The whole-colony morphology of the coral, Porites sillimaniani, varies dramatically in accordance with light intensity from plate-like shape to branching shape. To determine whether the variation is based on genetic difference or phenotypic plasticity, we conducted a field experiment by transplanting coral fragments taken from seven plate-like colonies to different light conditions. The fragments that transplanted to the high-light condition started to have branches in eight months period of the experiment, whereas those to the low-light condition remained to be flat. This result suggests that the morphological variation is the phenotypic plasticity in response to light availability. To examine the adaptive significance of this plasticity, a simple mathematical model of coral shape is constructed, in which the number of branches, their angles and lengths are morphological parameters. The circumference of the model coral is assumed to be uniformly covered with polyps which survive if its light flux is larger than a certain threshold value. The optimal morphology that can support a maximum number of viable polyps is calculated for a given light intensity. The optimal coral shape changes with light intensity, which agrees qualitatively with the observation of P. sillimaniani colony in natural habitats.
Department of Biology, Kyushu University,
Fukuoka, 812-8581 JAPAN