A source of fascination since it was first described, in 1776 by Carl Linnaeus, the electric eel was long thought to be a single species – Electrophorus electricus – that was widely distributed throughout the Amazon basin.
Now, a team led by C David de Santana of the Smithsonian Institution’s National Museum of Natural History, US, has published descriptions of two new electric eel species in Nature Communications.
Joining E. electricus are E. varii and E. voltai, the latter with a recorded electrical discharge of 860 Volts – nearly a third above the previously known upper-limit discharge for the genus of 650 V.
Some clarifications and context to begin: electric eels aren’t true eels; they’re Gymnotiformes, a group of bony fishes known as South American knifefish.
The Gymnotiformes include about 250 species of mostly small fish found throughout South America that have electricity-producing organs. In most cases, these organs generate vanishingly low-energy discharges, which are used mainly for navigation and to locate their bottom-dwelling prey.
Growing to over two metres long and to weights approaching 20 kilograms, electric eels are the bare-knuckle bouncers of the Gymnotiformes, their large electrical organs producing discharges with sufficient energy to stun prey and deter predators – and to make stun-gun-like shocks a genuine risk to biologists studying live specimens.
De Santana’s team examined 107 individuals collected from Brazil, French Guiana, Guyana and Suriname over the past six years.
The animals were all similar in appearance, so the research team first examined mitochondrial and nuclear DNA and found genetic differences that suggested the specimens were of three distinct species.
Given this genetic knowledge, de Santana’s team looked again at the animals and discovered subtle morphological differences, such as unique skull shape, pectoral fin characteristics and distinctive body-pore patterns.
Further work revealed that each species had its own geographic distribution.
E. electricus – once thought to be widely distributed – appears to be confined to clear-flowing waters on the highlands of the Guiana Shield, in the Amazon basin’s north. E. voltai primarily lives on the Brazilian Shield, a similar highland region in the basin’s south. E. varii dwells in the muddy, slow-flowing waters of the lowlands.
The team’s genetic study revealed that electric eels began to evolve in South America about 7.1 million years ago. The common ancestor of E. voltai and E. electricus lived in the clear waters of the ancient highlands, whereas E. varii lived in the lowlands, where the mineral-rich murky waters conducted electricity more efficiently.
This is an important distinction for electric eels, whose discharge won’t travel as far in environments where conductivity is low: de Santana believes that E. voltai’s spectacular electrical discharge of up to 860 V may be an adaptation to the lower conductivity of highland waters.
De Santana says the identification of two new species of electric eel highlights how much remains hidden in the Amazon rainforest – one of Earth’s biodiversity hotspots – and underlines the importance of protecting and preserving this threatened environment.
“These fish grow to be seven to eight feet [2.1–2.4 metres] long. They’re really conspicuous,” he says. “If you can discover a new eight-foot-long fish after 250 years of scientific exploration, can you imagine what remains to be discovered in that region?”
Ian Connellan is editor-in-chief of the Royal Institution of Australia.
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