50
2. ENVIRONMENTALLY DRIVEN PLASTICITY
The feedback that occurs between modular growth and the environment
creates high levels of morphological plasticity in virtually all octocorals. Although the mechanisms involved are generally unknown, responses of colony
form to currents and waveaction are well documented. Hydrodynamic forces
are particularly important among upright structures, where excessive drag
on the colony can lead to failure of either the colony (Lasker 1984) or of
the colony's attachment to the substratum. Many species have characteristic
orientations when they develop in unidirectional current/wave action (Wainwright and Dillon 1969). Sea fans, for instance, are almost always oriented
perpendicular to the primary current/wave action (Fig. 2.29 shows sea fans
growing parallel to each other). Some species such as plexaurid gorgon ians may vary from bush-like colonies where flows are slow or do not have
a preferred direction to candelabrum-like colonies in environments with unidirectional flow (Jordan and Nugent 1978). Other colony traits such as axial
core composition and stiffness also vary within species due to the level of
water movement (Lewis et al. 1992).
Form also affects the overall energetic/nutritive balance of a colony. Like
many of the bottom-dwelling taxa of coral reefs, most of the reef-dwelling
octocorals harbor symbiotic dinoflagellate algae, zooxanthellae. As in the
stony corals, the zooxanthellae probably provide much of the colony's nutrition, but quantitative estimates of that contribution are lacking. Flow by
controlling the exchange of essential inorganic nutrients such as bicarbonate
probably affects primary production across the colony as has been observed
among scleractinians (Helmuth et al. 1997). Furthermore, among densely
branched colonies, form may affect productivity via self-shading, as light
level within even loosely branched species is attenuated along the main axis
when compared with the colony edge (Goulet and Coffroth 1997).
Gorgonians and alcyonaceans have not been considered particularly effective feeders on zooplankton because of their lack of true stinging cells
(Mariscal and Bigger 1975). Some species can capture easily handled prey
(Lasker 1981), but most heterotrophic feeding may be concentrated on small,
passive particulates (Lasker 1981, Fabricius et al. 1995, Ribes et al. 1999).
Mariscal and Bigger (1975) also suggest that extensive villi on polyp surfaces may enhance the uptake of dissolved organic molecules. The relative
importance of the different modes of feeding are unknown. However, it is
clear that body form by controlling water flow through the colony will affect
colony-wide nutrition.
Encrusting forms by virtue of their veneer-like form escape many of the
surface-to-volume ratio restrictions that inevitably limit the size of unitary
(non -modular) organisms, but even among these forms size may be restricted
by the depletion of nutrients as water flows over the surface of a colony.
McFadden (1986) for instance suggested that the pattern of fission among
Alcyonium sp. allowed colonies to capture more prey than if the colony grew
as one continuous sheet. The presence of some form of environmentally
induced inhibition of growth among branching forms is suggested by the
experiments of Kim and Lasker (1997). They found that the growth of interior
branches of colonies was adversely affected by the presence of neighboring
branches, even when the adjacent branches were no longer attached to the
colony.
Flow acting through the hydrodynamic forces on colonies and through
the supply of essential nutrients to colonies generates variation in colony
2. ENVIRONMENTALLY DRIVEN PLASTICITY
The feedback that occurs between modular growth and the environment
creates high levels of morphological plasticity in virtually all octocorals. Although the mechanisms involved are generally unknown, responses of colony
form to currents and waveaction are well documented. Hydrodynamic forces
are particularly important among upright structures, where excessive drag
on the colony can lead to failure of either the colony (Lasker 1984) or of
the colony's attachment to the substratum. Many species have characteristic
orientations when they develop in unidirectional current/wave action (Wainwright and Dillon 1969). Sea fans, for instance, are almost always oriented
perpendicular to the primary current/wave action (Fig. 2.29 shows sea fans
growing parallel to each other). Some species such as plexaurid gorgon ians may vary from bush-like colonies where flows are slow or do not have
a preferred direction to candelabrum-like colonies in environments with unidirectional flow (Jordan and Nugent 1978). Other colony traits such as axial
core composition and stiffness also vary within species due to the level of
water movement (Lewis et al. 1992).
Form also affects the overall energetic/nutritive balance of a colony. Like
many of the bottom-dwelling taxa of coral reefs, most of the reef-dwelling
octocorals harbor symbiotic dinoflagellate algae, zooxanthellae. As in the
stony corals, the zooxanthellae probably provide much of the colony's nutrition, but quantitative estimates of that contribution are lacking. Flow by
controlling the exchange of essential inorganic nutrients such as bicarbonate
probably affects primary production across the colony as has been observed
among scleractinians (Helmuth et al. 1997). Furthermore, among densely
branched colonies, form may affect productivity via self-shading, as light
level within even loosely branched species is attenuated along the main axis
when compared with the colony edge (Goulet and Coffroth 1997).
Gorgonians and alcyonaceans have not been considered particularly effective feeders on zooplankton because of their lack of true stinging cells
(Mariscal and Bigger 1975). Some species can capture easily handled prey
(Lasker 1981), but most heterotrophic feeding may be concentrated on small,
passive particulates (Lasker 1981, Fabricius et al. 1995, Ribes et al. 1999).
Mariscal and Bigger (1975) also suggest that extensive villi on polyp surfaces may enhance the uptake of dissolved organic molecules. The relative
importance of the different modes of feeding are unknown. However, it is
clear that body form by controlling water flow through the colony will affect
colony-wide nutrition.
Encrusting forms by virtue of their veneer-like form escape many of the
surface-to-volume ratio restrictions that inevitably limit the size of unitary
(non -modular) organisms, but even among these forms size may be restricted
by the depletion of nutrients as water flows over the surface of a colony.
McFadden (1986) for instance suggested that the pattern of fission among
Alcyonium sp. allowed colonies to capture more prey than if the colony grew
as one continuous sheet. The presence of some form of environmentally
induced inhibition of growth among branching forms is suggested by the
experiments of Kim and Lasker (1997). They found that the growth of interior
branches of colonies was adversely affected by the presence of neighboring
branches, even when the adjacent branches were no longer attached to the
colony.
Flow acting through the hydrodynamic forces on colonies and through
the supply of essential nutrients to colonies generates variation in colony