rowth and form of marine organisms inhabiting hard substrata, the
G"marine sessile organisms", ischaracterized by anumber ofremarkable
properties. One remarkable feature of these organisms is that many
ofthem can be characterizedasmodularorganisms. Modularorganisms are
typically built ofrepeated units, the modules, which might be a polyp in
a coral colony or afrond in seaweeds. In most cases, the modulehas
adistinctive form, while the growth form of the entire colony is
frequently an indeterminate form. Indeterminategrowthindicatesthatthe
same growthprocess mayresult in an infinite
numberofdifferentrealizations ofthe growthform.This isincontrast to
unitaryorganisms such asvertebrates and insects, in which a
single-celled stage develops into a well-defined, determinate structure.
In many cases the growth process in modular organisms leads to complex
shapes, which are often quite difficult to describe in words. In most of
the biological literature these forms are only described in
qualitativeand rather vague terms, such as
"thinlybranching","tree-shaped" and "irregularlybranching". Anothermajor
characteristic ofmarine sessile organisms is that there is
frequentlyastrongimpactofthe physical environmenton the growthprocess,
leading to a variety of growth forms. Growth by accumulation of modules
allows the organism to fit its shape to its environment i.e., have
plasticity. In many seaweeds, sponges, and corals, differences in
exposure to water movement cause significant changes in morphology.
Agood example of this plasticity is the Indo-Pacific stony coral
Pocillopora damicornis(Veron and Pichon 1976) shown in Plg.r.i. In very
sheltered environments, this species has a thin-branching growth form.
The growth form gradually transforms to a more compact shape when the
exposure to water movement increases.
