Sexual differentiation and sex reversal in tilapia (Oreochromis niloticus) by hormone 17 α methyltestosterone similar to that used in cultivation systems

The hormone 17 α methyltestosterone (MT) is commonly used in the farming of tilapia, which is a gonochoristic species, for the obtainment of a male mono-sex population to avoid reproduction during grow-out and due to the fact that males present greater growth, making cultivation commercially more lucrative. Larvae were collected and experimentally received MT in the feed for 30 days in a similar way as that performed in commercial farming. Physiological events of sexual differentiation and the moment in which sex reversal promoted by MT occurred were documented with the use of histological and morphometric methods. On the first day of receiving feed with MT, the fish presented gonads with undifferentiated germ cells. On the 12th day of exogenous feeding with and without MT, sexual differentiation of the gonads was observed morphologically, demonstrated by the formation of the ovarian cavity in females. Males continued to have undifferentiated gonads for a longer period. Sex reversal occurred in all females treated with MT at 25 days after the onset of exogenous feeding. The results indicate that treatment for sex reversal used in commercial farming systems could be shorter than that customarily employed, diminishing the quantity of synthetic hormone released into the environment and reducing the harmful effects of MT on other aquatic organisms. Therefore, the data suggest the possibility of reducing the use of hormone MT by 5 days.