424
JOHAN B. STEEEj
to just how the muscles control the volume of the swim bladder vanished
when Moreau (1876) showed that they exert no control whatsoever.
Surprisingly enough, fishes are in fact in a state of unstable equilibrium,
as we are when standing upright.
Fishes compensate for transient changes of pressure by swimming.
Moreau showed that when a fish voluntarily changes depth, even by a
very small distance, the volume of gas within the swim bladder simply
changes in accordance with Boyle’s law. Even the gurnards, which have
particularly well-developed muscles around the bladder, do not use
them primarily for controlling the volume but for making sounds
( Moreau, 1876).
Further research has added details to Moreau’s conclusions. Alexander
(1959a-d) showed that only in Cyprinidae does the swim bladder restrict, although passively, the expansion of the gas when the external
pressure falls. For 22 other species he found that the swim bladder
obeyed Boyle’s law remarkably well. The sea horse, Hippocampus
breuirostris, has been shown to use the muscles of its swim bladder wall
to control not its density but its posture (Peters, 1951).
Moreau also demonstrated that in response to persistent changes in
pressure the fish can restore the original bladder volume by changing the
amount of gas in the bladder (the pressure of the gas is always equal
to the external hydrostatic pressure). It thus became easy to see why
codfish caught at 100-meter depth had very distended swim bladders
when brought to the surface while cod caught in shallow waters had no
such difficulties.
These early investigations thus drew up the contours of the hydrostatic function of the swim bladder. When a fish ascends the bladder
expands and gas is removed; when it descends the bladder is compressed
and gas is deposited. The crucial point is that the fish is able to vary
its gas content in such a way that the gas volume is almost constant
regardless of the hydrostatic pressure.
B. The Swim Bladder as a Float
In most common marine fishes the swim bladder occupies about 5%
of the total volume. This brings the density of the fish very close to
that of seawater, which is about 1.025. The density of muscle tissue is
about 1.05 and that of the skeleton 2 3 times that of water. The density
of fat, on the other hand, is about 0.9. In comparison the density of
air at 1 atm is about 0.00125.
The efficiency of the swim bladder as a float is reduced with depth.
Thus, while water and the organic components of animals are compressed
JOHAN B. STEEEj
to just how the muscles control the volume of the swim bladder vanished
when Moreau (1876) showed that they exert no control whatsoever.
Surprisingly enough, fishes are in fact in a state of unstable equilibrium,
as we are when standing upright.
Fishes compensate for transient changes of pressure by swimming.
Moreau showed that when a fish voluntarily changes depth, even by a
very small distance, the volume of gas within the swim bladder simply
changes in accordance with Boyle’s law. Even the gurnards, which have
particularly well-developed muscles around the bladder, do not use
them primarily for controlling the volume but for making sounds
( Moreau, 1876).
Further research has added details to Moreau’s conclusions. Alexander
(1959a-d) showed that only in Cyprinidae does the swim bladder restrict, although passively, the expansion of the gas when the external
pressure falls. For 22 other species he found that the swim bladder
obeyed Boyle’s law remarkably well. The sea horse, Hippocampus
breuirostris, has been shown to use the muscles of its swim bladder wall
to control not its density but its posture (Peters, 1951).
Moreau also demonstrated that in response to persistent changes in
pressure the fish can restore the original bladder volume by changing the
amount of gas in the bladder (the pressure of the gas is always equal
to the external hydrostatic pressure). It thus became easy to see why
codfish caught at 100-meter depth had very distended swim bladders
when brought to the surface while cod caught in shallow waters had no
such difficulties.
These early investigations thus drew up the contours of the hydrostatic function of the swim bladder. When a fish ascends the bladder
expands and gas is removed; when it descends the bladder is compressed
and gas is deposited. The crucial point is that the fish is able to vary
its gas content in such a way that the gas volume is almost constant
regardless of the hydrostatic pressure.
B. The Swim Bladder as a Float
In most common marine fishes the swim bladder occupies about 5%
of the total volume. This brings the density of the fish very close to
that of seawater, which is about 1.025. The density of muscle tissue is
about 1.05 and that of the skeleton 2 3 times that of water. The density
of fat, on the other hand, is about 0.9. In comparison the density of
air at 1 atm is about 0.00125.
The efficiency of the swim bladder as a float is reduced with depth.
Thus, while water and the organic components of animals are compressed