Gonopodia is a mobile unpaired copulative organ of some viviparous fish ( Poeciliinae ), formed as a result of a change in the structure of the anal fin. The gonopodium is equipped with a trough through which sperm enters the female’s genital opening and a hook with which the male can better hold during the transfer of sperm to increase the chance of fertilization. Only one out of ten copulation attempts succeeds.
In young males, the anal fins have the same structure as in females. During puberty, several rays of the anal fin lengthen, form hook-shaped outgrowths and turn into gonopodia. Gonopodia has a developed system of blood vessels and nerves, which allows the male to control the process of copulation. Several muscles are attached to the gonopodia, which lead it to a state of "erection".