, when he emerged onto the deck of the Beagle to find the ropes strewn with cobwebs and thousands of minute dusky red spiders running about the ship. He watched these “tiny aeronauts” stick their abdomens in the air, shoot out a thread and “sail away horizontally”. The Beagle itself was about 100 kilometres from the coast of Argentina, and it was a calm day—could such a light breeze really have carried them all that way? Perhaps electricity was involved, Darwin mused.

Scientists now know the air is electrically charged, and so Morley decided to test Darwin’s idea as part of her doctoral research at the University of Bristol in the United Kingdom. In 2018, she put dwarf Erigone spiders into a clear plastic box, isolated from any air currents. When she generated an electric field inside, tiny hairs on the spiders’ feet began to wave about. Imagine the feeling of your own hairs standing up when you rub a balloon along your arm, says Morley—but many times more sensitive.

The spiders began to rise up on their tiptoes like ballet dancers en pointe. Some raised their bums in the air and shot out a fan of silk, and a few even began to fly—inside the box, without a breath of wind. They were riding the electric field.

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Bumblebees can sense electric fields, too, but they’re more interested in the ones surrounding flowers, which have a tiny negative charge. The bees accumulate a slightly positive one just by flying around all floofy—think of how pulling a polyester jumper over your head can cause a spark of static electricity.

For sensory biologist Clara Montgomery, now at Harper Adams University in Shropshire, England, investigating the electrical interactions between flower and pollinator involved teaching bumblebees to fly through hoops that would measure their electrical charge before they visited petunia flowers. “Essentially a bee circus,” Montgomery says.

Bumblebees are very easy to train, as they’re suckers for sugar water. When Montgomery’s fuzzy circus performers visited the petunias, the flowers detected the bees’ electrical charge, and released more scent. (Montgomery checked the flowers weren’t simply responding to movement by tapping a control flower with an electrically grounded rod.) Producing scent takes energy, and it’s also risky—fragrance can attract predators as well as pollinators. So it makes sense for the flower to be smellier when it knows there’s an audience. Electricity just happens to be a useful cue.

Montgomery was amazed to discover that bees and flowers—some of the most studied organisms on Earth—were carrying on these invisible conversations under our noses this whole time. She suspects that many other insects—moths, for instance—probably have similar abilities. It’s just that no one has looked at them yet. And it’s a reminder, she says, that “animals have evolved to sense the world in completely alien ways”.

Kenneth Catania loves that there is still so much to discover about the electric eel, an animal that has enthralled scientists for generations. The eel’s unmatched power makes it an evolutionary outlier, he says—like Tyrannosaurus rex, it’s an extreme example of how far evolution can take a species. Still, humans are outliers, too. We might not be able to deliver 800-volt shocks with our bodies, but we’ve figured out how to harness the power of electricity to help with almost every aspect of our lives. “I think we don’t fully understand just how strange we are,” Catania says. “We’re too close to the subject.”

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