Form and Function of Sharks 39
smell, appearance, or sounds. This has been demonstrated in the field and
laboratory with sharks that were attracted to baits suspended next to active
electrodes. The sharks were clearly first lured in by the smell and tended
to approach the odor source. But at the last moment, they ignored the bait
and attacked the electrode. In fact, ampullary electroreceptors are located
around the mouth of many elasmobranchs—which is to say, in their blind
spot so far as vision is concerned. This positioning could allow a shark or
ray to home in precisely on a potential food source solely by electrorecep-
tion, effectively lining the food up in its “electrical sights” and then engulf-
ing the prey.
How fine-tuned is this electric sense? We can only detect electrical
fields of one-tenth of a volt (0.1 V) or stronger. Sharks can detect fields of
10 billionths of a volt (0.00000001 V), which is ten times what is needed
to sense the electrical output from a prey fish. It’s difficult to put this in a
meaningful context. But such sensitivity would be sufficient, at least theo-
retically, to allow a shark to detect the electrical output of a 1.5-V flashlight
battery over a distance of 1,300 km (800 miles), or roughly the distance
between Seattle and San Francisco (or New York City and Jacksonville,
Florida). More meaningful to us, a shark could theoretically sense the bio-
electric output of a swimmer 1 to 2 m (3–6 ft) away.
Such sensitivity means sharks can use electrical output to find prey.
This is well known from lab experiments with species such as the Atlan-
tic Stingray and two scyliorhinids, the Swell Shark (Cephaloscyllium ventrio-
sum) and Small-spotted Catshark. These animals can find and capture prey
fish buried in the sand under conditions that eliminate all other sensory
cues. Sharks can also sit motionless in the dark and snap up prey that swim
nearby by using electrical signals. Electrical cues are also used in social en-
The small holes on the head of this
Blue Shark are Ampullae of Lo-
renzini, the input structures for a
shark’s electrical sense. These pores
were discovered by Italian anatomist
Stefano Lorenzini in 1678. Lorenzini
had no idea of their function; that
understanding took another 300
years. Photo by Austin Gallagher, http://aust
ingallagher.com; used with permission