Use of 17-a-methyltestosterone for production of male secondary sexual characteristics in the adult female green swordtail (Xiphophorus hellerii).

The green swordtail X. hellerii Heckel, 1848 is a commercially important ornamental species. As phenotypic males with the sword extension of the caudal fin are more valuable economically than fish lacking the sword, masculinization by means of 17-α-Methyltestosterone (MT) was tested. Populations of female broodstocks were fed MT-treated feeds (20, 35 and 50 mg MT kg feed) at a rate of 50 g of MTtreated feed per kilogram of fish biomass (5% body weight [BW]) for 45 consecutive days to cause development of male secondary sexual characteristics, specifically the growth of the sword extension. Use of MT feed at 5% BW for 45 d resulted in 100% masculinization, whereas 0% masculinization occurred in green swordtails fed control feed. There were no significant differences between MT-treated groups in the proportion of phenotypic males. Also we found no significant differences in mortality among experimental groups. No degeneration of swords was seen 200 d after the last day of experimental feed treatment. According to our finding, the use of MT-treated feeds can increase the production of highervalue phenotypic male green swordtails and enhance profitability for aquaculturists.