Micro-CT observation of the final developmental stages of oocytes in white precious coral <i>Pleurocorallium konojoi</i>

Pleurocorallium konojoi (white precious coral), belonging to family Coralliidae, is harvested mainly off Kochi, Japan, but because it inhabits waters deeper than 100 m, fundamental information on its biological traits, including its reproduction, remains scarce. In particular, no oocyte data have been available for the period after June, the presumed spawning season, when gametes reach their final stage of development. In this study, we applied non-destructive micro-computed tomography (micro-CT) to a P. konojoi collected by ROV in July. A 99% ethanol-preserved branch was scanned using micro-CT. Besides externally visible oocyte, multiple siphonozooids contained spherical structures interpreted as oocytes. Twenty-five oocytes were measured at their largest sections; the mean diameter was 235.6±4.0 µm, significantly exceeding Stage III values reported previous study. Therefore, the oocytes observed here were classified as Stage IV or at the Stage III–IV transition. However, photographs taken before fixation indicated original diameters of 518.9 µm, indicating that the oocytes imaged after scanning had likely shrunk because of the preservative process, the passage of time, or partial dehydration during CT scanning. Although absolute oocyte diameters must be interpreted with caution, our findings show that micro-CT is a valuable, non-invasive tool for visualizing Coralliidae oocytes and reliably inferring their developmental stage.