11.3 Mechanics of Animal Swimming
357
Some aquatic animals control their buoyancy using a tissue fluid, less dense
than sea water, instead of fat as described above for the case of the shark.
Pelagic ocean squid replace some of the sodium ions by ammonium. Many
gelatinous planktonic animals, such as jellyfish, comb jellies, sea elephants and
sea butterflies keep body fluids of density lower than that of the surrounding
water by replacement of sulfate by chloride ions. Fish eggs often have tissue
fluids with a content of salt less than half of that in sea water. This allows
them to float close to surface and be readily dispersed by waves and currents.
The biochemical processes involved in ion transformation and preservation of
less dense fluid in animal tissues are complicated, and their details are beyond
the scope of this book.
11.3 Mechanics of Animal Swimming
11.3.1 Introduction
Animal swimming is a difficult subject for analysis due to the great morphological diversity in equipment used for swimming. Moreover, the dynamic processes
which are involved in transmission of forces between fish and water are very
complicated and not yet fully understood. We will start an examination of fish
swimming with classification of the swimming modes.
In each swimming mode, different swimming apparatus is involved. Efficient
swimming requires that most of the energy expended by the swimming muscles
be turned into appropriate motions of the propulsive surfaces with minimum
energy loss through elastic deformation and as little as possible kinetic energy
left in the water. The majority of fish species swim by undulation of body
and tail, powered by the lateral musculature, while other species use paired
or unpaired fins moved by intrinsic muscles. The complex architecture of fish
muscles is not treated here.
During steady fish swimming, tail movements are rhythmic and the distance
forward covered for every stroke is usually constant. Therefore, the speed is
determined by the stride frequency. Usually fish do not swim in a steady
fashion, but instead rapid starts and swimming with bursts of activity are
usually followed by coasting with a straight body.
A sound knowledge of the kinematics of swimming is fundamental for the
formulation of hydro dynamical models from which such quantities as thrust,
power and efficiency can be calculated. In the 1970s, hydrodynamicists developed reactive theories of swimming based on the inertial forces generated by the
propulsive surfaces of fish in a perfect fluid (for example, Lighthill, 1969, 1971,
1975; Wu, 1971). Basic results of the theory are described here for straight
forward swimming.
In the world of ciliary and flagellar propulsors, viscous shearing is the sole
mechanism available for generating thrust. For example, protozoan flagella are
small (0.2 /Lm in diameter) and swim slowly (10-10 4 /Lms- I ) with the corresponding Reynolds number, Re, being of the order of 10- 6 (10- 3 ). Locomotion
357
Some aquatic animals control their buoyancy using a tissue fluid, less dense
than sea water, instead of fat as described above for the case of the shark.
Pelagic ocean squid replace some of the sodium ions by ammonium. Many
gelatinous planktonic animals, such as jellyfish, comb jellies, sea elephants and
sea butterflies keep body fluids of density lower than that of the surrounding
water by replacement of sulfate by chloride ions. Fish eggs often have tissue
fluids with a content of salt less than half of that in sea water. This allows
them to float close to surface and be readily dispersed by waves and currents.
The biochemical processes involved in ion transformation and preservation of
less dense fluid in animal tissues are complicated, and their details are beyond
the scope of this book.
11.3 Mechanics of Animal Swimming
11.3.1 Introduction
Animal swimming is a difficult subject for analysis due to the great morphological diversity in equipment used for swimming. Moreover, the dynamic processes
which are involved in transmission of forces between fish and water are very
complicated and not yet fully understood. We will start an examination of fish
swimming with classification of the swimming modes.
In each swimming mode, different swimming apparatus is involved. Efficient
swimming requires that most of the energy expended by the swimming muscles
be turned into appropriate motions of the propulsive surfaces with minimum
energy loss through elastic deformation and as little as possible kinetic energy
left in the water. The majority of fish species swim by undulation of body
and tail, powered by the lateral musculature, while other species use paired
or unpaired fins moved by intrinsic muscles. The complex architecture of fish
muscles is not treated here.
During steady fish swimming, tail movements are rhythmic and the distance
forward covered for every stroke is usually constant. Therefore, the speed is
determined by the stride frequency. Usually fish do not swim in a steady
fashion, but instead rapid starts and swimming with bursts of activity are
usually followed by coasting with a straight body.
A sound knowledge of the kinematics of swimming is fundamental for the
formulation of hydro dynamical models from which such quantities as thrust,
power and efficiency can be calculated. In the 1970s, hydrodynamicists developed reactive theories of swimming based on the inertial forces generated by the
propulsive surfaces of fish in a perfect fluid (for example, Lighthill, 1969, 1971,
1975; Wu, 1971). Basic results of the theory are described here for straight
forward swimming.
In the world of ciliary and flagellar propulsors, viscous shearing is the sole
mechanism available for generating thrust. For example, protozoan flagella are
small (0.2 /Lm in diameter) and swim slowly (10-10 4 /Lms- I ) with the corresponding Reynolds number, Re, being of the order of 10- 6 (10- 3 ). Locomotion