44 Sharks: The Animal Answer Guide
forward. One possible exception is the Sand Tiger shark, which is known
to gulp air at the surface and hold it in its stomach, providing buoyancy
and allowing it to hover motionless in the water column. This has been
observed in public aquariums, where this species is often kept, and even in
the wild. Sand Tigers in captivity have also been known to expel bubbles
out their cloaca. Shark farts?
Forward swimming (no selachian shark can swim backwards) keeps
sharks from sinking because of the lift provided by their fairly rigid, out-
stretched pectoral fins, much in the way the fixed wings of an airplane pro-
vide lift. But the larger upper lobe of the typical tail fin would theoretically
force the nose of the shark down, because the body would tend to pivot
around the pectoral fins. As a result, sharks would have to brake with their
pectoral fins to counter the tendency to nosedive. This would be a very
inefficient and energetically costly way to move through the water, and
everything we know about sharks points out that they use and conserve
energy efficiently.
The answer provided by video analysis suggests that the upper and
lower lobes of the tail produce mostly forward thrust, any pivoting over-
come by the lift provided by the pectoral fins and flat underside of the
head. These forces are equal and opposite to the weight of the shark in
the water. Braking by the pectoral fins is unnecessary, although climbing,
diving, and turning are accomplished by adjustments to the pectoral fins,
which are more flexible than fixed, horizontal airplane wings.
Although most of the energy for swimming comes from muscular con-
traction, sharks are exceptional among fishes because their skin also aids in
propulsion. Shark skin includes inner layers of collagen fibers that act as if
they were tendons. Different fiber layers are oriented at angles to one an-
other, making a shark’s body a cylinder reinforced with wound fibers. Such
a cylinder is strong and bendable but not compressible. Pressure builds up
inside the cylinder, making the shark’s body a pressurized cylinder with an
elastic covering. As a shark swims faster, it basically pumps more pressure
into the cylinder with each tail stroke; the pressure during fast swimming
is about 10 times what it is during slow swimming. The higher the inter-
nal pressure, the stiffer the skin becomes, increasing the energy stored in
stretched skin.
Body muscles are attached not just to the vertebral column but also to
the inside of the skin. (For this reason it is very hard to “skin” a shark.) As
muscles on one side of the body contract, muscles and skin on the other
side are stretched. The stretched skin is very elastic, but stretched muscle
is less so. As muscles on, say, the right side relax, the energy stored in the
skin on the left side is released. In this manner, the shark’s skin helps start
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