The notion that sharks have the ability to smell a drop of blood in an Olympic-sized swimming pool now belongs together with stories about unicorns or the pot of gold at the end of the rainbow.
Tricia Meredith and Stephen Kajiura of the Florida Atlantic University dispelled this popular myth in a recent issue of the Journal of Experimental Biology after studying the olfactory capabilities of five species of elasmobranchs (the group that includes sharks, skates and rays) and concluded that sharks had a sense of smell no better than that of a typical fish.
Sharks were thought to have superior olfactory sensitivities compared to other fishes on the basis of their enlarged olfactory structures, including a greater surface area where olfactory receptors are located. However, this assumption has not been tested until now.
The five species that the authors studied were: the Clearnose skate (Raja eglanteria), the Yellow stingray (Urobatis jamaicensis), the Atlantic stingray (Dasyatis sabina), the Lemon shark (Negaprion brevirostris), and the Bonnethead shark (Sphyrna tiburo).
The authors placed each elasmobranch in a tank with tubes and an electrode that measured the electrical impulses in the nasal cavity generated in response to smells attached to each nostril. The tube released differing concentrations of 20 different kinds of amino acids (which lead sharks to their prey), and the authors recorded the responses of the elasmobranch.
Based on their results, the authors concluded that while elasmobranchs have a particularly acute sense of smell, and have a remarkable ability to detect odours at very low concentrations (about one part in a billion), they are no more sensitive at smelling than bony fishes.
One hypothesis that elasmobranchs are not more sensitive at smelling is because the smell threshold is very close to the background level of the amino acids that naturally occur in seawater. If they were any more sensitive, it would be difficult for them to distinguish the amino acids coming from potential prey, from the random background levels.
To put this in another way, according to lead author Tricia Meredith: "Imagine you were super-sensitive to sound, and you could hear whispering really well. That would be awful if you always lived in a room with a stereo blaring."
For more information, see the paper: Meredith TL and SM Kajiura (2010) Olfactory morphology and physiology of elasmobranchs. Journal of Experimental Biology 213, pp. 3449–3456.