The vulnerability of stony coral to environmental ocean acidification (OA) is of paramount concern in light of the imminent climate changes. We are studying the spatial distributions and arrangements of the CaCO3 crystalline phases within the early-formed coral polyps, for which sub-micrometer X-ray beams (< 100 nm) are essential. Previous FIB-SEM measurements on lamella containing rapid-accretion and thickening deposits lacked nanoscale tomographic imaging, preventing us from resolving the structures of larva grown under normal (pH 8.2) and acidic (pH 7.6) conditions. ID16B combines the possibility to collect nanoCT of our FIB lamella and perform XRD and XRF mapping, to identify Sr rich domains, and to map calcite within the aragonite-rich crystal domains. Lab-grown samples under normal and simulated OA conditions will reveal at ID16B the nano- and microstructural adaptations of stony corals, possibly revealing paths to sustain calcification in future acid-affected ocean environments.