Indigenous people in Venezuela called it arimna, or “something that deprives you of motion.” Early European naturalists referred to it as the “numb-eel.” And for 250 years, since it was first given a Latin name, Western scientists have known it as Electrophorus electricus, the electric eel, the sole member of its genus, the first and only of its name.
Throughout the animal’s storied history—as Alessandro Volta designed the first synthetic battery based on its body, as Alexander von Humboldt collected it by driving horses into eel-infested pools, as a young Charles Darwin dissected the creature aboard the HMS Beagle, as the physicist Michael Faraday placed his bare hands on it in his quest to understand electricity, and as modern researchers carried out an array of studies to show just how amazing (and sinister) its abilities are—Electrophorus electricus has always been regarded as a single species. The electric eel.
Carlos David de Santana, a Brazilian researcher at the Smithsonian National Museum of Natural History, thinks differently. By comparing 107 specimens pulled from museum drawers and the Amazon basin, he and his team of mostly Brazilian scientists have found that the infamous electric eel is actually three distinct species.
There are dozens of different ways of defining a species, and none are universally accepted. That said, de Santana says that his team “used many lines of evidence to prove that there’s more than one electric eel species.” This trinity differs not only in physique, but also in genetics, habitat, and electric power. Tellingly, the eels’ DNA suggests that they last shared a common ancestor 7 million years ago, which means that they started to diverge well before brown bears and polar bears, lions and tigers, and even humans and chimpanzees.
One of the trio retains the original name Electrophorus electricus, and de Santana now calls it Linnaeus’s electric eel, after the legendary Swedish taxonomist who classified it. The two others are now Volta’s electric eel (Electrophorus voltai), after the Italian physicist who built a battery based on the animal, and Vari’s electric eel (Electrophorus varii), after Richard Peter Vari, a famous ichthyologist who was part of de Santana’s team until his death in 2016. (Most of the eels used in previous research are likely to be Vari’s eels, since they’re the only species from Peru, the only country from which these animals can be legally exported.)
“These findings do not surprise me,” says Graciela Unguez from New Mexico State University. As researchers sample electric fishes from more parts of South America, she adds, they’re almost bound to find that currently known species harbor more diversity than people suspected.
The same goes for unusual animals whose outward distinctiveness can mask subtler differences that become clear only through genetic analyses. Such studies have shown that there are likely four distinct species of giraffe, three species of mola mola, and two species of African elephants. “We sort of lump the weirdos together,” says Prosanta Chakrabarty from Louisiana State University. “Oh, obviously, this thing is that thing, and no one looks more carefully. We all thought that an electric eel is an electric eel.”
Contrary to appearances, they’re not even eels. They’re knife fishes—a group of mostly small, gill-breathing species that have flattened bodies and that produce weak electric fields for navigation and communication. The misnamed eels buck all these trends—weirdos, even within their own family. They breathe by rising to the surface and gulping air, which makes them one of the only fish that you can drown. Their thick, cylindrical, meaty bodies can reach seven feet in length. And the electric organs that make up 80 percent of that length can produce shocks that are strong enough to incapacitate a human or a horse.
Collecting these animals from the wild, as de Santana did, is not easy. “I do it by myself, or with the help of really experienced fishermen,” he says. “I don’t allow students to do it. It’s never safe.” Even if he wears rubber gloves, the sweat that builds up inside them eventually links up with the water outside them, creating a continuous conductive layer. Bottom line: You can’t collect electric eels without suffering shocks, which de Santana compares to getting hit with a Taser. It’s even worse in the dry season, when more than 10 individuals can occupy a single stream. “When one starts to discharge, the others do too,” says de Santana. “You just get used to it. You do what you have to do.”
Once the samples were in, the team focused on 10 important genes. Immediately they saw that the eels clustered into three distinct groups, with very little genetic variation within each one, but substantial genetic differences between them. In one key gene, for example, the three species differ by 6 to 10 percent of their DNA, but individuals within each species differ by 0.3 percent at most.
Looking closely, the team realized that there are physical differences among these three species—not in obvious features such as size or color, but in subtler ones, like the flatness of their heads, or the number of pressure-sensitive pores on their flanks. With experience, de Santana can now tell the three species apart by eye.
In the wild, it’s even easier: The three eels live in different habitats, which might explain why they’re distinct. About 7 million years ago, some ancestral electric eel split into two populations. Vari’s eel lives in lowland floodplains, whose waters are usually murky, muddy, and oxygen-deprived. The two others live in highland rivers, where water is fast-flowing, well oxygenated, and clear. Though they share the same environment, their ranges don’t overlap: Linnaeus’s eel is restricted to northern Amazonia, while Volta’s eel lives in the south.
What separated them? Most likely, the Amazon River itself. Around 9 million years ago, after eons of flowing westward, the mighty river started reversing its course. Its modern eastward flow became entrenched around 2.5 million years ago—exactly when Volta’s and Linnaeus’s eel split into distinct species.
These different habitats have likely influenced the animals’ use of electricity. Clear water contains fewer dissolved minerals than muddy water, and is worse at conducting current. So to stun their prey, Volta’s and Linnaeus’s eels either need to get closer than Vari’s eel does or release stronger shocks. Volta’s eel certainly does the latter: De Santana’s team found that it can discharge up to 860 volts. That’s far higher than the 650 volts commonly cited for electric eels, and beyond the abilities of any other electric fish.
The three species might also behave differently. It’s commonly said that electric eels are solitary hunters that use electricity to locate prey in murky water, but de Santana’s team has evidence that the two clear-water species live in groups and hunt collectively.
These discoveries, made largely in Brazil and by Brazilian scientists, come at a difficult time for the nation’s researchers. The National Museum in Rio de Janeiro—the largest natural-history museum in Latin America—was gutted by a fire last year, destroying millions of priceless specimens in a preventable tragedy caused by inadequate funding.
The electric eels’ wild habitat is also on fire. About 40,000 blazes have swept through the Brazilian Amazon this year—an 80 percent rise from last year. Most of these were deliberately ignited to make way for agriculture by burning out forested lands, and the indigenous communities living there. That arson has been tacitly encouraged by Brazil’s far-right president, Jair Bolsonaro, who promised to undermine protections for the Amazon, open it up for economic development, and wrest control of land from indigenous groups. “It’s a really bad situation,” says de Santana, who is Brazilian himself. “I go to the Amazon twice a year. From what I’ve seen, I’d say that in 50 years’ time, we’ll only have fragments of what we have today.”
The electric eels should serve as reminders of what could be lost as the Amazon shrivels and smolders. “They’re eye-catching animals that have been known for 250 years, and that live in one of the Earth’s biodiversity hot spots,” says de Santana. “If you can find new species like that, what else could you find there?”
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ED YONG is a staff writer at The Atlantic, where he covers science.