“Gonadal development and sex differentiation in the cichlid fish Cichlasoma dimerus (Teleostei, Perciformes): A light- and electron-microscopic study”

F. MEIJIDE; F. LO NOSTRO; G. GUERRERO

Biloxi, Mississippi, USA

Congreso; 53rd Annual Meeting of the Gulf and Caribbean Fisheries Institute.; 2000

There is little descriptive information on the gonadal development of fishes, specially at the ultrastructural level. In the present study, gonads from larval and juvenile stages of laboratory reared Cichlasoma dimerus were analized both at the light-microscopic and ultrastructural level. C. dimerus, a perciform teleost, is classified as a differentiated gonochorist, in which an indifferent gonad develops directly into a testis or ovary. This freshwater species adapts easily to captivity and spawns with high frequency during eight months of the year, representing an appropriate model for developmental studies. Larvae were obtained from three pairs of adult fishes captured in Esteros del Riachuelo (Corrientes, Argentina) (27º 25’ S, 58º 15’ W). They were kept in 15 l aquaria, at 26,5 ± 1 ºC and 12:12 hours photoperiod. Gonadal development was documented from 15 to 90 days post-fertilization (5 mm - 25 mm TL), covering the period of histollogically evidenced sex differentiation. Gonadal tissue was processed according to classical techniques for light and electron microscopy. On day 15 gonadal primordium consists of few germ cells surrounded by somatic cells. Ovarian differentiation precedes testicular differentiation, as usual in teleost fishes. The earliest signs detected include oogonial proliferation and meiosis, and the somatic reorganization of the presumptive ovary. Ovarian cavity is completely formed by day 65. Numerous oocytes at various stages of development are observed by day 90. In contrast, signs of morphological differentiation are not observed in presumptive testis until day 75. By day 90, the unrestricted lobular configuration of the testis is evident. This basic research is a necessary first step for further studies on the effect of xenobiotics on gonadal sex differentiation.