Sexual reproduction of scleractinian corals is among the most important means of establishing coral populations.However,so far,very little is known about the mechanisms underlying coral gametogenesis.To increase our understanding of sexual reproduction in corals,we analyzed the germ cell development in a stony coral Euphyllia ancora in terms of histology,molecular biology and endocrinology.The histological analysis revealed that gametogenesis occurs in the specific site of mesentery.The germ cell development took approximately one year in females and half a year in males.We further characterized germline gene early germ cell markers(piwi and vasa)and major yolk protein genes(vitellogenin and egg protein)Immunohistochemical analysis with a specific antibody against E.ancora piwi and vasa revealed the immunoreactivity in the oogonia,early oocytes,and developing oocytes in females and spermatogonia and primary spermatocytes in males.RT-PCR analysis revealed that the expression levels of vitellogenin and egg protein increased when approaching the spawning season in females.High levels of both vitellogenin and egg protein transcripts were detected in the putative ovarian tissue.The sites of yolk synthesis are the mesenterial somatic cells,but not oocytes themselves on the basis of in situ hybridization and immunohistochemistry.Vitellogenins accumulated in eggs were most likely consumed during the coral embryonic development assessed by immunoblotting.Transcriptome analysis on the isolated gonadal regions revealed that various growth factors,such as activin and insulin,were expressed in the gonads,suggesting that these factors may be related to the germ cell development in E.ancora.Furthermore,we investigated the seasonal profiles of immunoreactive hormones(estradiol,testosterone and GnRH).The high levels of estradiol and testosterone were detected one month prior to the spawning period.Also,high concentration of immunoreactive GnRH was detected in the spawning seasons as compared to other seasons.Our recent data shed light on the mechanism of the sexual reproduction of corals and also the restoration of coral reefs.