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11 Locomotion of Marine Animals
weight exactly balances the buoyancy - the animal is neutrally buoyant. When
the density of animal body exceeds the density of water, then the animal is
negatively buoyant and tends to sink. When it is lighter (positively buoyant),
it tends to rise. The animal can remain at a particular depth in two possible
ways: by controlling its own density to achieve neutral buoyancy, or using lift
forces, generated by swimming, and controlling depth.
The animal's average density is the resulting density of the body components
which tend to sink, and the components which are less dense than sea water
and tend to float. From Table 10.1 it follows that sinking components are
principally proteins, animal skeleton and the muscles. The principal floating
components of marine animal body are fat, certain body fluids and chambers
filled with gases. The relative proportions of the different sinking and floating
components varies from animal to animal (Denton, 1974).
Some animals with bodies denser than sea water, such as octopi or lobsters,
live on the sea bottom and being neutrally buoyant is not a great need for them.
Another example is mackerel which has no special buoyancy mechanism. In
order to overcome its negative buoyancy, mackerel must swim continuously to
stay above the sea bed. To minimize the energy expenditure during swimming,
it has a streamlined shape with a drag coefficient, Cd, as small as 0.0043 (see
Fig. 2.25).
However, when animals swim quietly in the water column, they have to develop some mechanisms to bring them to neutral buoyancy. In general, the
buoyancy mechanisms developed by aquatic animals can be divided in three
groups: the use of gas chambers, the use of body fluids less dense than sea water, and the use of fat (Denton, 1974). Some marine fish, for example, cod, use
the first mechanism maintaining a gas space within an organ called the swim
bladder. The swim bladder volume usually amounts to about 5% of the total
volume of the animal. By varying the volume of this organ, the fish can adjust
its effective density to that of the surrounding water. It is now known that
swim bladder is used for controlling buoyancy down to 2000 m water depth.
To explore the effect of changing the swim bladder volume, consider a simple
example of a fish which, at a particular water depth, is neutrally buoyant. The
force balance is then:
(11.1)
in which V is the overall volume of the animal (inflated swim bladder included),
Pb is the density of the non-swim bladder body tissue, and a eq is the fraction of
the animal volume used by the swim bladder. The first term is the buoyancy
force and the second term represents the weight of the animal in sea water.
The necessary volume of the swim bladder (as a fraction of total animal
volume) to support the animal in the state of neutral buoyancy becomes:
Pb - Pw
aeq = Pw
(11.2)
11 Locomotion of Marine Animals
weight exactly balances the buoyancy - the animal is neutrally buoyant. When
the density of animal body exceeds the density of water, then the animal is
negatively buoyant and tends to sink. When it is lighter (positively buoyant),
it tends to rise. The animal can remain at a particular depth in two possible
ways: by controlling its own density to achieve neutral buoyancy, or using lift
forces, generated by swimming, and controlling depth.
The animal's average density is the resulting density of the body components
which tend to sink, and the components which are less dense than sea water
and tend to float. From Table 10.1 it follows that sinking components are
principally proteins, animal skeleton and the muscles. The principal floating
components of marine animal body are fat, certain body fluids and chambers
filled with gases. The relative proportions of the different sinking and floating
components varies from animal to animal (Denton, 1974).
Some animals with bodies denser than sea water, such as octopi or lobsters,
live on the sea bottom and being neutrally buoyant is not a great need for them.
Another example is mackerel which has no special buoyancy mechanism. In
order to overcome its negative buoyancy, mackerel must swim continuously to
stay above the sea bed. To minimize the energy expenditure during swimming,
it has a streamlined shape with a drag coefficient, Cd, as small as 0.0043 (see
Fig. 2.25).
However, when animals swim quietly in the water column, they have to develop some mechanisms to bring them to neutral buoyancy. In general, the
buoyancy mechanisms developed by aquatic animals can be divided in three
groups: the use of gas chambers, the use of body fluids less dense than sea water, and the use of fat (Denton, 1974). Some marine fish, for example, cod, use
the first mechanism maintaining a gas space within an organ called the swim
bladder. The swim bladder volume usually amounts to about 5% of the total
volume of the animal. By varying the volume of this organ, the fish can adjust
its effective density to that of the surrounding water. It is now known that
swim bladder is used for controlling buoyancy down to 2000 m water depth.
To explore the effect of changing the swim bladder volume, consider a simple
example of a fish which, at a particular water depth, is neutrally buoyant. The
force balance is then:
(11.1)
in which V is the overall volume of the animal (inflated swim bladder included),
Pb is the density of the non-swim bladder body tissue, and a eq is the fraction of
the animal volume used by the swim bladder. The first term is the buoyancy
force and the second term represents the weight of the animal in sea water.
The necessary volume of the swim bladder (as a fraction of total animal
volume) to support the animal in the state of neutral buoyancy becomes:
Pb - Pw
aeq = Pw
(11.2)