Introduction

Abstract

Growth and form of marine organisms inhabiting hard substrata, the “marine sessile organisms”, is characterized by a number of remarkable properties. One remarkable feature of these organisms is that many of them can be characterized as modular organisms. Modular organisms are typically built of repeated units, the modules, which might be a polyp in a coral colony or a frond in seaweeds. In most cases, the module has a distinctive form, while the growth form of the entire colony is frequently an indeterminate form. Indeterminate growth indicates that the same growth process may result in an infinite number of different realizations of the growth form. This is in contrast to unitary organisms such as vertebrates 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 qualitative and rather vague terms, such as “thinly branching”, “tree-shaped” and “irregularly branching”.