Electric eels use shocking tactics not just to incapacitate prey, but also control them, research has shown. The creatures vary their electrical discharges either to fire a single stunning zap, or high voltage volleys that cause fish to have massive involuntary muscle spasms. Another type of less powerful shock produces muscle twitches in prey that reveal their location to hunting eels. The electric eel is armed with muscle-derived biological batteries capable of generating a discharge of up to 600 volts. This is almost equivalent to the voltage created by the subway power system in New York. The eels are also known to use low-voltage electrical discharges as a kind of sonar to sense what is around them.