In vivo X-ray computed microtomography: A novel approach to assess coral skeletal construction in the reef building coral Stylophora pistillata

Scleractinian (stony) corals build reef frameworks through calcium carbonate deposition, yet all methods for assessing skeletal growth - staining, scanning electron microscopy (SEM), or ex vivo micro computed tomography (µCT) - are limited to endpoint measurements on dead specimens. Here, we utilise in vivo X µCT to non-destructively quantify skeletal growth in the reef-building coral Stylophora pistillata for the first time. In vivo µCT was used three times over a 16-day period to study a single S. pistillata fragment, cultured at controlled pH (8.0) and temperature (26°C) conditions. This approach revealed a volume increase from 630 to 700 mm³ that can be partitioned into external vertical extension and internal lateral thickening, generating high-resolution 4D reconstructions of the evolving skeletal architecture. These measurements were consistent with established approaches but provide additional unique insights into internal growth dynamics. We demonstrate that in vivo μCT enables micron-scale monitoring of calcification processes of living stony corals, thereby representing a powerful new tool to probe coral growth dynamics in the face of rapidly changing environmental conditions.