Sharks, as well as many other vertebrates that live in the water, can locate other creatures with the aid of electroreception. This means they can detect electrical impulses.
Sharks detect bioelectric fields with sensory cells, called ampullae of Lorenzini.
When a fish moves, its brain sends out a tiny nerve impulse to the muscles that contract. Those impulses create an electric field that extends into the water and can be detected.
Sharks detect these bioelectric fields with their ampullae of Lorenzini. The ampullae look like black pores on the shark’s snout. These pores are distributed symmetrically across the shark’s head, allowing the shark to easily determine the direction where the electric field originated. Each pore is full of sensory cells surrounded by a specialized jelly. When exposed to a bioelectric field, the sensory cells are stimulated, alerting the shark to a potential food source.
Electroreception is found typically in animals that live in salt water. Even if humans had the ability to detect electric fields emanating from contracting muscles, it probably wouldn’t be very sensitive. Air isn’t a very good medium for carrying electric fields. However, salt water is because it has a higher concentration of ions.
Sharks have the most sensitive electroreception abilities in the animal kingdom. They can detect an electric field as weak as five billionths of a volt per centimeter, which is significantly weaker than the electric field produced by a ticking wristwatch.