2.1. THE PHYSICAL ENVIRONMENT
resist high inertial loads in wave-swept environments can have a significant
effect on their likelihood of experiencing large forces (Denny et al. 1998).
While some sessile organisms (such as stony corals) are very stiff, and
some (such as thin blade-like algae) are very flexible, others (such as sea
fans, Fig. 2.1, or stipitate kelp) are of intermediate stiffness. Measurements
of hydrodynamic forces experienced in a wave tank by models of organisms of the same blade-like shape, but different flexural stiffnesses, showed
that as the stiff models "grew" the peak force increased, whereas as the
very flexible models "grew" the force remained low (Koehl 2000). A third
type of size-dependent behavior was shown by the models of intermediate
stiffness : lengthening increased hydrodynamic forces on short models, had
no effect on models of intermediate length, and decreased forces on long
models . Since the deflection of the free end of an organism attached to the
substratum (like a cantilever) depends on length", the longer the models of
intermediate stiffness become, the more they bend over and go with the flow
(Koehl 2000) .
CONSEQUENCES OF HYDRODYNAMIC FORCES AS ORGANISMS GROW. Hydrodynamic forces can deform sessile organisms (which can in turn affect
performance of functions such as light or food interception), and can break
or rip them off the substratum. How much the tissues in a body deform, and
whether or not they break, depends not only on the stiffness and strength
of the tissues, but also on the stresses to which they are subjected (stress is
the force imposed on a material divided by the cross-sectional area of material bearing that force). The stresses within an organism's body when being
stretched, bent, or compressed by a hydrodynamic force can be calculated
using the same techniques engineers use to calculate the stresses in manmade structures (for details, see e.g. Roark and Young1975, Wainwright et al.
1976). Such analyses reveal that , not only do the shape and size of attached
organisms determine the magnitude of the hydrodynamic forces which they
experience when exposed to ambient currents, but shape and size also determine the distribution and magnitude of stresses within their bodies when
bearing those forces (e.g. Koehl 1977b, 1986).
Whether organisms grow geometrically (i.e. maintain the same shape
as they get bigger) or allometrically (i.e, change shape as they grow) determines whether or not the local stresses in their tissues increase, decrease ,
or stay the same during their ontogeny. Although the scaling of the proportions of terrestrial and locomoting organisms of different sizes has received
much attention, the scaling of attached sessile organisms is less well studied
(Denny 1988, 1999, Johnson and Koehl 1994, Koehl 2000). The "safety factor" of a structure is the ratio of the strength of the material composing it to
the maximum stress it experiences during its lifetime . Since ambient water
flows on sessile organisms often vary with season and since the size, shape,
and mechanical properties of their tissues can change with age, the "environmental stress factor" is a biological version of safety factor that relates the
ability of organisms at their particular stages in ontogeny to resist breakage
relative to the maximum loads that they experience in nature at those stages.
For example, the giant kelp Nereocystis luetkeana adjusted their shapes and
material properties as they grew in different hydrodynamic habitats in such
a way that the "environmental stress factor" was the same in all the habitats
and was maintained as the kelp grew during the summer months, but de21
resist high inertial loads in wave-swept environments can have a significant
effect on their likelihood of experiencing large forces (Denny et al. 1998).
While some sessile organisms (such as stony corals) are very stiff, and
some (such as thin blade-like algae) are very flexible, others (such as sea
fans, Fig. 2.1, or stipitate kelp) are of intermediate stiffness. Measurements
of hydrodynamic forces experienced in a wave tank by models of organisms of the same blade-like shape, but different flexural stiffnesses, showed
that as the stiff models "grew" the peak force increased, whereas as the
very flexible models "grew" the force remained low (Koehl 2000). A third
type of size-dependent behavior was shown by the models of intermediate
stiffness : lengthening increased hydrodynamic forces on short models, had
no effect on models of intermediate length, and decreased forces on long
models . Since the deflection of the free end of an organism attached to the
substratum (like a cantilever) depends on length", the longer the models of
intermediate stiffness become, the more they bend over and go with the flow
(Koehl 2000) .
CONSEQUENCES OF HYDRODYNAMIC FORCES AS ORGANISMS GROW. Hydrodynamic forces can deform sessile organisms (which can in turn affect
performance of functions such as light or food interception), and can break
or rip them off the substratum. How much the tissues in a body deform, and
whether or not they break, depends not only on the stiffness and strength
of the tissues, but also on the stresses to which they are subjected (stress is
the force imposed on a material divided by the cross-sectional area of material bearing that force). The stresses within an organism's body when being
stretched, bent, or compressed by a hydrodynamic force can be calculated
using the same techniques engineers use to calculate the stresses in manmade structures (for details, see e.g. Roark and Young1975, Wainwright et al.
1976). Such analyses reveal that , not only do the shape and size of attached
organisms determine the magnitude of the hydrodynamic forces which they
experience when exposed to ambient currents, but shape and size also determine the distribution and magnitude of stresses within their bodies when
bearing those forces (e.g. Koehl 1977b, 1986).
Whether organisms grow geometrically (i.e. maintain the same shape
as they get bigger) or allometrically (i.e, change shape as they grow) determines whether or not the local stresses in their tissues increase, decrease ,
or stay the same during their ontogeny. Although the scaling of the proportions of terrestrial and locomoting organisms of different sizes has received
much attention, the scaling of attached sessile organisms is less well studied
(Denny 1988, 1999, Johnson and Koehl 1994, Koehl 2000). The "safety factor" of a structure is the ratio of the strength of the material composing it to
the maximum stress it experiences during its lifetime . Since ambient water
flows on sessile organisms often vary with season and since the size, shape,
and mechanical properties of their tissues can change with age, the "environmental stress factor" is a biological version of safety factor that relates the
ability of organisms at their particular stages in ontogeny to resist breakage
relative to the maximum loads that they experience in nature at those stages.
For example, the giant kelp Nereocystis luetkeana adjusted their shapes and
material properties as they grew in different hydrodynamic habitats in such
a way that the "environmental stress factor" was the same in all the habitats
and was maintained as the kelp grew during the summer months, but de21