X-ray micro-CT observation of the apical skeleton of Japanese white coral Corallium konojoi

• Corallium konojoi was three-dimensionally visualized by a synchrotron X-ray micro-CT.
• Living tissues were identified using tomographic images of a high edge-contrast.
• Skeletal morphologies reflected growth mechanism of axial skeleton.

Precious corals within Japanese waters have long been commercially traded as valuable marine resources. To manage and recover coral resources toward sustainable use, the accumulation of scientific knowledge including coral biology and ecology is essential. This study aimed to obtain biological and structural information on the white coral Corallium konojoi, a Japanese precious coral, using a synchrotron X-ray micro-computed tomography. Tomographic images of a high spatial resolution and edge-contrast showed morphological features of not only the skeletal structures, axial skeleton, and sclerites but also of living tissue, leading to the identification of polyps (autozooids and siphonozooids) and gastrovascular canals. A coenenchyme with a multilayered structure of the dispersed sclerites and the gastrovascular canals were also observed. External and internal forms of the axial skeleton suggested that skeleton formation at an apical part was in combination with incorporations of the sclerites and a biomineralization through a skeletogenic epithelial layer surrounding the axial skeleton.