BITERS, SHAKERS, AND GRAZERS.
Above is the unmistakable jaw of
a Great white shark. This large adult specimen measuring about 60cm across is
perfectly constructed to do the job for which it was designed, i.e. to quickly
sheer through the tough hide of pinipeds causing devastating wounds and
rendering them immobile. The jaw characteristics that support this diet include
both the heavy built cartilaginous jaw itself that will not buckle under the
enormous force exerted by the surrounding heavy musculature, and the wide based
serrated teeth in both the upper and lower jaw. When closed these strong cutting
tools meet like interlocking steak knives. In younger white sharks the lower
teeth are narrower reflecting the difference in their more fish based diet.
There is no stereotypic shark jaw that is representative of
the group as a whole. This is because of the vastly different diets and
lifestyles of the various groups. Most sharks and rays roughly fit into the
categories of 'crushers', 'biters and shakers', and grazers. Many sharks sit on the fence
when it comes to a dietary group for example tiger sharks (which are obviously
biters) will also happily slurp up a conch shell if they need some food. Some
sharks such as horn sharks have even developed different teeth from front to
back much like our own dentition allowing them to grasp and crunch as the food
demands. Horn sharks constitute the taxonomic order Heterodontiformes and the
family Heterodontidae, which literally means different teeth.
Many bottom feeding elasmobranches live on hard shelled
animals such as crabs and snails. If you can imagine how difficult it would be to eat
a walnut (still in its shell) with a knife and fork, you can see the need for
adaptation in order to utilize this source of nourishment. Just as we have
developed nut cracking tools for that very specific purpose, so too have sharks
and rays that depend on hard shelled organisms for sustenance. The teeth of most
rays and some sharks have evolved into what appears to be a roughened pad that
has high friction surfaces capable of hanging onto awkward smooth objects such
as shelled mollusks. Close inspection reveals that these pads are actually made
up of hundreds of tiny flattened teeth. This form of dentition is referred to as
Many of the worlds pelagic sharks have very sharp, pointed
teeth that are specifically designed for piercing through fish in order to hold
onto them as they struggle. The classic example of this dentition pattern is the
Mako shark which has wickedly sharp, inwardly curving, spindle shaped teeth protruding from both
jaws. The Mako hunts fast swimming tuna and other pelagic fishes. Its ability to
punch through the skin of a fleeing tuna and hold tightly onto it as it weakens
has an obvious benefit for this hunting strategy.
Some sharks have teeth
designs that are more blade like as in the above white shark or the tiger. These
teeth are perfect for slicing and are capable of severing large pieces of flesh
and bone from a prey animal. Although the sharpness of the serrated teeth is
often enough to completely remove chunks of flesh, these sharks also employ
another strategy for tearing away food. Once the jaw is closed onto the desired
area, the shark thrashes its head from side to side. The resistance of the water
and the force of the shark's head swinging, aid to saw through the remaining
flesh allowing the shark to remove enormous crescents out of large prey animals.
One of the times that this behavior becomes most evident is when viewing sharks
feeding on a whale carcass. The sharks can be seen thrashing their bodies from
side to side wildly at the surface while their jaws are latched onto the whale's
All sharks are carnivores but not all hunt large prey.
Ironically, the largest of the sharks and rays are plankton feeders. This
includes the Whale shark, Basking shark, Megamouth shark, Manta ray, and Mobula
rays. Each species has a different method for collecting plankton but the
resultant supply is the same. Some of these species also occasionally supplement
their planktonic diet with small fishes. Plankton filtering sharks do not
require teeth to collect their food. Basking shark for example, possess a line
of thick brushlike strands that cover the gill openings and keep any food from
escaping (not unlike the baleen in some whales). These 'gill rakers' which get
battered throughout the summer feeding season are shed during the winter months
only to be regrown before the plankton blooms again in the spring.
Interestingly, these plankton feeders possess rows of tiny residual teeth that
at first sight appear useless. It has now been suggested that these tiny teeth
are used to help the male gain purchase on the female during mating. It must
be quite a sight to see two basking or whale sharks joined in such a way.
At the other end of the spectrum one of the smallest 'biters'
in the shark world is the ectoparasitic Cookiecutter shark. This little monster
which matures at a little over 30cm, is responsible for the round hollows of
flesh that are sometimes missing from large bony fishes and cetaceans. There are
actually at least 3 members of the Dalatiidae (kitefin sharks) family that feed
this way, the largest being the Kitefin shark itself (Dalatias licha)
which reaches almost 2 meters in length. Most sharks have more pointed teeth in
their lower jaws for grasping and sharper sawing teeth in the upper but this is
reversed in the Cookiecutters. It is assumed that the Cookiecutter first bites
down with the top jaw to gain a secure purchase. Then it slices upwards with the
lower more knife like teeth while simultaneously twisting its body around. This
maneuver cores out a neat plug of flesh rather like the indent left when
scooping out a spoonful of ice cream.