Gonadal sex differentiation in the Senegalese sole (Solea senegalensis) and first data on the experimental manipulation of its sex ratios

Like other flatfishes, the Senegalese sole exhibits sexual growth dimorphism in favor of females and thus all-female stocks would be desirable. However, reproduction-associated problems persist and basic aspects of its biology such as sex determination and differentiation are still unknown. In this study, we histologically examined the process of gonadal sex differentiation in the Senegalese sole and tested the effects of treatment with sex steroids and an aromatase inhibitor on sex ratios. Sex differentiation in females started before 98days post fertilization (dpf), when fish had an average total length (TL) of ~33mm. In contrast, males began sex differentiation by 127dpf with a mean TL of ~44mm. At this time, an incipient growth advantage in favor of females was already observed. All fish were sexually differentiated by ~48mm TL (~170dpf). Treatment between 98 and 169 dpf with the synthetic androgen 17α-methyldihydrotestosterone (10mg/kg) or the aromatase inhibitor fadrozole (100mg/kg) significantly increased the proportion of males up to ~95% while the natural estrogen, estradiol-17β (10mg/kg), increased the proportion of females by about 50% but reduced growth with respect to the control values. Together, these results show that the Senegalese sole is a differentiated gonochoristic species in which gonads start to form within the first three months and sex differentiation is completed after animals reach their fifth month of age, with females differentiating earlier than males. Results also indicate that the labile period starts at <98dpf and lasts until ~125dpf. These results show that sex control can be achieved in the Senegalese sole, an important step towards the establishment of all-female stocks to improve the currently compromised production of this economically important species.