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In-Depth Information
of a figure in a boat than swimming by oscillation, as most commonly seen in
the powered swimming of fish. The water boatman (
Sigara striata
)—no prizes
for guessing how it got its name—uses its legs to make a backward stoke,
during which they are then propelled forward. As the legs are drawn forward
again to make a subsequent stroke, the legs are furled or bent to reduce the
amount of drag. Even so during this process the animal clearly decelerates
at this point. The amount to which deceleration occurs is to some degree
determined by the relative size of the animal and the viscosity of the water.
Larger animals such as fully adult tuna will decelerate at a slower rate than
very small animals such as the water flea, though the surface area of the tuna
is far greater than that of the flea and thereby subject to higher drag. The
momentum of a larger animal moving at speed, particularly an animal with a
more streamlined profile that reduces drag, will be maintained over greater
relative distances.
Some fish use their pectoral fins as oars to create the same rowing action.
They push the fins backward at right angles to the body, thereby powering
the fish forward. The paired fins are then angled and furled into a position
with a lower profile to reduce drag during the forward movement that
completes the cycle. The forward movement of the fins also produces a
degree of drag, but this is far less than that of the backward power stroke of
the action.
Swimming by Lift on Paired Fins
Swimming with a rowing action using paired fins is not the only option open
to fish. Paired fins can also be used to produce lift and forward thrust, perhaps
best illustrated in sharks. Many sharks have rather large, rigid pectoral fins of
cartilage that are not veined nor capable of being furled or backward motion,
as with many of the species of bony fish. Instead these fins are held rigidly
out from the body and act in the same way as wings do. Because sharks are
heavier than water, they sink if they are not continuously swimming forward,
so the large pectoral fins assist them in maintaining their position in the water
and in forward motion. Their large caudal in and extended caudal peduncle
provide the power to ensure that water passes over the pectoral fins to
create the necessary lift. The degree of lift varies depending on the angle of
attack of the pectoral fins in exactly the same manner as discussed when we
looked at bird wings. These large pectoral fins allow for a very efficient form of
locomotion because they use the strong currents to create the necessary lift.
Flying fish have another and rather unique use for their pectoral fins. As
with many other fish, the flying fish uses its tail to provide the power for
the majority of its locomotion, but occasionally it uses its pectoral fin as
a principle source of locomotion. These fish spend much of their time
swimming close to the ocean's surface and, as with most fish that are prey
to larger predators, they leap from the water to escape becoming a meal. As
a result of this regular behavior, over thousands of years the flying fish's fins
have evolved to become almost as long as their bodies and, once unfurled,
