Can a shark really smell a drop of blood from miles away?
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No, the image of a drop of blood detected “miles away in an Olympic swimming pool” is an exaggeration. The great white shark can indeed perceive certain molecules at concentrations of the order of a few parts per billion, but the actual detection distance depends mainly on dilution and ocean currents, not on a fixed range.
A powerful sense of smell, but subject to the physics of water
The nostrils of the great white shark open onto richly innervated olfactory sacs, capable of detecting minute chemical traces from living tissue or blood. In the laboratory, concentrations of the order of a billionth have been able to trigger a reaction. But in the open sea, an odorous molecule does not spread in a straight line or homogeneously: it dilutes quickly and follows the plume created by the currents, forming a sinuous corridor rather than a regular circle. A shark can therefore cross this plume very far from the source... or pass a few meters without detecting anything if the current has carried it elsewhere.
This is why smell is only one piece of the sensory puzzle. Once it enters the odorous plume, the animal moves up the concentration gradient by zigzagging, a behavior well documented in sharks.
The other senses take over at short distance
A few meters from the target, smell gives way to more precise systems. The lateral line, a band of vibration-sensitive receptors along the body, detects the movements of an injured fish or a struggling seal. Then, at very short range, the ampullae of Lorenzini — pores filled with gel located on the snout — perceive the tiny electric fields generated by the muscular and cardiac activity of any prey, even immobile or hidden in the sand. This electroreception allows almost millimeter localization just before the bite, an essential relay where the sense of smell, diluted by currents, becomes ineffective.
