How Sharks Smell Blood – Superbly, But Not Miles Away

how sharks smell blood
The nares, or nostrils, are clearly visible on the snout’s underside on this great white photographed in South Africa.

HOW SHARKS SMELL BLOOD IS FAR MORE INTERESTING than their mythical ability to detect the scent of “a drop of blood miles away.” Scientists disproved that legend long ago, although you may still find it in cheesy movies and on shark-week type reality shows.

Sharks do have an amazing sense of smell, but their long-range detection capabilities are limited to several hundred yards rather than miles, many authoritative sources suggest. And one study suggests that they’re not particularly better at smelling things than bony fish.

And actually, the rate and direction of a scent’s dispersal in the ocean currents is probably more key to how sharks smell blood than their odiferous superpowers.

how sharks smell blood
Dogfish, or Mustelus canis, were the species used for research on shark scenting techniques, These were photographed at an aquarium.

ANATOMY AND THE SHARK SCENTING PROCESS

Sharks start out with anatomy that would seem built to facilitate scent detecting. Two-thirds of their brains are packed with highly sensitive olfactory tissues. Folded over plates called lamellae, scent-detecting tissues account for much greater surface areas than comparable tissues in bony fish.

And, while we mammals both breathe and smell through our noses, sharks receive their oxygen supply through the gills on the sides of their heads. Their nostrils focus on drawing in water for scent evaluation by those brainy two-thirds. Called nares, the side-by-side nostrils on the underside of their snouts work in parallel to help sharks follow the trail of a scent plume.

TIMING, NOT CONCENTRATION KEY TO HOW SHARKS SMELL BLOOD

On the other hand, the tracking process appears to be based more on timing rather than on an odor’s concentration, as was long believed. Research published in 2010 found that the animals react to whichever nare detects the scent first.  In response to a scent encountered by one between a 1/10th and ½ a second before the other, they turn their heads toward the side that first detected the smell.

The most wonderful part of this is that the scientists found it out by fitting several species of sharks with special headgear in order to supply them with pulses of marinated squid.

how sharks smell blood
If sharks “smell in stereo,” scalloped hammerheads like this one seemm likely to have an advantage due to their widely spaced nosdtrils.

STEREO SNIFFING

The side-by-side architecture of shark nostrils has led to the suggestion that they “smell in stereo,” using the information taken in through one nare versus the other to direct their scent tracking, moving from one odor patch to another as they encounter them.

And their swimming speed may be geared to the spacing of their nares, avoiding encountering a scent plume so quickly both nares react simultaneously, leading to no information. This concept suggests that hammerhead sharks can track down prey more quickly than other sharks. Farther apart than pointy-nosed sharks, they benefit from a longer delay between the two nostrils, letting them swim faster.

AND, OFACTORY SIZE DOESN’T MATTER

Similar headgear was used to study the idea that sharks were superior at detecting smells because of their extensive olfactory tissues. This project found that sharks may have enormous portions of tissues devoted to scenting but among species with varying amounts of tissues, their abilities to detect faint scents were about the same. And, the researchers suggested, not that much different than for bony fishes.

ALSO, HOW SHARKS SMELL BLOOD DEPENDS ON STUFF

Keep in mind that much of the understanding of how sharks smell blood is based on research in tanks. Scents move through air and water through the dispersal of molecules from the amino acids – the basic elements of animal proteins that the sharks are responding to.

In the actual ocean, the strength of a scent and the rate at which it moves depend on factors like the speed, direction, tides, temperature gradients and turbulence of currents. And, of course, on the amount of blood or urine or whatever is released at the source.

how sharks smell blood
Regrettably – or perhaps fortunately – when dive boats arrive these days, Caribbean reef sharks like this one show up not tracking a prey but looking for handouts. This one was photgrahed in the Bahamas.

ALL OF WHICH TRANSLATES TO…

The legend of sharks detecting blood from “miles away” has been universally debunked by most scientists. But truth to tell, otherwise, estimates of distance-detection capabilities are all over the place. And while there’s research about how sharks smell blood, there’s nothing to cite about the distance estimates.

Some sources like to measure odor detection in swimming pool distances – as in, a lemon shark can detect tuna oil at one part per 25 million, (equivalent to 10 drops in a home swimming pool). For other species, detection is estimated at one part per 10 billion (an Olympic-size pool). Or, if you prefer an open ocean reference, one part per 25 billion – suggesting an upper detecting limit of about 600 meters distance.

All of this, of course, depends on the “stuff” mentioned above.

PRINCIPAL SOURCES: Olfactory morphology and physiology of elasmobranchs,” Journal of Experimental Biology;  “The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks,” Current Biology; “Timing is everything for sharks that smell in stereo,” Nature; “How do sharks smell blood underwater?” Science Focus, BBC Focus Magazine; “The Superlative, Sensitive Shark,” PBS.org; Shark Smell Myth Found Fishy,” Inside Science; “Myth 5: Sharks Can Detect a Single Drop of Blood in the Ocean,” American Museum of Natural History; “Do Sharks Smell in Stereo?” Smithsonian Ocean Life;

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