48
2. ENVIRONMENTALLY DRIVEN PLASTICITY
Morphological plasticity enables rapid, adaptive responses to local environmental variation, such as supplies of space and food, and the adaptive
significance of this plasticity has long been recognized. The physiological
processes that regulate plasticity of colony form and function are now beginning to be understood largely due to the work by Dr. Neil Blackstone.
In hydractiniids, the transport of gastrovascular fluid interacts with redox
chemistry to generate an adaptive morphogenetic response (Blackstone and
Buss 1992, Blackstone 1996, 1997, 1998, 1999, Dudgeon and Buss 1996) Low
rates of flow in the gastrovascular system or a relative oxidation state of cells
in a colony typically increase stolon branching and hydranth production,
generating sheet growth (Blackstone 1998, 1999) . In contrast, high rates of
flow or a relative reduction state of cells reduce hydra nth and stolon branch
formation, thereby generating runner growth. Redox gradients arise within
a colony associated with the pumping activity of polyps and the availability
of metabolic substrate (Blackstone 1999). Factors regulating the production
of other polyp types (gonozooids, dactylozooids, and tentaculozooids) are
presently unknown.
Environments occupied by hydrozoans commonly vary in the availability of space, frequency of competitive encounters, and food supply. This
environmental information is translated by polyp and stolon behaviors into
emergent patterns of gastrovascular flow and redox gradients that specify local "physiological states" within the colony. These physiological states
regulate adaptive plastic morphogenetic responses. Colony form itself in fluences characteristics of gastrovascular flow and may affect redox chemistry
by influencing the pumping behavior of polyps. In this way, the architecture of a colony determines its future morphological trajectory. Colony
form , gastrovascular transport and redox chemistry are clearly inextricably
linked.
OCTOCORALS: MODULAR GROWTH IN Two AND THREE DIMENSIONS. The
two most abundant octo coral groups are the alcyonaceans, which reach their
greatest abundance and diversity on Indo-Pacific coral reefs, and the gorgonians, which are most common on Caribbean coral reefs (Fig. 2.29 shows
a montage of two species) . Although they range in form from delicate branching structures to massive forms, octo corals all share a simple body plan of
individual polyps embedded in a matrix of tissue, the coenenchyme. Colony
form is maintained by some combination of support from calcium carbonate
sclerites embedded in the coenenchyme, hydrostatic forces created by pumping of water into the polyps, and among the gorgonians a proteinaceous axial
skeleton . In octo corals colony form is controlled by a feedback that occurs
between growth and the environment. Among octo corals the end product of
that interaction ranges from lacy forms that are restricted to protected waters to massive colonies that blanket tens of square meters and can withstand
heavy surf.
The expansion of encrusting and nodular colonies typically occurs
through the production of polyps along the colony margin. This gives colonies
the potential of continuous expansion and monopolization of space. However, among most species this process is functionally inhibited by grazing
and disturbance, and by allometric factors that may constrain resource supply. A solution to allometric constraints among some of these species is the
formation of new colonies from fragments of colonies. In some cases the
2. ENVIRONMENTALLY DRIVEN PLASTICITY
Morphological plasticity enables rapid, adaptive responses to local environmental variation, such as supplies of space and food, and the adaptive
significance of this plasticity has long been recognized. The physiological
processes that regulate plasticity of colony form and function are now beginning to be understood largely due to the work by Dr. Neil Blackstone.
In hydractiniids, the transport of gastrovascular fluid interacts with redox
chemistry to generate an adaptive morphogenetic response (Blackstone and
Buss 1992, Blackstone 1996, 1997, 1998, 1999, Dudgeon and Buss 1996) Low
rates of flow in the gastrovascular system or a relative oxidation state of cells
in a colony typically increase stolon branching and hydranth production,
generating sheet growth (Blackstone 1998, 1999) . In contrast, high rates of
flow or a relative reduction state of cells reduce hydra nth and stolon branch
formation, thereby generating runner growth. Redox gradients arise within
a colony associated with the pumping activity of polyps and the availability
of metabolic substrate (Blackstone 1999). Factors regulating the production
of other polyp types (gonozooids, dactylozooids, and tentaculozooids) are
presently unknown.
Environments occupied by hydrozoans commonly vary in the availability of space, frequency of competitive encounters, and food supply. This
environmental information is translated by polyp and stolon behaviors into
emergent patterns of gastrovascular flow and redox gradients that specify local "physiological states" within the colony. These physiological states
regulate adaptive plastic morphogenetic responses. Colony form itself in fluences characteristics of gastrovascular flow and may affect redox chemistry
by influencing the pumping behavior of polyps. In this way, the architecture of a colony determines its future morphological trajectory. Colony
form , gastrovascular transport and redox chemistry are clearly inextricably
linked.
OCTOCORALS: MODULAR GROWTH IN Two AND THREE DIMENSIONS. The
two most abundant octo coral groups are the alcyonaceans, which reach their
greatest abundance and diversity on Indo-Pacific coral reefs, and the gorgonians, which are most common on Caribbean coral reefs (Fig. 2.29 shows
a montage of two species) . Although they range in form from delicate branching structures to massive forms, octo corals all share a simple body plan of
individual polyps embedded in a matrix of tissue, the coenenchyme. Colony
form is maintained by some combination of support from calcium carbonate
sclerites embedded in the coenenchyme, hydrostatic forces created by pumping of water into the polyps, and among the gorgonians a proteinaceous axial
skeleton . In octo corals colony form is controlled by a feedback that occurs
between growth and the environment. Among octo corals the end product of
that interaction ranges from lacy forms that are restricted to protected waters to massive colonies that blanket tens of square meters and can withstand
heavy surf.
The expansion of encrusting and nodular colonies typically occurs
through the production of polyps along the colony margin. This gives colonies
the potential of continuous expansion and monopolization of space. However, among most species this process is functionally inhibited by grazing
and disturbance, and by allometric factors that may constrain resource supply. A solution to allometric constraints among some of these species is the
formation of new colonies from fragments of colonies. In some cases the