Page 29 - Fish and Amphibians (Britannica Illustrated Science Library)

24 LIFE IN THE WATER FISH AND AMPHIBIANS 25

The Art of Swimming Forward The Upward and

dorsal fin
Motion keeps the Downward
fish
o swim, fish move in three dimensions: forward and back, left results from the upright. The angle of the fins relative to the body allows
synchronized S- the fish to move up or down. The paired fins,
and right, and up and down. The main control surfaces that fish curve movement of located in front of the center of gravity, are used
T use for maneuvering are the fins, including the tail, or caudal fin. To the muscles The pectoral for this upward or downward movement.
fins maintain
surrounding the
change direction, the fish tilts the control surfaces at an angle to the water UPSIDE-DOWN spinal column. These balance and Ascent
can act as
muscles usually make
current. The fish must also keep its balance in the water; it accomplishes CATFISH alternating lateral brakes.
Synodontis nigriventris motions. Fish with large
this by moving its paired and unpaired fins. pectoral fins use them like
This fish swims upside down, oars for propulsion.
seeking food sources that are
less accessible to other species.
The ventral fins
stabilize the fish
The oarlike movement for proper balance.
of the tail is the main
MUSCLES GREAT WHITE SHARK Red muscles are for slow force used for forward
The tail has powerful muscles that Carcharodon carcharias or regular movements. motion.
enable it to move like an oar. Balance Paired
fins Descent
When the fish is moving slowly or is still in
the water, the fins can be seen making small
movements to keep the body in balance.
Larger white muscles are
for moving with speed, but
they tire easily.
1 2 3
When the crest
reaches the area
Starting Out Forceful Stroke between the two Complete 1 second
Muscles on both sides of the spinal column, dorsal fins, the tail Cycle
The movement of a fish through the The crest of the body's wave especially the tail muscles, contract in an fin begins its push
water is like that of a slithering snake. moves from back to front. alternating pattern. These contractions to the right. When the tail moves back The amount of time it takes for this
Its body goes through a series of power the wavelike movement that propels toward the other side and shark to complete one swimming cycle
wavelike movements similar to an S In its side-to-side the fish forward. The crest of the wave reaches the far right, the head
movement, the tail
curve. This process begins when the displaces the water. reaches the pelvic and dorsal fins. will once again turn to the
fish moves its head slightly from right to begin a new cycle.
side to side.
The crest of the
wave passes to the
first dorsal fins.
CAT SHARK
Scyliorhinus sp.
The resulting
STREAMLINED SHAPE impulse moves
Like the keel of a ship, the rounded contours of a fish the fish
At first the are instrumental. In addition, most of a fish's volume forward.
tail is even is in the front part of its body. As the fish swims
with the head. forward, its shape causes the density of the water The head
ahead to be reduced relative to the density of the moves from
water behind. This reduces the water's resistance. side to side.

The fish on the outside,
guided by those in the
THE FISH'S KEEL THE FASTEST middle, are in charge of
A ship has a heavy keel in the keeping the group safe.
lower part to keep it from The powerful caudal The unfurled dorsal School
capsizing. Fish, on the other hand, fin displaces large fin can be up to 150
have the keel on top. If the paired amounts of water. percent of the width Swimming in Groups A group of fish, usually of the same
fins stop functioning to keep the SAILFISH of the fish's body. species, that swim together in a
fish balanced, the fish turns over Istiophorus Only bony fish can swim in highly coordinated manner and with
because its heaviest part tends to platypterus coordinated groups. Schools of fish specific individual roles The fish in
the middle
sink, which happens when fish die. include thousands of individuals that control the
move harmoniously as if they were a school.
single fish. To coordinate their motion
they use their sight, hearing, and lateral 1cubic mile
Its long upper jaw line senses. Swimming in groups has its
enables it to slice advantages: it is harder to be caught by a (4 cu km)
KEEL LIVE DEAD 70 miles per hour (109 km/h) through the water, predator, and it is easier to find The area that can be taken up by
aiding this fish's
FISH FISH a school of herring
The maximum swimming speed it attains hydrodynamics. companions or food.