138 Sharks: The Animal Answer Guide
Although seemingly specialized for certain food types, some sharks can
adjust their teeth to take advantage of a meal. Hemiscylliid bamboo sharks
can turn their spiky, fish-eating teeth into more pavement-like dentition by
allowing the functional row of teeth to bend toward their throat, creating
a flat rather than a pointy contact surface (see “Do a shark’s teeth change
during its life?” in chapter 2).
Some sharks prefer certain types of food over others. For example,
White Sharks prefer prey with a high fat content—thus, their attraction to
whale carcasses. Tests with floating baits show that White Sharks will bite
and reject furry sheep carcasses but will bite and consume the carcass of a
nice fat pig. White Sharks offered baits of seal fat and seal muscle will eat
the fat baits but reject the muscle. The initial bite (the attack) is apparently
when the shark tastes its prey and assesses fat content. This preference for
fatty foods may be why furry sea otters are attacked but not eaten, whereas
seals and sea lions, with all their blubber, are favored prey items (or maybe
White Sharks just don’t like a mouth full of fur). A preference for fat has
prompted speculation that one of the reasons people are attacked but often
not consumed is because we aren’t fat enough. An alternative explanation
is that we’re smart enough to get out of the water, or that someone comes
to help before the shark finishes its meal. Needless to say, the controlled
experiments necessary to test these ideas have not been performed.
The biggest sharks and rays eat the smallest items. Whale, Basking, and
Megamouth sharks and manta rays feed on zooplankton (a collective term
for very small animals or larvae that are suspended in the water) and small
fishes.
Chimaeras have beaklike teeth. Not surprisingly, the limited informa-
tion available on their food habits indicates that they crush shelled prey.
Diet studies point to hard-bodied, bottom-living invertebrates—particu-
larly clams, scallops, snails, and crabs—as the main prey items. But as good
chondrichthyans, they take advantage of a variety of available food types,
including small fishes, worms, echninoderms (brittle stars, sea urchins),
and even sea anemones and an occasional jellyfish. Smaller holocephalans
eat smaller prey or prey types with softer shells.
Finally, given all the strange and seemingly inedible things that have
been found in some sharks, you might think that they do not have the abil-
ity to reject a food item once it has been swallowed. On the contrary, sharks
have a much more effective system for expelling something distasteful than
we do. (We rely on stomach muscle contraction and reverse peristalsis in
our esophagus, pushing items slowly or sometimes not so slowly back out.)
A study of the Shortfin Mako found that these predators can actually evert
their entire stomach, basically pushing it up and out of the mouth in as
little as three-tenths of a second. They do this by contraction of abdominal
http://www.ebook3000.com