A newly discovered species of electric eel produces an 860 volt shock, the highest charge of any known animal in the world.
The creature is one of two extra species of the fish identified by scientists, who examined 107 specimens from across the Amazon rainforest.
The newly discovered eel delivers a zap more than 200 volts higher than ever previously recorded, but is unlikely to kill a human, according to experts.
It was previously believed there was just one type of electric eel.
The only species already known to science was the Electrophorus electricus, which Swedish naturalist Carl Linnaeus recorded in 1766.
But researchers have now found evidence to add two new species to the genus: E. varii and E. voltai, the latter of which is the eel that produces the 860-volt shock.
Eels use their electric shock both to protect themselves and for hunting.
The new study, published in the journal Nature Communications, not only provides new knowledge about the animal more than 250 years after it was first described but also opens avenues of research into the origin and production of strong electric discharges in other fish species.
Electric eels are naked-back knifefishes, known as Gymnotidae, and are more closely related to catfish and carp than to other eel families.
Gymnotiformes, the knifefish family to which Gymnotidae belong, are native to Mexico and South America and are found almost exclusively in freshwater habitats.
They are mostly nocturnal and all are capable of producing a weak electric field for communication and navigation and most have very small eyes.
Study author Dr Carlos David de Santana, an associate researcher at the US National Museum of Natural History (NMNH), said: 'The electric eel, which can reach 2.5 metres in length, is the only fish that produces such a strong discharge; it uses three electric organs.
'The shock is used for defence and predation.'
After studying the animals' DNA, bodies and environments, and measuring the power of the shocks, the researchers decided the formerly single species needed to be reclassified into three.
Professor Naércio Menezes, from the University of Sao Paulo in Brazil, said: 'We used voltage as the key differentiation criterion.
'This has never been done before to identify a new species.'
During field measurements using a voltmeter, he said the research team recorded a discharge of 860 volts, the highest found in any animal, from a specimen of E. voltai.
The strongest shock previously recorded was 650 volts.
The name of the species pays homage to Italian physicist Alessandro Volta, who invented the electric battery in 1799, basing its design on the electric eel.
E. varii is named after zoologist Richard P. Vari, an American researcher who died in 2016.
Dr Santana added: 'He was the foreign researcher who most influenced and helped Brazilian students and researchers with the study of fish in South America.'
Dr Santana, who has entered several rivers to collect electric eels for research purposes and been shocked more than once, said the discharge is high voltage but low amperage – around one amp, so it is not necessarily dangerous to humans.
As a comparison, a shock from a power outlet can be 10 or 20 amps but is of a lower voltage – 240v in the UK.
But Dr Santana explained that the electric eel emits not a direct current but an alternating current - in pulses, and its charge is depleted after a strong shock.
Its electric organ takes some time to recharge.
But, even so, Dr Santana said an encounter with a bed of electric eels in the water can be quite perilous.
He said the shock will not kill a healthy person, but it can be hazardous if you have a weak heart. It can also contribute to a fall or drowning.
Dr Santana said: 'The shock stuns the victim. It's sufficiently strong to help the fish capture prey or scare off a predator.'
The research conducted by the group has shown that electric eels communicate to convene groups that can electrocute a potential threat.
Contrary to what had been previously claimed, the eels are not solitary and frequently associate in groups of up to 10 during adulthood.
The new classification was based on an analysis of 107 specimens collected in different parts of the Amazon in Brazil, Surinam, French Guiana and Guyana.
Dr Santana said: 'Their body shape is highly conserved. It has not changed much during 10 million years of evolution.
'Only a few details of their external morphology distinguish them, and only an integrated analysis of morphology, genetics and ecology was able to make robust distinctions between the species.'
He added: 'The discovery of new electric eel species in Amazonia, one of the planet's biodiversity hotspots, is suggestive of the vast amount of species that remain to be discovered in nature.
'Furthermore, the region is of great interest to other scientific fields, such as medicine and biotechnology, reinforcing the need to protect and conserve it, and is important for studies involving partnerships among Brazilian researchers, and between us and groups in other countries, to explore the region's biodiversity.'
The species that kept the name E. electricus is present in an area far north of Amazonia known to geologists as the Guiana Shield, encompassing the northern regions of three Brazilian states Amapa, Amazonas and Roraima, and Guyana, French Guiana and Suriname.
Meanwhile, E. voltai inhabits the Brazilian Shield, which is in the south of Pará and Amazonas, as well as Rondonia and the north of Mato Grosso.
Shield regions are quite elevated, exceeding 300 meters in altitude.
In contrast, E. varii inhabits the lowest part of the Amazon Basin, living in murky rivers with relatively little oxygen and sandy or muddy bottoms.
The researchers also estimate that the species diverged twice, with the first time being approximately 7.1 million years ago, when they separated from their common ancestor.
It was not until approximately 3.6 million years ago, that E. voltai and E. electricus reached their present status.
There are only a few animals in the world that produce electricity.
They include the black ghost knifefish, electric rays, the northern stargazer and the electric catfish.