The Electric Eel (Electrophorus electricus) is truly a wonder of the animal kingdom, and an amazing work of millions of years of evolution.
Despite its name it is in no way closely related to eels, it is a member of the Knifefish family and is the only member of its genus. The electric eel lives in fresh water in the Amazon, as well as other river basins in south america. They can grow to about 2m in length (6 and a half feet) weighing 20 kg. It can produce an electric shock of up to 600 Volts!
It produces this shock using 3 organs, the Main organ, the Hunters organ, and the Sachs organ. The total size of these 3 organs make up an amazing four fifths of the eels body! The organs are made of electrocytes, and are lined up so that a current can be passed from one organ to the next. When the eel wants to produce a shock it opens up glands in and between the organs allowing sodium ions to flow between them, creating a sudden change in potential difference (voltage.)
The shock only lasts approximately 0.2 milliseconds meaning it is not very likely to be lethal to an adult human despite it being 600 volts. That being said, it has been known to kill if the shock is, for example, directed towards the heart.