Native to tropical river systems in South America, electric eels are no strangers to the record books, having long been recognized as the most electric fish, and indeed, the most electric animals overall. However, a new paper published in Nature Communications has revealed that certain electric eels – also known as poraquês – have more spark than others.
Since they were first scientifically described in 1766, it has been thought that only one species of electric eel exists: Electrophorus electricus.
Now, a multinational team of experts representing several universities and museums across Brazil, Suriname, the USA and as far as Switzerland have rewritten the electric eel family tree.
A comprehensive study of the internal anatomy and geographic distribution of these unusual freshwater fish throughout the Greater Amazonia region has concluded that there are (at least) two more species beyond E. electricus, which have been provisionally named E. varii and E. voltai.
What’s more, early signs indicate that electrical organ discharge (EOD) intensity – in other words, maximum electrical output – varies between the different types, with one 1.21-m (4-ft) specimen of E. voltai unleashing an unprecedented 860 volts.
Until now, the highest output documented for any electric eel is between 550 and 650 volts, which means that E. voltai officially assumes the mantle of nature’s most electric animal.
Despite their name, electric eels are not true eels. They belong to the knifefish family, which itself falls under a wider group of South American fish known as the Gymnotiformes. A defining feature of Gymnotiformes is their ability to generate bioelectricity. Different fish use this "superpower" (which varies in strength) for different functions, including to stun prey, navigating the murky waters of rainforest waterways and as a form of self-defence against predators.
Ichthyologist Dr William Crampton of the University of Central Florida – one of the lead authors of the new paper and a specialist in Gymnotiformes – was the scientist who recorded the shocking 860-volt poraquê in 2014. "I found the record-breaking eel while making an episode of the National Geographic TV show Monster Fish, with Zeb Hogan," he begins.
"We made the documentary in the beautiful Rio Tapajós of the Brazilian Amazon. A memorable incident was that the cinematographer, Colm Whelan, got very badly shocked by an eel that emerged from a hole in a stream bank.
"I've been shocked a few times, and seen others get shocked, but this was different," Dr Crampton continued. "For several minutes after the shock, Colm was as white as a sheet and trembling. We were quite worried about him for a while, but fortunately he recovered.
"At that time, I had no idea that the eels in those streams belong to a completely distinct species from the ones I'd previously worked with, and I had no idea that this species reaches a much higher voltage than others.
"When I saw a reading of 860 volts… I thought initially that something must be wrong," Dr William Crampton.
"I checked all the equipment, made sure all the calibration settings were correct, and then repeated the whole process – several times, including the next day. The result remained the same."
By comparison, measurements of the other two species in the study wielded results of 480 volts for E. electricus and up to 572 volts for E. varii. The authors acknowledge that there is scope for more in-depth research into electrical output across the genus with a larger sample set to establish "norms" for each species.
One myth that Dr Crampton is eager to dispel is that electrical potential increases proportionately to the size of the fish. "A popular misconception is that the bigger an eel is, the stronger its voltage. That’s not the case," he stated.
"Eels do increase in voltage when younger, at about 100 volts for every 10 cm [4 in] of their length, but only until they reach about 0.5 m [1 ft 7 in] long. Thereafter, as they continue to grow, they do not increase voltage. If anything, the voltage goes slightly down with size.
"Electric eels can reach huge sizes – up to at least 2 m [6 ft 6 in] in length – but these monsters often have lower voltage than ones that are much smaller."
Dr Crampton suggests there are wider lessons to be taken from this study too, in the light of the fact that two far-from-inconspicuous animals have – to all intents – been swimming right under our noses unnoticed for more than 250 years: "The discovery of two new giant species goes to show how the Amazon continues to surprise us with its extraordinary diversity of life," he said.
"I hope that these kinds of discoveries help people understand how much humankind has to lose if the Amazon’s forests and rivers continue to be damaged by unsustainable resource use.
"Who knows what other discoveries are to be made in the Amazon? If we can still find new giant electric eel species after a quarter of a millennium of collections, what else might be out there?"
Thumbnail and header image credits: Colm Whelan; L Sousa