Electric fish, like knifefish, produce electrical signals used for sensing where they and their schoolmates are. Electric eels produce, in addition, more powerful currents for hunting and defense.
The weakly-electric fishes generate signals of just one or two volts, which have an effective range of a few centimeters for navigation, and one or two body lengths for communication.
The electric eel also produces low-voltage electricity for electro-location, using what's called the Sachs organ. Inside the organ are many muscle-like cells, called electrocytes. Each can only produce 0.15 volt, though together the organ transmits a signal of about 10 volts at around 25 hertz. Electrosensors in the eel's skin detect distortions in the field caused by nearby objects, giving the eel another sense of its environment.
Using two other organs, the Main organ and the Hunter's organ, the eel can also kick out a stunning charge that it uses in hunting and self-defense. It generates its electrical pulse in a manner similar to a battery, in which stacked plates produce an electrical charge. In the eel, some 5,000 to 6,000 stacked electroplaques are capable of producing a shock at up to 500 volts and 1 ampere of current (500 watts). It's enough to deter just about any other animal, except possibly the alligator-like caiman. “I have heard reports that a caiman will bite an electric eel in half, and then devour it,” said eel expert Will Crampton.
Are these electric fish a danger to humans? “Theoretically it could be enough to stop someone's heart and kill them,” he said, “but I've never heard of that happening. I don't think there's a single documented case of anyone being killed or even seriously injured by an electric eel. Not one.”