This fish is not a true eel, but a giant member of the knifefish order with an eel-like body. When fully grown, it can be as thick as a human thigh, and is one of the largest freshwater fishes in South America. It has an elongated square body with a flattened head, and an overall dark grayish green colour shifting to yellowish on the bottom. The electric eel can grow up to 2.5m (8.2 ft) in length and weigh up to 20kg (44 lb). It has a continuous fin that runs along its underside, but no fins on its back.
Electric eels have poor eyesight, and find their way by using weak pulses of electricity. However, using modified muscles, or electric organs, that run the entire length of their bodies, they can also produce sudden jolts of up to 600 volts – powerful enough to kill other fishes and potentially lethal to humans, making it a top predator in its range.
Juvenile eels feed on invertebrates, while adult eels feed on fish and small mammals. First-born hatchlings will even prey on other eggs and embryos from later batches.