Growth forms of hermatypic corals: stable states and noise-induced transitions

Abstract

Field observations show that some coral species may assume different growth forms, or morphotypes, within apparently uniform habitats. The present paper examines the causes of this morphological diversity in hermatypic cnidarians and more particularly the following questions: Are there stable morphotypes among all the possible growth forms of a coral; if so, can a colony belonging to a certain stable morphotype change its growth form, and what can induce transition from type to type when the environment is relatively steady and only limited random (and short-term) variations of the external conditions occur? The approach adopted was to construct a mathematical model of the form dynamics of a coral colony and validate it by field data on the Red Sea hydrocoral Millepora dichotoma. The ratio between the volume and the surface area of a coral served as an index of its form and enabled treatment of the coral morphology in terms of dynamical systems. Such a dynamical system must have certain stable states corresponding to specific growth forms of a coral colony. Computer simulations showed that limited stochastic disturbances in the processes of biomass and skeletal growth caused by random fluctuations in environmental conditions may induce transitions of a coral from one stable growth form to another.