Many fish produce their own electricity, and the first batteries were inspired by electric fish. Electric eels produce electricity and have large electric organs. They send energy out into the world, which causes the muscles of other animals to twitch, revealing their location. The eel then sends stronger pulses out that paralyze its prey.

Most fish, however, are “weakly electric” and do not use electricity for hunting or for defense; these fish perplexed Charles Darwin. It is now widely believed that weakly electric fish use their electric fields as other animals use their senses, to navigate and to communicate, electrolocating as bats echolocate. Their electroreceptors can detect the difference between conductors, such as flesh, and insulators, such as rocks. Black ghost knifefish have electroreceptors all over their bodies; just as humans create images as a result of light falling on their retinas, electric fish “see” by perceiving the varying voltages that flash onto their skin, with conductors shining and insulators casting shadows.

Active electrolocation, like echolocation, takes a lot of effort. The stimulus must be created, not simply received. Unlike echolocation, however, electric fields do not move. This means that when electric organs are fired, electric fields immediately occur.