Sexual Reproduction in Precious Corals (Coralliidae) Collected in the Ryukyu Archipelago

Little is known about the basic life history of most species of precious corals (Octocorallia: Coralliidae). Three commercially valuable precious coral species (Paracorallium japonicum, Corallium elatius, and C. konojoi) were sampled from the Ryukyu Archipelago to study their reproductive biology. To determine features of their gamete differentiation, samples were thin-sectioned for examination with a digital light microscope, and diameters of sperm sacs and oocytes were measured. Sexual reproduction strategy in all three species was determined to be gonochoristic broadcast spawning. Almost all gonads were found to be differentiating in siphonozooids, not in autozooids. Sex ratio of C. konojoi was around 1 : 1; that of P. japonicum and C. elatius seemed biased toward females but did not deviate significantly from 1 : 1. Number of gonads per polyp in these three species was fewer and they were generally smaller in diameter in comparison with other octocorals reported elsewhere, suggesting comparatively low fecundity for these species of Coralliidae from southern Japan. Spawning of these three species appears to occur during summer, mostly from May to August.