How Sharks’ Amazing Seven Senses Actually Work
Sharks can’t actually smell blood from a mile away. But they do have two more senses than humans, and their sense of detection is legendary.
The following is an excerpt from Why Sharks Matter: A Deep Dive with the World’s Most Misunderstood Predator by David Shiffman.
Why Sharks Matter: A Deep Dive with the World's Most Misunderstood Predator
Sharks possess an impressive array of senses that they use to navigate through the underwater world and to find prey. They have all the same five senses that people have, plus two more.
Sharks’ sense of smell is legendary. In some species, nearly one-quarter of their entire brain is devoted to processing scents in their watery home. Some older books even refer to sharks as “swimming noses.” No, that doesn’t mean that they can smell blood from a mile away or that they can detect a drop of blood in an Olympic-sized swimming pool, two common myths I’ve encountered. In fact, whenever you smell something, it’s because tiny particles of whatever you’re smelling are interacting with chemoreceptors inside your nose—think of that the next time you smell something gross! It’s worth pointing out, too, that sharks aren’t just smelling blood when tracking prey, but also various chemicals associated with injured or dead prey animals. And while you probably shouldn’t go in the ocean if you’re actively bleeding for a lot of reasons, human blood does not smell the same as fish blood to a shark.
Sharks can, however, hear the sounds of a fish struggling or something mechanical, like the sound of a boat motor, from nearly a mile away. Their lateral line, a mechanosensory organ that can detect vibrations in the water, is sensitive enough to feel the difference between a healthy fish swimming strongly or a sick and injured fish for which escape would be difficult.
Sharks have a reputation for poor vision; this assumption is often erroneously used to explain why they may accidentally bite humans. The reality is that, while the huge diversity in shark species means that eyesight quality depends heavily on which species you’re talking about, some sharks have fairly advanced vision—it’s just not designed to tell the difference between a seal and, say, a human wearing a seal blubber–like wetsuit. Sharks, after all, are extraordinarily well-adapted to the oceans that have been their homes for tens of millions of years, and it’s perhaps not fair to blame them for being imperfectly adapted for conditions that have appeared in an evolutionary blink of an eye. Sharks can see better than humans in just about every underwater environmental condition, which is to say, just because you can’t see a shark doesn’t mean that a shark can’t see you.
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In my opinion, the most amazing sense that sharks and their relatives have is the electric sense; that is, their ability to detect electromagnetic fields. Platypuses also have this sense (because what’s one more random addition to an animal that lays eggs and produces milk and can therefore make its own custard?). Some other bony fishes here and there also have an electric sense, but it’s relatively uncommon outside of the chondrichthyanlineage. This sense is housed in jelly-filled pores on a shark’s snoot called Ampullae of Lorenzini, which look almost like the chin stubble of a five o’clock shadow. (If you ever get the chance to witness a shark necropsy—basically the equivalent of an autopsy on humans—you’ll see that a tiny bit of gooey gel comes out when the face is squeezed. Please do not try this on a living shark for what I hope are obvious reasons.)
You may wonder what use it is to have an electric sense. Sometimes prey animals hide under the sand where a predator wouldn’t be able to see, smell, or hear them. Sharks, however, can still detect their presence by sensing their body systems’ electricity, and can dig them up for a meal. This electromagnetic sense is also helpful when it comes to long-distance open-ocean navigation. Not only are there no street signs in the middle of the Pacific, there aren’t any landmarks. Sharks can navigate the featureless open ocean by detecting the Earth’s magnetic field, which lets them end up exactly in the right spot after a long migration.
From Why Sharks Matter: A Deep Dive with the World’s Most Misunderstood Predator by David Shiffman. Copyright 2022. Published with permission of Johns Hopkins University Press.