KELLY, D. A.: Rapid Gonopodium Movement in the Sailfin Molly (Poecilia latipinna)

Male poeciliid fish have a highly modified anal fin used to inseminate females during copulation. This fin, called the gonopodium, has hypertrophied musculature compared to the female’s anal fin, and its anterior fin rays are lengthened and laterally compressed to form an approximately cylindrical rod. High-speed video of copulating sailfin mollies (Poecilia latipinna) show that the gonopodium is elevated for copulation by swinging ventrally nearly 180� in the dorsoventral plane of the fish’s body; the tip of the fin moves anteriorly and laterally during elevation. Gonopodium elevation takes an average of 0.4 seconds, and the fin moves at an average velocity of 0.15 m/s. The gonopodium appears to bend elastically during elevation, implying that drag forces on the fin are at least temporarily larger than its critical force for Euler buckling.