Histological characterization of gonadal sex differentiation in N ibea albiflora

The aim of this study was to investigate the period of sex differentiation and the pattern of gonadal differentiation in N ibea albiflora using histological techniques. The fish were raised in temperature between 19.5 and 21.5°C. The promordial gonads formed at the posterior region of the abdomen and the fish remained undifferentiated till the 36 day post hatching (dph) with a total length ( TL ) of 21.20 ± 3.90 mm. 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 ± 4.61 mm at 40 dph, and the formation of the ovarian cavity began simultaneously at 46 dph with total length of 49.40 ± 4.83 mm. When the mean total length reached 78.05 ± 5.24 mm (61 dph), 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 ± 5.26 mm at 55 dph. The unrestricted lobular organization of the testis is evident at the stage of TL  = 78.05 ± 5.24 mm (61 dph). 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 ± 3.90 mm (36 dph).