Electric organ
Muscle turned into living batteries, used by electric eels, torpedo rays and knifefish to stun prey or sense their surroundings.
Also: electrocytes
An electric organ is made of thousands of flattened cells called electrocytes, changed from muscle or nerve tissue. Each makes a tiny voltage, and stacked in series they add up. The electric eel of South America, which is a knifefish rather than a true eel, can deliver around 600 volts for an instant, enough to stun fish and deter large animals. Torpedo rays and the electric catfish give strong shocks too. Many weakly electric fish, such as the elephantfish of Africa, give off low voltages to sense objects and signal to each other.