Sexual Dimorphism in the Anal Fin of Ayu Plecoglossus altivelis

Abstract

Sexual dimorphism in the anal fin of ayu Plecoglossus altivelis was studied by serial sampling of ayu. Changes in the shape of the anal fin were triggered by sexual maturation in both sexes; however, secondary sexual characteristics developed drastically in males while they did so incon-spicuously in females. The anal fin base became larger in both sexes during sexual maturation, but grew much faster in males than in females, and consequently males have larger anal fin bases than females. Secondary changes in the anal fin height occur only in males, and the change is negative, and consequently females have larger anal fin height than males. The relationship between the shape of the anal fin and the sex hormone levels in diploid and triploid ayu suggested that the development of secondary sex characteristics in the anal fin of ayu is controlled by androgens.