A shark attacked and killed a 14-year-old girl near a Florida beach on Saturday, despite a surfer's attempts to save her. The surfer pulled the girl from bloody water, fended off the predator (probably a bull shark) with his fists, and tried to confuse the predator by splashing around away from the girl's body. Can sharks really smell blood in the water?
Yes. Sharks have a keen sense of smell, and blood happens to be a very attractive odor—they can sniff out even a tiny amount diluted into a large body of water. Sharks also use sight and hearing to detect their prey. A splashing sound might tempt them, as would the sight of a flailing animal. In particular, hungry sharks like to hear low-frequency, irregular sounds, like those made by a struggling fish. They also look for flickering shapes, like a fish's glinting scales. Attacks on people can sometimes be a case of mistaken identity. For example, a shark could confuse reflective jewelry for scales.
The shark's eyes, ears, and nose are all situated near its mouth. But sharks also detect their prey with sensory receptors that run along their sides. These receptors make up the "lateral line," an organ similar in function to the ear that can feel pulses or vibrations in the water. A shark might sense the flailing of an animal in distress and swim closer to investigate.
When a shark gets very close to potential prey, it can utilize yet another sense: electroreception. Electroreceptive organs (or "ampullae of Lorenzini") sit inside little pores on the shark's snout. Living things submerged in salty seawater produce a faint electrical field that the shark can feel at short distances, allowing it to suss out creatures that bury themselves in the sea floor. Muscle contractions also produce little surges of electrical activity that a shark can detect using electroreception. (Research suggests that some sharks may use electroreception like a compass, to help navigate underwater.)
Bonus Explainer: The surfer who tried to save this weekend's shark attack victim says he defended himself by punching the shark with his bare fist. Is that a good idea? Shark attack experts suggest punching a shark only as a last resort. Rapid retreat tends to be a better plan. It won't help to play dead if a shark has you cornered. Instead, a smack to the face or snout—where sharks, like humans, have a high concentration of sensory receptors—can stun your attacker and give you enough time to escape. When a shark has you in its jaws, try poking at its eyes or gills.
Explainer thanks Robert Heuter of the Mote Marine Laboratory.