Sharks are impressive predators, capable of detecting the slightest scent in the depths of the ocean. Their olfactory system is extremely sophisticated, allowing them to track prey several meters underwater. But how does a marine animal, without air to transport odors, manage to detect such odorous particles?
An olfactory system adapted to water
Sharks have exceptional olfactory abilities. Unlike humans, who breathe and smell through their nostrils, sharks do not use their nasal cavities to breathe. Their two nostrils, located at the front of their heads, allow seawater to circulate from one to the other and bring odorous particles with it. These particles are dissolved in the water, and this is how the odor is transported.
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Inside each nasal cavity is a structure called an olfactory rosette. This rosette is made up of folds of skin, covered with specialized sensory cells called lamellae. When seawater seeps in, the lamellae capture odor particles and send a signal to the olfactory bulb in the shark’s brain. This is where the information is analyzed, allowing the animal to identify the nature and source of the odor.
So sharks’ noses work much like humans’, but with one key difference: They sense odors in the water, while humans pick them up in the air. This mechanism allows sharks to sense and locate potential prey or dangers in their environment.
— © Elias Levy / Wikimedia Commons
Do sharks smell blood from miles away?
A myth claims that sharks can detect a drop of blood from a kilometer away. However, this myth is greatly exaggerated. In reality, sharks can only smell odors when the odor particles reach their noses, meaning those particles must first be transported through the water.
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The distance at which a shark can detect a scent depends on ocean currents. If blood spills into the water, it will take some time to travel the distance from its source to the shark. However, sharks are equipped with a unique sensory tool that allows them to very precisely locate the source of an odor.
Indeed, they smell “in stereo”. This means that they are able to perceive slight differences in the time it takes for the odor to reach each of their nostrils. This small difference allows them to determine the direction from which the odor comes with formidable precision. Of course, not all shark species have the same sensitivity, but some can detect a tiny amount of blood dissolved in millions of liters of water.
— © Sharkcrew / Wikimedia Commons
The best nose among sharks
Not all sharks are equal when it comes to smell. Among them, some species are more sensitive than others. Hammerhead sharks, for example, are often believed to have a more keen sense of smell due to their elongated snouts. This could allow them to better capture odors and therefore be more efficient in finding prey.
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A study carried out in 2014 analyzed the size of the olfactory bulb of 58 species of cartilaginous fish, including sharks. This bulb is the part of the brain dedicated to managing olfactory signals. The results showed that the great white shark, tiger shark and nurse shark have the largest olfactory bulbs. This suggests that they are probably the best at detecting odors. In contrast, species like the sickle lemon shark or the bigeye thresher shark have smaller olfactory bulbs.
The researchers noted that sharks with more developed olfactory apparatus were often those that hunted odorous prey, such as seals or rotting whale carcasses. These prey give off powerful odors, which has probably pushed certain species to develop a more refined sense of smell to capture these scents at great distances in sea currents. However, sharks are not particularly interested in the smell of humans. Indeed, their preference is generally towards fatter prey, such as fish or marine mammals. Elsewhere, a nurse shark bites a tourist in this incredible video.
