Figure 1

Diagrammatic summary of the morphological changes involved in Acropora settlement, metamorphosis and the initiation of calcification. (A) Initially the planula swims horizontally, well away from the bottom. (B) After it has become competent to settle it explores the substratum with its aboral end. (C) If it encounters appropriate chemical cues it rounds up into a sphere, which then flattens against the substratum. As this is happening the cells against the substratum change their morphology and begin to secrete a disc of organic material. (D) Perpendicular walls, the protosepta, are then secreted on the plate in multiples of six. (E) The protosepta become more elaborate and more are added. (F) The walls become joined laterally and grow upward, forming a crown shaped structure upon which the polyp sits. In all stages the skeleton is external to, but enveloped by, the living tissue which secretes it. By the stages shown in (E) and (F) a polyp has formed, initially with tentacle buds and then with tentacles. However, these have been omitted from the drawing in order to focus on the skeleton. The drawings are based on drawings in Goreau and Hayes [34].