Foods and Feeding 141
have another type of touch sense. Sharks can detect minor variations in
water movement, often called a “distant touch” sense. Receptor cells in
canals along the lateral line and scattered around the body react to minor
changes in the flow of water pushed toward a shark, such as those com-
ing off a nearby swimming fish, or water flowing across its body as the
shark swims. This distant touch sensitivity includes directional informa-
tion, which means that as a shark moves through the water, it can detect the
presence, location, direction of movement, and relative speed of moving
objects that displace water.
Sharks are extremely sensitive to bioelectric fields (see “How do sharks
detect electric fields?” in chapter 2). Such fields are created by all living
things due to muscular contractions, nerve impulses, or just because the
salt concentration of an animal’s body fluids differs from the salt concen-
tration of the water around it. Sharks have electrosensory organs, called
the Ampullae of Lorenzini, scattered around their bodies but concentrated
in the head and mouth region. Another hypothesis about the function of
the expanded skull of hammerhead sharks is that the hammer allows for a
wider distribution of ampullae, which increases both sensitivity to and ac-
curacy of detection of electrical impulses.
Sharks and rays are so sensitive to electric fields that, theoretically, they
could detect the output of a single flashlight battery several miles away if
it were not for electrical interference from the earth’s geomagnetic field.
They are more than sensitive enough to detect the bioelectric output of
common prey such as fishes or less common prey such as humans. Sharks
can find prey buried in the sand by homing in on the bioelectric output of
the prey. Sharks also dive into the sand after buried active electrodes.
When a shark makes its final approach to food, electrical output appears
to be the primary cue it uses to direct its bite. When sharks are offered a
choice between the fish slurry that initially attracted them (the bait) and a
nearby activated electrode, they prefer to bite at the electrode.
Touch and taste are the last senses employed in the search for a meal.
Smell and taste are related senses in that both rely on detection of chemi-
cals. Olfactory cues may be used to sense food at great distances, whereas
taste buds come into play much closer (very close) to the food item. Taste
buds concentrated inside the mouths of sharks apparently play a role in
accepting or rejecting food, as when demonstrably hungry sharks reject
the Red Sea Moses Sole (Pardachirus marmoratus) after biting it. The sole
secretes a combination of milky skin toxins called pardaxin and mosesins
when attacked. Sharks quickly release a sole but feed actively on other less
chemically defended fishes. Sharks also bite into and quickly release hag-
fish, presumably in response to the hagfish’s rapid secretion of mucous,
which the shark finds somehow distasteful, if not just repulsive. Noxious