10. THE SWIM BLADDER AS A HYDROSTATIC ORGAN
425
to half the volume by 20,000 atm, gases obey Boyle’s law fairly well.
Thus their volume is halved when the pressure is doubled, for example,
on going from the surface to 10-meter depth of the sea. According to
Alexander (1966) the specific gravity of O2 at 700 atm is 0.7, while the
density of water is changed insignificantly. Thus even at 7000-meter
depth a gas-filled swim bladder gives some buoyancy.
C. Energy Saved by Neutral Buoyancy
The possession of neutral buoyancy enables a fish to save energy in
two ways: (1) It can remain motionless in midwater, and ( 2 ) the
energy expenditure to swim horizontally is reduced.
(1) Since a fish without a bladder is about 5% heavier than water, it
will have to exert a force of 5% of its body weight downward on the
water if it wishes to stay at one level. A few percent of the body weight
may seem quite a small load, but a weight easily supported on land can
only be sustained in water by continuous movement. Even an active
pelagic fish very .infrequently exerts a swimming force corresponding
to more than 25% of its weight (Gray, 1953). To exert continuously
a force of a few percent of its body weight seems therefore to be a
formidable task (see Denton and Shaw, quoted by Denton, 1961).
( 2 ) Alexander (1966) has made an interesting calculation of the
amount of energy saved by the presence of a swim bladder during
horizontal swimming. When a fish moves at a velocity of one body length
per second, the power needed to overcome the sinking tendency is 60%
of the total power of movement. When it moves at 10 lengths per second,
only 5% of the total power is used to keep it at the same vertical position.
At normal cruising speeds, which according to Bainbridge (1960) is
3-4 lengths/sec, about 20% of the total power is expended to overcome
sinking. Since we do not know the efficiency of horizontal versus vertical
movements, we can only guess that the energy partition must also be of
this magnitude.
At present we are not in a position to make a quantitative comparison
between the energy needed to stay at a certain depth by swimming as
compared to the cost of maintaining a gas-filled swim bladder. It appears,
however, that the swim bladder solution is by far the least expensive.
D. The Cost of Depositing Gas
Kanwisher and Ebeling (1957) investigated the metabolic cost
connected with vertical migrations. On the assumption that the
fish must keep a constant bladder volume during vertical migrations,
425
to half the volume by 20,000 atm, gases obey Boyle’s law fairly well.
Thus their volume is halved when the pressure is doubled, for example,
on going from the surface to 10-meter depth of the sea. According to
Alexander (1966) the specific gravity of O2 at 700 atm is 0.7, while the
density of water is changed insignificantly. Thus even at 7000-meter
depth a gas-filled swim bladder gives some buoyancy.
C. Energy Saved by Neutral Buoyancy
The possession of neutral buoyancy enables a fish to save energy in
two ways: (1) It can remain motionless in midwater, and ( 2 ) the
energy expenditure to swim horizontally is reduced.
(1) Since a fish without a bladder is about 5% heavier than water, it
will have to exert a force of 5% of its body weight downward on the
water if it wishes to stay at one level. A few percent of the body weight
may seem quite a small load, but a weight easily supported on land can
only be sustained in water by continuous movement. Even an active
pelagic fish very .infrequently exerts a swimming force corresponding
to more than 25% of its weight (Gray, 1953). To exert continuously
a force of a few percent of its body weight seems therefore to be a
formidable task (see Denton and Shaw, quoted by Denton, 1961).
( 2 ) Alexander (1966) has made an interesting calculation of the
amount of energy saved by the presence of a swim bladder during
horizontal swimming. When a fish moves at a velocity of one body length
per second, the power needed to overcome the sinking tendency is 60%
of the total power of movement. When it moves at 10 lengths per second,
only 5% of the total power is used to keep it at the same vertical position.
At normal cruising speeds, which according to Bainbridge (1960) is
3-4 lengths/sec, about 20% of the total power is expended to overcome
sinking. Since we do not know the efficiency of horizontal versus vertical
movements, we can only guess that the energy partition must also be of
this magnitude.
At present we are not in a position to make a quantitative comparison
between the energy needed to stay at a certain depth by swimming as
compared to the cost of maintaining a gas-filled swim bladder. It appears,
however, that the swim bladder solution is by far the least expensive.
D. The Cost of Depositing Gas
Kanwisher and Ebeling (1957) investigated the metabolic cost
connected with vertical migrations. On the assumption that the
fish must keep a constant bladder volume during vertical migrations,