Sniffing out the truth on shark senses

4 06 2007

Perhaps one of the longest-standing myths about sharks is that they can “smell a drop of blood up to a mile away.” This little pseudo-fact shows up in nearly every popular work on the subject (particularly children’s books) and even in museums (like the newly re-opened Hall of Human Origins at the American Museum of Natural History). This all stems from a misunderstanding of what scents are and how they are detected; different scents and odors are actually chemicals of varying amounts present in a medium like air or water, different animals having differing sensitivities to varying concentrations of the chemicals. Sharks devote a large part of their brain to olfaction (sense of smell) and are able to detect certain odors in as little a concentration as one part per million in seawater. But in order to do this, however, they actually need to come across the chemicals in the water causing the odor, which they then follow up the “scent corridor” to the potential prey. In other words, they can’t smell anything until they actually come across the scent, so if you cut yourself in the ocean sharks don’t have some kind of 7th sense (they do have a 6th; electroreception) that tells them that someone a mile away is now bleeding. Even if some of your blood does enter the water, it is becoming more apparent that sharks are more interested in oils and certain body fluids than blood, which seems less able to travel far in seawater. Hopefully research will be done on this topic soon, but it is important to separate the truth about sharks from the modern mythology often confused with fact.

This all brings me to a newly published paper in the Journal of Experimental Biology by Jayne M. Gardiner* and Jelle Atema, in which it was determined that smooth dogfish sharks (Mustelus canis) rely on more than simply smell to find their prey. When the sharks were subjected to some squid-odor dispersed in the tank alone, they had some amount of difficulty finding the source of the scent even though you would think their sense of smell alone would be enough. When a brick was added to “jumble” the scent, however, the sharks did much better. Why? Along the body of a shark are mechanoreceptors known as the lateral line (which is present in other fish as well). These mechanoreceptors not only allow the shark to detect the movements of other fish around it, but to detect “odor plumes” (a three dimensional signature of a scent) as well, and the study seems to show that “feeling” an odor is just as important as smelling it. Likewise, when the sharks lateral lines were disabled by the researchers, they seemed to rely on vision for their lost sense, and while it took the sharks longer to find the source of the scent they had the same amount of success when in the light (they did not do well at all while in the dark).

Through much of my time in grade school, sharks were treated as “primitive” fish, a sort of “living fossil” that has gone “virtually unchanged for millions of years.” Perhaps it was my preferential bias towards these animals, but I always rejected this particular view; how could such amazing animals, with senses lacking in many of the more evolved fish, be primitive throwbacks? No one ever put them in context of evolution or pointed out that sharks have been evolving since they first appeared and have not always looked like they do now, and instead the “origin time” for each group of animals was used as a benchmark for how evolved (and therefore “important”) it was. I would love to say that such misunderstandings are no longer being taught in our schools, but I can’t be so sure. Indeed, just as we fight to keep creationism out of schools, so too should we fight for better elementary science education. It’s foolish to assume that in the absence of creationism, evolution is being taught correctly to students, and with the current focus on standardized test scores understanding science has become less and less of a priority for many schools. We are doing a great disservice to ourselves and students if we idly allow long-refuted myths to continue.



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