Japanese red coral, Paracorallium japoniucm is a member of precious corals, living in deep waters at depths of more than 100 m. Therefore, the specifics of its biology are little understood, especially about its reproduction such as which parts of the body it develops reproductive organs and when it breeds.　
　What we see as a red coral is a colony consisting of many polyps. A thin film-like membrane covering the axis is actually the living tissues of coral. There are two kinds of polyps in its tissues. One is particularly visible with its bumped shape and tentacles (Fig. 1). However, the other is smaller and has no tentacles or no bumps so this polyp is hard to spot. There are numerous tiny sclerites in coral tissues and therefore it is not easy to see the inside.

　Sclerites and axes can be removed with a chemical treatment, which provided a clear inside view.
　When the tissues of red coral were examined carefully after the chemical treatment, it was found that some colonies have oocytes (Fig. 2) and that others have sperm cysts (Fig. 3). Oocytes and sperm cysts grew in different colonies and there were no colonies with both oocytes and sperm cysts. This indicates that red coral is gonochoric.
　Oocytes and sperm cysts in tissues were observed in cross section, which showed that both of them develop inside the polyps with no tentacles.
　Japanese red corals have been collected off Tosa Bay, Kochi Prefecture at least once a month for 2 years. Most of those with oocytes or sperm cysts were caught in April, May and June. During summer, specifically after late July, few such corals were found. From this, it can be presumed that red corals off Tosa Bay attain maturity from spring to early summer and that they release oocytes and sperms all at once. （Kazuo Okuda of Kochi University）