66
2. ENVIRONMENTALLY DRIVEN PLASTICITY
all branches of a specific colony, independent of the age of the polyps (Rinkevich and Loya 1979; Fig. 2-40). The only case where this synchronization in
reproductive activities was distorted has been documented in regenerative
colonies (Rinkevich and Loya1989). In old, senescent colonies, reproductive
activity was decreased (as calcification rates) , synchronically, in all branches.
New and old polyps exhibited senescence simultaneously leading to complete
tissue mortality (Rinkevich and Loya 1986, Fig. 2-40).
Although the above S. pistillata's life history characterizations (Figs. 2.39
and 2.40) are probably genetically controlled, they were clearly affected by
environmental and a variety of biological challenges. Colony architecture is
probably a character shaped by selection, so trade-offs between that trait
and other traits may define a suite of morphological responses, some influenced by genetics, others by epigenetic impacts. In S.pistillata, a developing
colony responds to perturbations by "canalysing" (Waddington 1942)growth
pattern to the typical species morphology. This was critically illuminated by
Loya (1976) who demonstrated that in harmed S. pistillata colonies, where the
spherical-like structure is lost through partial branch breakage, the pattern
formation is maintained by fast growth in regenerating parts as opposed to
reduction in growth in the intact branches, until the colony regains its former
shape (Fig. 2.40). Photosynthetic products are channeled to the regenerating
parts (Rinkevich and Weissman 1987) and as a result of the whole colony investment, the synchronization in reproductive activities (Rinkevich and Loya
1979) is lost (Rinkevich and Loya 1989). Changes in the colony morphology
however, may be induced by intraspecific interactions (Rinkevich and Loya
1985a, Frank et al. 1997), by the employment of intracolonial morphological rules , such as the "no fusions between branches" concept (Rinkevich and
Loya 1985a), or by biological interferences, such as symbiotic, mutualistic
and parasitic interactions (Abelson et al. 1991).
Even a single separated polyp from as. pistillata colony has the capacity
to survive and develop a new colony. Therefore, as implied by the outcomes
presented in this section, a single colony may be regarded as a "whole", where
intrinsic orders of a branch's growth and related physiological parameters
(such as sharing of resources between different branches, simultaneous aging processes, and reproductive activities) "produce" the structure we are
familiar with .
More than two decades of studies on different life history traits of S. pistillata have resulted in much biological information on a variety of aspects
that can be of help when analyzing colony architecture. Flexibility and variations in colony formation can be controlled by specific genetic rules, as can
the general sphere-like structure of the colony and the interactive processes of
branching. Although there is as yet no direct evidence for that proposal, phenotypic plasticity in the colonial organism, which may also be constructed
by genetic rules for morphologies (Zilberberg and Edmunds 1999), should
be seriously taken into consideration when studying marine invertebrate
branching forms.
2. ENVIRONMENTALLY DRIVEN PLASTICITY
all branches of a specific colony, independent of the age of the polyps (Rinkevich and Loya 1979; Fig. 2-40). The only case where this synchronization in
reproductive activities was distorted has been documented in regenerative
colonies (Rinkevich and Loya1989). In old, senescent colonies, reproductive
activity was decreased (as calcification rates) , synchronically, in all branches.
New and old polyps exhibited senescence simultaneously leading to complete
tissue mortality (Rinkevich and Loya 1986, Fig. 2-40).
Although the above S. pistillata's life history characterizations (Figs. 2.39
and 2.40) are probably genetically controlled, they were clearly affected by
environmental and a variety of biological challenges. Colony architecture is
probably a character shaped by selection, so trade-offs between that trait
and other traits may define a suite of morphological responses, some influenced by genetics, others by epigenetic impacts. In S.pistillata, a developing
colony responds to perturbations by "canalysing" (Waddington 1942)growth
pattern to the typical species morphology. This was critically illuminated by
Loya (1976) who demonstrated that in harmed S. pistillata colonies, where the
spherical-like structure is lost through partial branch breakage, the pattern
formation is maintained by fast growth in regenerating parts as opposed to
reduction in growth in the intact branches, until the colony regains its former
shape (Fig. 2.40). Photosynthetic products are channeled to the regenerating
parts (Rinkevich and Weissman 1987) and as a result of the whole colony investment, the synchronization in reproductive activities (Rinkevich and Loya
1979) is lost (Rinkevich and Loya 1989). Changes in the colony morphology
however, may be induced by intraspecific interactions (Rinkevich and Loya
1985a, Frank et al. 1997), by the employment of intracolonial morphological rules , such as the "no fusions between branches" concept (Rinkevich and
Loya 1985a), or by biological interferences, such as symbiotic, mutualistic
and parasitic interactions (Abelson et al. 1991).
Even a single separated polyp from as. pistillata colony has the capacity
to survive and develop a new colony. Therefore, as implied by the outcomes
presented in this section, a single colony may be regarded as a "whole", where
intrinsic orders of a branch's growth and related physiological parameters
(such as sharing of resources between different branches, simultaneous aging processes, and reproductive activities) "produce" the structure we are
familiar with .
More than two decades of studies on different life history traits of S. pistillata have resulted in much biological information on a variety of aspects
that can be of help when analyzing colony architecture. Flexibility and variations in colony formation can be controlled by specific genetic rules, as can
the general sphere-like structure of the colony and the interactive processes of
branching. Although there is as yet no direct evidence for that proposal, phenotypic plasticity in the colonial organism, which may also be constructed
by genetic rules for morphologies (Zilberberg and Edmunds 1999), should
be seriously taken into consideration when studying marine invertebrate
branching forms.