Despite its name, the electric eel is more closely related to carp and catfish than to other eels. It produces electricity in electrocytes – special cells arranged like stacks of batteries -formed in three separate organs.
The rapid transfer of sodium ions along the length of these electrocytes generates an electrical current at either high or low voltage, depending on the organ producing the charge.
A high-level shock can carry 600 volts (V) at a current of 1 amp – enough to kill a human (though this rarely occurs).
Certain species of catfish and ray also generate electricity using the same method; catfish can produce up to 400V, rays up to 200V.
The ability of these different fish to produce electricity is a nice example of convergent evolution – the independent development of similar adaptations in unrelated species.