Significance of Sexual Dimorphism of the Anal Fin of Polypteridae

Abstract

THE anal fin of the female of both Polypterus and Calamoichthys is narrow and pointed, whereas that of the male is wider and rounded, and can be erected into a cup-shaped organ. This diiference was described for P. bichir by Leydig¹, who suggested that internal fertilization occurred, and subsequently Harrington² stated that the anal fin of the male “is undoubtedly a copulatory organ”. Budgett³, noting sexual dimorphism of the anal fins of P. senegalus and P. bichir (= P. lapradei), wrote that “it remains to be seen to what use the male Polypterus puts this modified anal fin”, while he dismissed Leydig's suggestion that fertilization is internal. Traquair⁴ described sexual dimorphism of the anal fin for C. calabaricus but did not comment on the possible function of the anal fin of the male. Recent work by Arnoult^5,6, who described the spawning behaviour of P. senegalus in aquaria, shows that the anal fin of the male undoubtedly forms a copulatory chamber. During spawning the male swims alongside the female with its anal fin folded up like a gutter towards the vent of the female, fertilizing the eggs as they are laid.