gonopodia

A gonopodium is a modified anal fin found in male bony fish, specifically within the order Cyprinodontiformes. This specialized structure serves as an external reproductive organ, analogous to a penis in other animals, and is used to transfer sperm to the female during internal fertilization. The gonopodium is typically an elongated, rod-like structure formed by the fusion or modification of several rays of the anal fin. Its exact shape and size can vary significantly between different species, often playing a role in species recognition and mate selection. The presence of a gonopodium is a defining characteristic of males in groups such as livebearers like guppies, mollies, and platies. The development of the gonopodium is hormonally controlled and occurs during the sexual maturation of the male fish. Its function is crucial for the reproductive success of these species, enabling internal fertilization and often leading to the birth of live young rather than the laying of eggs.