Most people know that sharks have an acute chemosensory system that enables them to detect very dilute amounts of blood in the ocean water, but the ability to 'smell' blood only gets a shark so far. Once the shark gets closer to its prey, the chemical trail left by the blood floating in the water is no longer accurate. The prey, now alert to the shark's presence, can dart around and distort the signal by distributing blood in all directions. The shark now needs to rely on some other sense, and vision is usually not sufficient. Darkness, turbidity of the water, disturbed sand and other debris, and other factors become an issue. Instead, the shark can actually sense the teeniest, tiniest electrical signal that the very cells of the prey organism itself are producing, and use that to hone in on the animal when all other senses are virtually useless.
To understand how sharks do this, we need to go back and look at the lateral line in fish. The lateral line is a mechanosensory system, meaning it detects movement, specifically the direction of the displacement of water around the fish. To put it simply, the lateral line is a line of pores running down each side of the fish and clustering about the head. These pores open into a tube system lying just under the surface of the skin. The tubes are filled with liquid and lined with cells that have special hair-like projections called cilia. Fish use these cilia to detect motion in the water. When the water moves, it pushes the water in the tubes, which in turn push the cilia (imagine kelp swaying underwater). When the cilia bend, the cell turns the mechanical signal into an electrical signal that can be sent to the brain and interpreted as the direction of water disturbances nearby.
In humans, this system is retained in the inner ear and is important in hearing and proprioception. Proprioception, by the way, is one of the 'forgotten senses'. It is your ability to understand where your different body parts are and how they're moving with respect to one another and gravity at al times. In hearing, sound waves disrupt the water in the cochlea of the ear, and those disruptions bend the cilia, just like the cilia in the lateral line. When the cilia bend, this opens ion channels in the cilia itself, changing the charge distribution between the inside and the outside of the ciliated cells and thus turning a mechanical message into an electrical message. This electrical message is carried to the brain and interpreted as sound. Similar events are occurring in our sense of proprioception. Every time we turn our head, we are causing water disturbances in the semicircular canals of the ear, which bends the cilia in there as well and is interpreted by the brain as our direction of motion.
In sharks, this system has been modified in certain places to detect electrical signals instead of mechanical signals. Clustered about the face of sharks are specialized pores of the lateral line called ampullae of Lorenzini. They're named after the man who discovered them and determined that they must serve some sensory function, although it wasn't until much later that biologists determined that they're used for electroreception.
In the ampullae of Lorenzini, sharks retain the ciliated sensory cells of the lateral line. However unlike the mechanosensory cells of the lateral line of fishes or the mammalian ear, the ampullae aren't activated by water disturbances. Instead they actually sense tiny changes in the voltage of the seawater around the animal. The ampullae are sensitive enough to pick up the minute currents generated by the cells of nearby animals. In fact, they are so sensitive that they can register one millionth of a volt in a cubed centimeter of water! The ampullae detect changes in membrane potential of the electrosensory cells, which interface with the seawater. Membrane potential is the difference in voltage between the inside and outside of the cells. The cells have ion channels that are triggered to open at a certain threshold, which is the smallest change in voltage that the shark can detect. Like in the lateral line of fish and the inner ear in humans, this message is then sent to the brain, which determines the strength of the signal and the direction it is coming from. This process is instantaneous, which makes it a very effective mechanism for tracking the movements of prey animals at close ranges. This is what allows sharks to strike with deadly accuracy!
Check out this great video on how sharks detect their prey (this video focuses more on the lateral line than the ampullae, but it's still a great watch).
Or, if you're like me and you prefer sensationalism, blood, and heavy metal, watch this short clip instead. Then go watch the full version, since the official video doesn't allow embedding.
Enjoy the rest of shark week!
MURRAY RW (1962). The response of the ampullae of Lorenzini of elasmobranchs to electrical stimulation. The Journal of experimental biology, 39, 119-28 PMID: 14477490
Bullock, T. (1982). Electroreception Annual Review of Neuroscience, 5 (1), 121-170 DOI: 10.1146/annurev.ne.05.030182.001005
Obara, S. (1972). Mode of Operation of Ampullae of Lorenzini of the Skate, Raja The Journal of General Physiology, 60 (5), 534-557 DOI: 10.1085/jgp.60.5.534
Montgomery, J., Coombs, S., & Halstead, M. (1995). Biology of the mechanosensory lateral line in fishes Reviews in Fish Biology and Fisheries, 5 (4), 399-416 DOI: 10.1007/BF01103813