Form and Function of Sharks 29
mostly zooplankton, which they strain from the water with their gill rakers
as water passes from their mouths and out their gill slits.
Chimaeras (Holocephali) feed on hard-bodied prey and have evolved an
entirely different solution with their rabbit-like teeth. Holocephalan teeth
are not replaced, as in sharks, but instead continue to grow to make up for
the wear they experience from eating hard-bodied prey. A ratfish has three
pairs of tooth plates, one pair in the lower jaw and two pairs in the upper
jaw. Ratfish also have tooth plates inside on the roof of their mouth, which
push against the tongue during a bite.
How many teeth does a shark have during its lifetime?
Mammals, including humans, have two sets of teeth during their lives—
baby or milk teeth when young, and adult or permanent teeth later. The
normal total for a person is 20 milk or primary teeth and 32 permanent
teeth, for a total of 52. Sharks, in contrast, replace their teeth throughout
their lives, so their teeth are categorized as either functional or replace-
ment. Functional teeth are used daily in capturing and processing prey.
They sit at the jaw margins and are exposed when a shark opens its mouth
to bite something. Replacement teeth lie in rows farther back and deeper
in the jaws, hardening as they develop and ready to move forward in con-
veyor-belt fashion as functional teeth are lost during feeding or are shed as
a natural process. Shedding occurs in part because the teeth are embedded
in a shark’s gums rather than anchored to the jawbones as in mammals and
bony fishes.
A functional tooth and the replacement teeth lined up behind it make
up a row. In most sharks, only the one or two outermost teeth in a row are
Functional and replacement teeth in
the lower jaw of a Bull Shark, Car-
charhinus taurus. The teeth pointing
upward are the functional teeth. At
least five series of replacement teeth
are visible lying against the inside
of the jaw cartilage, ready to move
into position if a functional tooth is
shed. In the third row from the left, a
replacement tooth appears to be ro-
tating into functional position. Photo
courtesy of Phil Motta; used with permission