40 Sharks: The Animal Answer Guide
counters. Male Round Stingrays (Urolophus halleri) find females buried in
the sand by detecting the electrical output produced by the female’s breath-
ing activity. In electrogenic skates (those that have special tail organs for
producing electricity), the output is used during social interactions.
Theoretically, sharks are sensitive enough that they could detect the
earth’s magnetic field and determine their compass headings when crossing
oceans. It is thought that sharks such as hammerheads navigate to and from
particular areas on a daily basis by homing in on electrical properties of the
seafloor, including variation in the earth’s magnetic field. Experimental evi-
dence supports this idea. Stingrays in lab pools can learn to orient to food
in fields weaker than the earth’s field. When experimenters reverse the field
polarity, the stingrays reverse the location where they search for the food, a
behavior which suggests that geomagnetic cues can be used in normal daily
activities. Elasmobranchs are thought to have small particles of magnetite
(an iron substance) in their inner ears that could be used in geomagnetic
orientation. When magnets are placed over the predicted region of magne-
tite in the head, rays lose their ability to orient to a magnetic field.
In addition to detecting electricity, some rays (but not selachian sharks
or holocephalan chimaeras) produce unusual amounts of electricity. Best
known and understood is the strong electric output of the torpedo rays
(family Torpedinidae). Also called numbfish, torpedo rays produce electri-
cal discharges of up to 50 V and 50 A, producing an output approaching 1
kW. Muscle contractions in all vertebrates involve some electrical produc-
tion; torpedo rays (and electric eels and their relatives) have evolved muscle
systems that don’t contract but are wired together to maximize this elec-
tric output. In the case of the torpedo rays, the muscles around their gills
and pectoral fins are arranged and modified to produce strong discharges.
With the pectoral area given over to electricity production, torpedo rays
are among the few raylike fishes that swim with their tail fins instead of
their pectoral fins.
Torpedo rays can use electric discharges to catch prey. At night, torpe-
dos swim slowly above the bottom; when they encounter a small fish, they
“embrace” it with their pectoral fins and discharge electricity, stunning the
prey and making it easy to capture. This was first discovered by research
divers who placed prey fish on the ends of poles that were wired to flash
bulbs. When the ray embraced the prey, the flashbulb fired.
Electric output is used in defense as well as offense. A captured or
speared torpedo ray will shock you when you handle it. Unfortunate divers
who bump into a torpedo at night may fall victim to this defensive response.
Mysterious instances of drownings have occurred among scuba divers in
southern California. The victims were night diving alone and were found
on the bottom with plenty of air in their tanks and no visible signs of injury.
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