11 Locomotion of Marine Animals
11.1 Introduction
In Part 1 we discussed all kinds of forces acting on a body submerged in flowing
water and forces induced by the surrounding fluid on a body moving in calm
water. A body can move only when sufficient energy is available to overcome
resistance forces. The availability of energy denotes the work that has to be
done by a body to overcome its drag. Organisms may either oppose drag by
being rigid or they may avoid excessive drag by being flexible and bending
with the flow, thus reducing the area perpendicular to flow. When the force,
or thrust, by which the body tends to move itself is larger than the sum of the
resistance forces, then the body can achieve an acceleration which is inversely
proportional to the body's mass. This is essentially Newton's second law of
motion. According to Newton's first law, when the body thrust is equal to its
drag, the body moves at a constant speed for as long as the thrust is maintained.
There are many ways in which animals move through the ocean. We begin
our discussion with the simplest mode of motion, when aquatic animals utilize
their special properties to stay in mid-water or move up and down without any
work. This situation is closely related to the physical law of buoyancy.
Probably the most interesting locomotion mode of marine animals is swimming. Lift and drag are the mechanisms by which the necessary thrust is
produced. It is intuitively correct to expect some relationship between speed
of swimming and animal shape and size. Another way to produce thrust is
through propulsion, when some mass is forced away, similar to gases expelled
by a jet engine. To see how thrust is created by moving animal, we will examine
the physics of swimming and jet propulsion.
11.2 Buoyancy in Marine Animals
The physics of the buoyancy force has been explained in Sect. 2.2. Now we
will examine how the buoyancy phenomenon is used by aquatic organisms to
minimize their energy expenditure for transport in the water column. If the
density of an animal is exactly equal to the average density of water, then its
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999
11.1 Introduction
In Part 1 we discussed all kinds of forces acting on a body submerged in flowing
water and forces induced by the surrounding fluid on a body moving in calm
water. A body can move only when sufficient energy is available to overcome
resistance forces. The availability of energy denotes the work that has to be
done by a body to overcome its drag. Organisms may either oppose drag by
being rigid or they may avoid excessive drag by being flexible and bending
with the flow, thus reducing the area perpendicular to flow. When the force,
or thrust, by which the body tends to move itself is larger than the sum of the
resistance forces, then the body can achieve an acceleration which is inversely
proportional to the body's mass. This is essentially Newton's second law of
motion. According to Newton's first law, when the body thrust is equal to its
drag, the body moves at a constant speed for as long as the thrust is maintained.
There are many ways in which animals move through the ocean. We begin
our discussion with the simplest mode of motion, when aquatic animals utilize
their special properties to stay in mid-water or move up and down without any
work. This situation is closely related to the physical law of buoyancy.
Probably the most interesting locomotion mode of marine animals is swimming. Lift and drag are the mechanisms by which the necessary thrust is
produced. It is intuitively correct to expect some relationship between speed
of swimming and animal shape and size. Another way to produce thrust is
through propulsion, when some mass is forced away, similar to gases expelled
by a jet engine. To see how thrust is created by moving animal, we will examine
the physics of swimming and jet propulsion.
11.2 Buoyancy in Marine Animals
The physics of the buoyancy force has been explained in Sect. 2.2. Now we
will examine how the buoyancy phenomenon is used by aquatic organisms to
minimize their energy expenditure for transport in the water column. If the
density of an animal is exactly equal to the average density of water, then its
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999