Histological characterization of gonadal sex differentiation in Nibea albiflora

The aim of this study was to investigate the period of sex differentiation and the pattern of gonadal differentiation in Nibea albiflora using histological techniques. The fish were raised in temperature between 19.5 and 21.5 degrees C. The promordial gonads formed at the posterior region of the abdomen and the fish remained undifferentiated till the 36day post hatching (dph) with a total length (TL) of 21.20 +/- 3.90mm. Ovarian differentiation precedes testicular differentiation, as usual in teleost fish. Oocytes undergoing meiosis at zygotene (bouquet) and pachytene stages were firstly observed when the total length reached 33.05 +/- 4.61mm at 40dph, and the formation of the ovarian cavity began simultaneously at 46dph with total length of 49.40 +/- 4.83mm. When the mean total length reached 78.05 +/- 5.24mm (61dph), ovarian cavity grew bigger, and oocytes were firstly detected. By contrast, signs of morphological differentiation in the presumptive testis are not observed until their total length reached 69.18 +/- 5.26mm at 55dph. The unrestricted lobular organization of the testis is evident at the stage of TL=78.05 +/- 5.24mm (61dph). These findings indicate that N.albiflora is a differentiated gonochorist and the critical period of sex differentiation in N.albiflora occurs at the stage of TL=21.20 +/- 3.90mm (36dph).