What IS an electric fish?
A fish is considered to be an electric fish if it can generate electric fields. In this case it is said to be electrogenic. If a fish has the ability to detect electric fields, it is said to be electroreceptive. Most fish that are electrogenic are also electroreceptive. Some electric fish live in the ocean and some live in freshwater rivers of South America and Africa. The figure below (from Moller 1995) provides a summary of the geographical distribution of electric fishes (click on the image to see a larger version). The figure also shows the location of the electric organ in each fish, and a sample of the waveform of the electric organ discharge.
What's NOT an electric fish?
Certain fishes like sharks, skates, rays and catfish can detect electric fields, and are thus electroreceptive, but they don't generate an electric field so they are not classified as electric fish. Most common bony fish (teleosts), including most fish that you would find in a tropical fish store or at the fish market, are neither electrogenic nor electroreceptive.
The electric organ discharge (EOD)
The electric field of an electric fish is produced by a specialized structure called an electric organ. The electric organ is shown shaded in black in the figure above. The electric organ is made up of modified muscle or nerve cells, which became specialized for producing electric fields. For most electric fish, the electric organ is located in the tail. The output of the organ is called the electric organ discharge and is abbreviated EOD.
Strongly electric fish have an EOD that is powerful enough to stun their prey. Typical EOD amplitudes for these animals range from about 10 to 150 volts. Strongly electric fish are grouped at the top of the figure. Weakly electric fish generate an EOD that is typically less than one volt in amplitude. These discharges are too weak to stun prey, but are used for navigation, object detection (electrolocation) and communication with other electric fish (electrocommunication). Weakly electric fish are shown grouped together near the middle of the figure.
In different species of electric fish, the EOD waveform takes two general forms. In some species the waveform is continuous and almost sinusoidal (e.g., Apteronotus, Eigenmannia, Gymnarchus). These fish are said to have a wave-type EOD. In other species, the EOD waveform consists of brief pulses separated by longer gaps (e.g., Gnathonemus, Gymnotus, Raja). These fish are said to have a pulse-type EOD. The figure above illustrates the different types of EOD waveforms observed in different species.