Efficacy of 17-α-Methyltestosterone for Expression of Male Secondary Sexual Characteristics in the Green Swordtail

Abstract

The green swordtail Xiphophorus hellerii is a commercially important ornamental species. Because phenotypic males (specifically, fish 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 market-size experimental fish, a mix of females and late-maturing males (i.e., lacking male secondary characteristics) were fed MT-treated feed (60 mg MT/kg feed) at a rate of 30 or 50 g of MT-treated feed per kilogram of fish biomass (3% or 5% body weight [BW]) for 28 consecutive days to cause development of male secondary sexual characteristics, specifically the growth of the sword extension. Use of MT feed at 3% or 5% BW for 28 d resulted in 100% masculinization, whereas 0% masculinization occurred in green swordtails fed control feed. No regression of swords was seen 28 d after the last day of experimental feed treatment. There were no significant differences in mortality among treatments. The use of MT-treated feeds can increase the production of higher-value phenotypic male green swordtails and enhance profitability for aquaculturists.