Gonopodium

The gonopodium is a modified anal fin in males of the family Poeciliidae (guppies, mollies, swordtails) and some other live-bearing teleosts. The anterior rays of the anal fin are elongated and joined, forming a rigid, rod-like structure with a hooked tip. It is used to deliver sperm into the female's genital pore during mating. The gonopodium is highly mobile and can rotate rapidly during mating attempts. Its length relative to standard body length varies among species and populations.

The gonopodium of a male guppy can rotate up to 180° within 50 milliseconds during a copulation attempt.