Paradoxical Masculinization of Female Western Mosquitofish during Exposure to Spironolactone

The steroid Spironolactone acts primarily as an aldosterone antagonist in human and other mammalian models, but it often has antiandrogenic or feminizing side effects. However, when Spironolactone was added to aquarium water containing adult female western mosquitofish (Gambusia affinis) all specimens exhibited growth of a complex, malelike anal fin, the gonopodium, within 21 d. This unexpected androgenic response to an antiandrogen represents the first report of “paradoxical masculinization” in fishes. This finding indicates that Spironolactone may be of value in the culture of commercially important fish species and possibly in the aquarium industry to enhance formation of secondary sexual characteristics in sexually dimorphic species.