An electric fish is a fish that can make electric fields. Many fish (sharks, rays, and catfish) can detect electric fields: they are 'electroreceptive'. They cannot make electric fields, and are not called electric fish.
Strongly and weakly electric fish
Electric fish have a special organ that makes electricity. This organ is called an electric organ. It is usually near the tail of the fish. It is made from specialised muscle or nerve cells. When the organ releases electricity this is called an electric organ discharge (or EOD for short).
Based on the strength of the EOD, the electrogenic fish are two types:
Strongly electric fish: Their discharge is so strong they can stun (and capture) prey with it. Examples of such fish are the electric eel (which is a knifefish, not a true eel), electric catfishes, or electric rays Weakly electric fish: Their discharge is not strong enough to stun prey with it; rather, they use the electricity to locate objects, to navigate, or to communicate with other electric fish. Examples of weakly electric fish are Peters' elephantnose fish or the black ghost knifefish.
In total, there are 348 known species of electric fish, in about 56 genera. The only known species that lives in salt water are the electric rays, with 23 species.
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The duck-billed platypus also detects movement by an electric field.