From The Aquarium Wiki
A gonopodium is a modified anal fin (it looks rather like a thin rod) that allows male fish of the families Anablepidae and Poeciliidae to briefly hook into the vent of a female fish to deposit sperm.
This fin is normally held back and the male fish turns it down and forward just before mating.
Typical male fish with this fin are:
- See also Andropodium