10.3. The Density of Marine Organisms
351
Fibres such as in mesogloea of M etridium belong to the class of materials
known as composites. The theory of these materials is beyond the scope of this
book and the interested reader should consult Wainwright et al. (1976) and
Vincent (1982) for an in-depth discussion.
Rubbers. Rubbers or elastomers are materials which function in many biological systems when being deformed. They have a low modulus, but can be
substantially deformed without breaking. They often show reversible elastic
properties, similar to those of rubber. The rubber maintains a linear relationship between stress and strain up to very large strains.
Biological rubber type materials, such as resilin, abductin and elastin, are
mostly proteins organized in cross-linked polymer chains. Generally, rubber
type materials and protein rubbers have Young's moduli about 1 MPa. They
are not stiff enough to be primary structural elements in organism skeletons and
their main use is to be energy storage devices. For example, resilin provides an
elastic mechanism that stores the kinetic energy of the moving wings of insects
and helps to decelerate the wing at the end of its stroke and then accelerate
it in the opposite direction (Wainwright et al., 1976). However, not all energy
put into deformation is recoverable. The ratio of energy recovered to energy
put in is the resilience. Resilin has resilience as high as 96-97%, which is as
good as, or better than very high quality synthetic rubbers (Vincent, 1982).
Another type of protein rubber, abductin, has been found in the inner hinge
ligament of the bivalve mollusc shell. Swimming bivalves open and close their
shells several times per second and require an efficient hinge ligament and high
resilience level. The resilience level is lower (about 80%) for digging bivalves
which do not move their shells so often. The abductin in the inner ligament
of scallops is compressed when the shell is closed and its compressed material
provides the force to reopen the shell. During shell closing, these animals
also expel water, creating a thrust to escape from predators. We will consider
locomotion by jet propulsion in the next chapter.
10.3 The Density of Marine Organisms
In Sect. 1.2, we have defined the density of fluid and provided a computer
program (Dll) to calculate density dependent on water temperature, T, and
salinity, S. Now, we will examine the density of organisms living in the ocean.
The previous discussion of forces imposed on marine animals by flowing water,
or drag induced by moving, showed that both densities should be taken into
account. More important than the absolute densities of both the organism
and water is the difference between these two densities. For example, a fish
can hang motionless above the sea bed without moving a fin, by bringing its
specific weight close to that of the sea. At the same time, a hovering bird must
frantically beat its wings to stay aloft.
As was shown in Sect. 1.2, the density of sea water, Pw, is very close to 1026
kg/m 3 . For example, for salinity S = 35 ppm and temperature T = 10°, the
351
Fibres such as in mesogloea of M etridium belong to the class of materials
known as composites. The theory of these materials is beyond the scope of this
book and the interested reader should consult Wainwright et al. (1976) and
Vincent (1982) for an in-depth discussion.
Rubbers. Rubbers or elastomers are materials which function in many biological systems when being deformed. They have a low modulus, but can be
substantially deformed without breaking. They often show reversible elastic
properties, similar to those of rubber. The rubber maintains a linear relationship between stress and strain up to very large strains.
Biological rubber type materials, such as resilin, abductin and elastin, are
mostly proteins organized in cross-linked polymer chains. Generally, rubber
type materials and protein rubbers have Young's moduli about 1 MPa. They
are not stiff enough to be primary structural elements in organism skeletons and
their main use is to be energy storage devices. For example, resilin provides an
elastic mechanism that stores the kinetic energy of the moving wings of insects
and helps to decelerate the wing at the end of its stroke and then accelerate
it in the opposite direction (Wainwright et al., 1976). However, not all energy
put into deformation is recoverable. The ratio of energy recovered to energy
put in is the resilience. Resilin has resilience as high as 96-97%, which is as
good as, or better than very high quality synthetic rubbers (Vincent, 1982).
Another type of protein rubber, abductin, has been found in the inner hinge
ligament of the bivalve mollusc shell. Swimming bivalves open and close their
shells several times per second and require an efficient hinge ligament and high
resilience level. The resilience level is lower (about 80%) for digging bivalves
which do not move their shells so often. The abductin in the inner ligament
of scallops is compressed when the shell is closed and its compressed material
provides the force to reopen the shell. During shell closing, these animals
also expel water, creating a thrust to escape from predators. We will consider
locomotion by jet propulsion in the next chapter.
10.3 The Density of Marine Organisms
In Sect. 1.2, we have defined the density of fluid and provided a computer
program (Dll) to calculate density dependent on water temperature, T, and
salinity, S. Now, we will examine the density of organisms living in the ocean.
The previous discussion of forces imposed on marine animals by flowing water,
or drag induced by moving, showed that both densities should be taken into
account. More important than the absolute densities of both the organism
and water is the difference between these two densities. For example, a fish
can hang motionless above the sea bed without moving a fin, by bringing its
specific weight close to that of the sea. At the same time, a hovering bird must
frantically beat its wings to stay aloft.
As was shown in Sect. 1.2, the density of sea water, Pw, is very close to 1026
kg/m 3 . For example, for salinity S = 35 ppm and temperature T = 10°, the