Gonadal sex differentiation in the Malabar grouper, Epinephelus malabaricus

To obtain basic information on sex differentiation in groupers, the process of gonadal sex differentiation in the Malabar grouper, Epinephelus malabaricus, was followed using histology for one year from hatching. Gonads of fish less than 39 days post-hatch (DPH) were undifferentiated and consisted of a few germ cells surrounded by somatic cells. There were no morphological indications of sex differentiation at this stage. By approximately 47 DPH, however, two elongations of somatic cells had formed in the gonads on the sides facing the gonad lateral walls. These elongations correspond morphologically to the initiation of ovarian cavity formation, a characteristic of ovarian differentiation in other teleost fish. By 74 DPH, the elongations had developed further and were evident in the ovaries of all fish. However, by 144 DPH, ovarian germ cells had not yet begun active division. Oocytes were first evident in the ovaries at 243 DPH, distributed mainly within the somatic tissue on the distal sides of the ovaries facing the ovarian cavity. The oocytes increased in number and in size from 243 to 360 DPH. These observations demonstrate that morphological changes associated with ovarian differentiation in the Malabar grouper begins at approximately 47 DPH and that in all individuals the gonads differentiate directly into ovaries.