a modification of the anal fin of some male fishes, that is used to transfer reproductive products to the female 
Modified Anal Fin in male Livebearers, used to transfer sperm to females.
a modification of the anal fin of males of certain live-bearing species in the families Anablepidae and Poeciliidae. It is used to transfer reproductive products to the female during mating.
An modified anal fin which is elongated. Only appears on the males, which helps to fertilize the females. You would expect to see these organs in Livebearers. Go Livebearers!!