How Do Sharks Sense Their Prey? Smell, Electricity and a Line Along the Body
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A shark can find a flatfish buried in sand with no smell, no sound and no sight of it, because the fish's heartbeat produces an electric field and the shark's snout is covered in pores that detect fields of a few billionths of a volt. It can hear the thrashing of a wounded fish from a kilometre away, follow a scent trail across the current by comparing the timing of its arrival at each nostril, and feel the wake of a swimmer through a line of sensors along its flanks. Sharks have been hunting with this equipment for four hundred million years, and most of it has no equivalent in a human body.
Smell
The shark's reputation as a swimming nose is partly earned. Its nostrils, on the underside of the snout, are for smell only, since sharks breathe through their gills, and water flows through them continuously as the shark swims, over folds of tissue packed with receptors; a large part of the brain is given to processing the result, and some species can detect blood at about one part in ten billion, a drop in a swimming pool. The old claim that a shark smells a drop of blood from a mile away is an exaggeration, since the molecules must physically reach the nose and a scent plume spreads slowly; what the shark does is steer up a plume by the difference between its two nostrils, turning toward the side where the scent arrived first, which experiments with sharks wearing tubes on their noses have shown.
Electricity
Every muscle contraction and every nerve impulse in a living animal produces a tiny electric field, and in seawater, which conducts, the field spreads a short distance from the body. Sharks and their relatives detect it with the ampullae of Lorenzini, several hundred to several thousand jelly-filled pores around the snout and head that lead to sensory cells, and they are sensitive to about five billionths of a volt per centimetre, the most acute electrical sense in the animal kingdom. The range is only about a metre, so the sense is for the last moment of a hunt: a hammerhead sweeps its wide head over the sand like a metal detector for buried rays, and a great white attacking a seal rolls its eyes back for protection and homes on the electric field for the final bite. The Earth's magnetic field, moving through seawater, induces fields that the same organs may read, which would explain how sharks navigate across oceans.
The other senses
The full set, roughly in the order they come into play as prey gets closer:
- •Hearing: an inner ear tuned to low frequencies, 10 to 800 hertz, the range of struggling fish, detected from hundreds of metres; sharks come to recordings of injured fish
- •Smell: for scent plumes over tens to hundreds of metres
- •The lateral line: a canal along each flank and around the head, open to the water through pores, lined with hair cells that feel pressure changes and water movement, so that the shark senses a swimming animal's wake and its own approach to a wall in the dark
- •Sight: eyes with a reflective layer behind the retina, as in cats, for dim light, and in most species poor at colour; good at contrast and movement over the last few tens of metres
- •Electroreception: for the final metre, and for prey hidden in sand or in murky water
- •Touch and taste: a test bite, which is why most shark attacks on humans are a single bite and a release
What the myths got wrong
Sharks do not smell fear, do not go into a frenzy at a drop of blood, and do not hunt humans; the roughly ten fatal attacks a year worldwide are mostly a great white, tiger or bull shark mistaking a surfer or swimmer for a seal or investigating with a bite, and the shark usually leaves after the first taste, since a human is bony and lean. The sensory story explains the pattern: a person splashing at the surface makes the low-frequency sound and the pressure wake of a distressed animal, a silhouette on a board looks like a seal from below, and the shark, having closed in on sound and sight, bites to find out. The senses that make a shark a good hunter of fish make it a poor identifier of people, and the animal that has the finest electrical sense on Earth cannot tell a surfboard from a sea lion until it has it in its mouth.
Why it matters
The shark's senses are being copied and defended. Electrical sensors modelled on the ampullae are studied for detecting the heartbeats of people buried in rubble; the lateral line is a model for underwater vehicles that must navigate without sonar; and electric and magnetic repellents, which overload the ampullae, are sold to surfers and tested on long-line fishing gear to keep sharks off the hooks, since about a hundred million sharks a year are killed by fishing, and a third of species are threatened. The animals whose senses are the most refined in the sea are being removed from it faster than they can breed, by a predator that they cannot detect at all.
The takeaway
Sharks hunt with a layered set of senses: hearing tuned to the low frequencies of struggling prey from hundreds of metres, smell that steers up a plume by comparing the two nostrils, a lateral line of pressure sensors that feels wakes and movement, eyes built for dim light and contrast, and the ampullae of Lorenzini, pores that detect the electric field of a living body over the last metre. The equipment is superb for fish and poor at telling a person from a seal, which is why attacks are rare, single bites.