|Many fishes swim by contracting and relaxing a succession of muscle blocks, called
myomeres, alternately on each side of the body, starting at the head and progressing down
toward the tail. The alternate shortening and relaxing of successive muscle blocks, which
bends part of the body first toward one side and then toward the other, results in a
series of waves traveling down the fish's body. The rear part of each wave thrusts against
the water and propels the fish forward. This type of movement is quite clearly seen in the
freshwater eel. Because movement of the head back and forth exerts drag, which consumes
additional energy and slows travel, a great many fishes have modified this snakelike
motion by keeping the waves very small along most of the length of the body, in some cases
showing no obvious movement at all, and then increasing them sharply in the tail region.
It is the end of the traveling waves that moves the tail forcefully back and forth,
providing the main propulsion for forward motion. A simpler form of tail propulsion is
seen in such inflexible-bodied fishes as the trunkfish, which simply alternates
contractions of all the muscle blocks on one side of the body with those on the other
side, causing the tail to move from side to side like a sculling paddle.
Some of the predatory bony fishes are the fastest swimmers; they can cruise at speeds that are between three and six times their body length per second and may be able to reach 9 to 13 body lengths per second in very short bursts. Some fishes, such as the blenny, which has been timed at 0.8 km/hr (0.5 mph), swim very slowly; others, such as the salmon, which may reach a sustained speed of 13 km/hr (8 mph), move much faster; and it has been estimated that tuna may reach speeds of 80 km/hr (50 mph), and swordfish, 97 km/hr (60 mph).
Distribution - Anatomy - Circulation -
Body Temperature - Water Balance - Gas Bladder
Lateral Line System - Evolution - Reproduction
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