Electric eels are hard to miss. They’re eight feet long, have to surface to breathe oxygen every ten minutes and produce electric shocks that are enough to kill prey and light up a Christmas tree. But in the more than 250 years since the electric eel was first described, scientists have missed something about the fish: There isn’t just one unique species of electric eel, but three. In a paper in Nature Communications, researchers from the Smithsonian’s National Museum of Natural History and other institutions describe two new species of electric eel, Electrophorus varii and Electrophorus voltai, tripling the known number of species. And one of the new species also boasts a record-setting shock ability of 860 volts, which makes E. voltai the world’s strongest known bioelectric generator.
The name “electric eel” is a misnomer, explains C. David de Santana, a zoologist with the Natural History Museum. The animals are actually eel-shaped knifefish; unlike proper eels, they dwell in freshwater, not salt water, and need oxygen to survive. Three electric organs make up 80 percent of their body and emit electric pulses that can be weak (to communicate and navigate) or forceful (to hunt or defend themselves).
Before this research, zoologists considered the electric eel’s habitat to cover a large portion of northern South America around the Amazon and Orinoco rivers. The size of that range stuck out as anomalous, says de Santana: “If you take the distribution of neotropical fishes, they’re really rare to have one unique species broadly distributed across the continent.” But the giant fish are hard to collect, and technology like DNA testing and 3-D CT scans are relatively recent innovations, so for centuries, scientific consensus held that there was only one species of electric eel, he says.
De Santana and his colleagues wanted to look more closely at the single known species of electric eel, Electrophorus electricus, and collected 107 specimens by going to South America and tracking down the fish. They asked local communities to help by identifying known habitats, and they used a “fish detector” made of a microphone cable and amplifier that picked up electric pulses in the water. They needed the fish alive to measure the voltage of their electric organ discharge and to get DNA samples. Once the fish were collected, the scientists sent tiny samples of the animals’ flesh to Washington, D.C., for genetic testing.
That testing, plus detailed electronic scans of the electric eels’ internal anatomy, revealed that there was enough of a genetic difference between different populations that they were actually three distinct species. While the three species have subtle physical differences, de Santana says that “without having the DNA, it would be almost impossible to distinguish them” and to be sure that these external differences weren’t just variations within E. electricus.
Ichthyologist Nathan Lovejoy, whose lab at the University of Toronto Scarborough has researched the electric eel’s mitochondrial genome but was not involved in this research, calls the collection of 107 electric fishes “unprecedented.”
“Despite more than a century of scientific field work on fishes of South America, our understanding of the taxonomy of the electric eel has remained very limited,” he writes in an email, so “the discovery of two new species of electric eel is particularly exciting, given how iconic and impressive these fishes are.”
The scientists measured the strength of the fishes’ electric shocks in inflatable swimming pools, and noticed that E. voltai stood out with a fierce 860 volts of electricity, more than 200 volts above the previously recorded maximum. (The average car battery is around 12 or 13 volts, while most electric fences run at a minimum of 2,000 volts.) The scientists named E. voltai for the early 19th-century physicist Alessandro Volta, who invented the eel-inspired electric battery. The other species, E. varii, honors the late Smithsonian ichthyologist Richard Vari, who contributed to this research and was one of de Santana’s post-grad supervisors.
To better understand how the three species were related, the researchers reverse-engineered their ancestral family tree using maternally-inherited mitochondrial DNA as well as nuclear DNA, which comes from both parents. Scientists expect genetic change to accumulate at a steady rate, like a clock, so more genetic differences mean that more time has passed since two species shared a common ancestor. Based on these calculations, E. varii diverged from the ancestors of the other electric eel species 7.1 million years ago—slightly before the first known hominins lived. E. electricus and E. voltai split into two species later, around 3.6 million years ago.
While de Santana cautions that at this point, scientists can only hypothesize about what might have caused different species to arise, the development of the Amazon River’s current course could have created a geographic barrier isolating the populations that would eventually diverge into E. voltai and E. electricus.
Today, E. varii lives in the lowlands of the Amazon basin, where the slow-flowing water conducts electricity better, while E. electricus and E. voltai live in the highlands of Guiana and Brazil, respectively. Electric eels’ status as “top-of-chain predators,” says de Santana, means it’s “quite unlikely to have the species occurring together,” even though such overlaps do occasionally happen.
While de Santana and his colleagues entered into their research with a healthy skepticism that a single species of electric fish could range across all of upper South America, they were surprised by the jump in electric shock voltage that they found between species. De Santana says further research might reveal more about the evolution of these high-powered electric discharges, and speculates that perhaps hunting behavior or water conductivity could have helped shape the trait.
Lovejoy, meanwhile, says the finding is a reminder that South America offers more zoologic and scientific mysteries to unravel. “What other large animals await discovery?” he writes, “And will we find them before they and their habitats are wiped out by human activity?”
De Santana similarly positions his team’s findings as part of a larger story about unexplored biodiversity. The research, he says, “indicates that an enormous amount of species are waiting to be discovered.” And, as the release of the study follows a surge of wildfires burning the Amazon rainforest, the zoologist says the discovery underscores that there’s a “critical need to protect the Earth’s hotspots of biodiversity.”