How Sharks Smell Things Underwater

A shark’s nostrils are for smelling, not breathing — and they are some of the most sensitive in the animal kingdom.

Start with the surprise: a shark’s nose has nothing to do with breathing. It breathes through its gills. The two small openings under its snout are nostrils, and their only job is to sniff.

Water flows into each nostril, across folded sheets of smell-sensing tissue, and back out. Anything dissolved in that water — the scent of a fish, the drift of something interesting — gets read almost immediately.

How sensitive is it? Very. Sharks can pick up some scents at extremely low concentrations, the kind you’d describe in parts per billion, which is like noticing a single drop of dye in a swimming pool. The exact numbers scientists measure in a lab don’t always match the popular stories, and different species are better or worse at it, so it’s worth treating the most dramatic claims with a bit of caution.

Smell in stereo

Here’s the clever part. A shark has a nostril on each side of its head. If a scent reaches the left one a fraction of a second before the right, the shark can tell which side the smell came from. That tiny time difference is enough to steer by. It’s smell in stereo.

Smell tells a shark that dinner exists. It doesn’t tell the shark exactly where dinner is, or what dinner plans to do about it. For the finish, the shark switches senses: it feels the water move with the sensors along its body, and it picks up the faint electricity of a hiding fish with special pores on its snout.

And the famous line about smelling a single drop of blood from a mile away? It’s a good story, but the numbers behind it are shakier than they sound. Sharks are excellent smellers. Their legends, like most legends, grew a little in the retelling.

Did you know? Sharks can close their nostrils, which is handy when they’re swimming fast or rooting around in the sand.