Sharply Angled Lateral Curvature Modification in Anal Fin of a Viviparous Fish, (Cyprinodontiformes: Goodeidae).

Almost all viviparous species possess male external genitalia; for example, the mammalian penis is an intromittent organ. Some live-bearing bony fish use their anal fins to assist in mating and internal fertilization. We previously reported a male-specific asymmetric curvature at the posterior end of the anal fin in , a viviparous fish of the family Goodeidae. However, three other goodeid species, , , and , examined in that study possessed lesser anal fin curvature modifications as compared to those in the anal fin of . Here, we report the second case of acute-angled curvature modification of the male anal fin in the family Goodeidae. We obtained a dead specimen of the goodeid species from a city zoo in Japan, and the morphological and histological analyses indicated an acute-angled asymmetric curvature of the posterior end of the anal fin in , similar to that observed in in the previous study. However, in our previous report, obtuse-angled modification was only observed in one other species, , and two species belonging to other genera, and Therefore, our findings suggest that the acute-angled curvature in the male anal fin has been developed in the genus .