Abstract
We quantified the phenotypic variation in weight and architectural parameters of fan-like colonies of the gorgonian Eunicella cavolini. For the quantification of the morphological plasticity with fan size, we developed a method for the precise determination of colony size with fan-like growth forms. The cortex mineralization as well as the percentage by weight of the architectural parameters axis, sclerites and organic content increased towards the base of the colonies. Compared to the center the periphery of colonies is characterized by longer, thinner and less ramified branches and by a higher distance between branches (branch distance). With increasing colony size the branch distance increases in the periphery and decreases in the center. This results in more or less constant branch distances (and thus porosity values) at any given fan size. When colonies are getting larger, the end branches in the periphery get longer and less ramified, whereas in the center the branch length increases and ramification density remains constant. Generally, branches become thicker, longer and less ramified, and increase in width becomes more important as colonies increase in size. Because flow velocity increases with the distance from the substratum, the variation in architectural parameters in different regions of a colony and in colonies of different size is probably a morphological adaptation to reduce drag forces. This study corroborates data from the literature showing that colonies from extremely sheltered and very exposed habitats have very porous branching networks with long branches and few ramifications compared to colonies from intermediate habitats, which are characterized by strongly ramified branches with low porosity values. Differential growth and ramification rates were shown to be the major reasons for the variations of growth forms in relation to colony region and colony size. The intensity of water movement is discussed as a potential force influencing growth and ramification rates.
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