Gonadal Sex Differentiation of Hatchery-Reared Longtooth Grouper (Epinephelus bruneus)

Abstract

This study was investigated the gonad development of the longtooth grouper (Epinephelus bruneus) from 1 to 365 days after hatching (DAH). Fertilized eggs were obtained from Marine and Environmental Research Institute, Cheju National University. Fish were sampled for histological analyses every five days from 1 to 50 DAH and then randomly until 365 DAH. The migrating primordial germ cells were first appeared in the area on the dorsal side of the intestine at 10 to 15 days after hatching (DAH). A diameter of the primordial germ cells ranged from 11.7 to 25.2 ㎛² at 10 and 15 DAH. The primordial gonad (PG) was first observed in 20 to 30 DAH larvae containing only somatic cell. The gonocytes were first observed at 45 DAH. At 50 DAH, the primordial gonad containing germ cells was surrounded by somatic cells and gonad remained undifferentiated. The initial ovarian cavity which was indicated by the presence of two elongated aggregations of somatic cells in the gonad at 60 DAH and then increased in size at 90 DAH. The ovarian cavity was formed at 110 DAH with large space. The oogonia were first observed in the edge of ovarian cavity of gonad at 140 DAH with average size was 13.3 ± 2.0 ㎛ in diameter. Primary-growth stage oocytes were observed in different lobules along of ovarian cavity at 270 DAH. The size of there primary-growth stage oocytes was 12.5 ± 4.7 ㎛ in diameter. The present observations indicates that ovarian differentiation associated with gonadal morphological begins at approximately 60 DAH and the gonads differentiate directly into ovaries in all experimental fish. 17α-methyltestosterone dose tested in this study could not induced masculinization of Longtooth grouper during gonadogenesis. The most effective period for sex reversal using androgen hormone treatment in Longtooth grouper (E. bruneus) could be at 50 DAH before formation of ovarian cavity.