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.
Read: A scientist’s shocking discovery about electric eels
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?”