356
11 Locomotion of Marine Animals
into the ballast tank of a submarine is different from the 'osmotic' mechanism
used in the case of cuttlefish and into the shell in the case of the Nautilus, but
this problem will not be discussed here. The cautious readers should consult
Alexander (1968) or Denton (1974) for an in-depth discussion.
Table 10.1 shows that fat and oil densities are less than that of sea water.
Some aquatic animals, for example shark Centrophorus squamosus, and deepdiving whales or dolphins, make use of low-density fat to give buoyancy. Sharks
of this genus have enormous fat reservoirs in their liver, accounting for about
20 percent of their total volume (Denton, 1974). Hence, the balance of vertical
forces for shark can be written as:
PwgV - [0.2PIgV + (1 - 0.2)PbgV] = 0, or:
(11.5)
Pw - (0.2PI + 0.8pb) = 0,
( 11.6)
in which PI is the density of fat, while Pb is the density of non-fat components
of the body. Let us assume that PI = 930 kg/m 3 , Pw = 1026 kg/m 3 , and
Pb = 1060 kg/m 3 . The effective force acting on the shark body is only -78
N /m 3 of the animal's volume. This small negative buoyancy is easy rectified
by the shark due to the high amount of the low density hydrocarbon, squalene,
in the liver's fat.
In the balance of forces given by Eq. (11.6), we tactically assumed that both
forces act along the same vertical line. However, in a resting shark the centre
of buoyancy (B) and the centre of weight (G) are shifted with respect to each
other. Thus, similarly to the case of stability of the cylinder examined in
Sect. 2.2.3, a pair of weight and buoyancy forces tends to rotate the animal in
a counterclockwise direction. This makes the shark unstable and it has to use
its oily liver to rectify the buoyancy force and improve stability.
Instability of marine organisms can also be caused by ocean waves. As was
shown in Chap. 4, dynamic pressure due to waves is transmitted from the
surface into the ocean body and is changing in time. Pressure becomes highest
under the wave crest and smallest under the wave trough. The increase in
pressure under the wave crest compresses the swim bladder and the fish starts
to sink. Under the wave trough, due to decreasing pressure, the swim bladder
expands and increasing buoyancy forces the fish to ascend. As was indicated
by Denny (1993), fish that rely on a swim bladder alone are as unstable under
waves as they are in still water.
In order to improve stability while swimming slowly near the surface, blue
marlin, swordfish, and perhaps other istiophorids, have a large swim bladder
providing sufficient buoyancy. Blue marlin, for example, have an unusual swim
bladder in the form of a thin-walled, multichambered sack extending from the
level of the first pectoral fin to the first anal fin (Block et al., 1992). However,
a large swim bladder creates swimming problems at the beginning of a descent
and at greater water depths.
11 Locomotion of Marine Animals
into the ballast tank of a submarine is different from the 'osmotic' mechanism
used in the case of cuttlefish and into the shell in the case of the Nautilus, but
this problem will not be discussed here. The cautious readers should consult
Alexander (1968) or Denton (1974) for an in-depth discussion.
Table 10.1 shows that fat and oil densities are less than that of sea water.
Some aquatic animals, for example shark Centrophorus squamosus, and deepdiving whales or dolphins, make use of low-density fat to give buoyancy. Sharks
of this genus have enormous fat reservoirs in their liver, accounting for about
20 percent of their total volume (Denton, 1974). Hence, the balance of vertical
forces for shark can be written as:
PwgV - [0.2PIgV + (1 - 0.2)PbgV] = 0, or:
(11.5)
Pw - (0.2PI + 0.8pb) = 0,
( 11.6)
in which PI is the density of fat, while Pb is the density of non-fat components
of the body. Let us assume that PI = 930 kg/m 3 , Pw = 1026 kg/m 3 , and
Pb = 1060 kg/m 3 . The effective force acting on the shark body is only -78
N /m 3 of the animal's volume. This small negative buoyancy is easy rectified
by the shark due to the high amount of the low density hydrocarbon, squalene,
in the liver's fat.
In the balance of forces given by Eq. (11.6), we tactically assumed that both
forces act along the same vertical line. However, in a resting shark the centre
of buoyancy (B) and the centre of weight (G) are shifted with respect to each
other. Thus, similarly to the case of stability of the cylinder examined in
Sect. 2.2.3, a pair of weight and buoyancy forces tends to rotate the animal in
a counterclockwise direction. This makes the shark unstable and it has to use
its oily liver to rectify the buoyancy force and improve stability.
Instability of marine organisms can also be caused by ocean waves. As was
shown in Chap. 4, dynamic pressure due to waves is transmitted from the
surface into the ocean body and is changing in time. Pressure becomes highest
under the wave crest and smallest under the wave trough. The increase in
pressure under the wave crest compresses the swim bladder and the fish starts
to sink. Under the wave trough, due to decreasing pressure, the swim bladder
expands and increasing buoyancy forces the fish to ascend. As was indicated
by Denny (1993), fish that rely on a swim bladder alone are as unstable under
waves as they are in still water.
In order to improve stability while swimming slowly near the surface, blue
marlin, swordfish, and perhaps other istiophorids, have a large swim bladder
providing sufficient buoyancy. Blue marlin, for example, have an unusual swim
bladder in the form of a thin-walled, multichambered sack extending from the
level of the first pectoral fin to the first anal fin (Block et al., 1992). However,
a large swim bladder creates swimming problems at the beginning of a descent
and at greater water depths.