- We studied the effect of light availability on the skeletal aragonite microstructure of the reef-building coral Fungia simplex. Scanning electron microscopy (SEM) of samples transplanted from light to shade conditions showed that the latter promoted a well-defined crystal alignment and dense packing. An X-ray diffraction (XRD) study of newly formed coral skeleton tissue showed a twofold increase in coherence length along the c-axis under shade conditions, while coherence length along the perpendicular direction remained essentially unchanged. These findings indicate that under shade conditions controlled calcification occurred by extension of the crystalline domains along the c-axis of the prismatic aragonite crystals. The reduced calcification rate under shade conditions was in keeping with the lower algal densities and the higher chlorophyll a content of the zooxanthellae that mediate the calcification process.