How Do Sharks Sense Their Prey? Smell, Electricity and a Line Along the Body

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.

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 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

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.

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 electrical sense was demonstrated rather than assumed, through experiments in the late 1960s and early 1970s. A flatfish was buried in the sand of a tank, and sharks found it easily. When it was enclosed in a chamber of agar, a jelly that lets electric fields through but blocks scent, they still attacked the right spot; when it was wrapped in a thin plastic film that blocked the electric field, they swam past it. Finally, electrodes buried in the sand and carrying a weak current like a fish's drew attacks even when a piece of fish lay nearby, showing that at close range the field overrode smell.

Sharks are not alone in sensing electricity, and the other examples show how often the ability has evolved. Rays and skates share the ampullae of Lorenzini with sharks, and many freshwater fish have separate electroreceptors of their own. Among mammals, the platypus hunts along stream beds with its eyes, ears and nostrils shut, finding shrimps and insect larvae with thousands of electroreceptors in its bill, and a dolphin of South American coastal waters has been shown to detect weak electric fields through pits on its snout that once held whiskers. Water conducts, which is why the sense is almost entirely confined to animals that feed in water or wet mud.

The organs behind the electric sense were noticed long before anyone knew what they did. Marcello Malpighi saw them in the seventeenth century, and the Italian physician Stefano Lorenzini described the pores and their jelly-filled tubes in detail in 1678, after which they remained a puzzle for almost three centuries. Suggestions included producing mucus, sensing temperature or detecting pressure, and each had some support, since the organs do respond weakly to temperature. Only when researchers began recording the nerve signals coming from them in the twentieth century did their extraordinary sensitivity to voltage become clear.

The design works like a set of wires. Each pore on the skin leads to a canal filled with jelly, ending in a small chamber lined with sensory cells, and those cells compare the voltage at the pore with the voltage inside the shark's body. Because the canals run in many directions and to different lengths, the pattern of signals across them tells the shark which way a field is coming from. The jelly itself is remarkable: a study published in 2016 found that it conducts electric charge, carried by protons, better than any other biological material yet measured, so faint signals arrive with little loss.

Sharks also find their way across huge distances, and satellite tags have revealed journeys nobody expected. In 2003 scientists tagged a female great white shark off South Africa, nicknamed Nicole. Over about nine months she swam to the coast of Western Australia and then back to South Africa, a round trip of some twenty thousand kilometres, the fastest return migration known for any marine animal at the time.

Her track was strikingly direct, and she spent much of the time near the surface, which led researchers to suggest she might navigate partly by the stars or by sensing the Earth's magnetic field, which sharks' electrical senses could detect. The journey also showed that great white populations on different continents are connected, which matters for protecting them.

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.