The mating organ of the male of most livebearing toothcarps is called the gonopodium.
With the onset of sexual maturity, the anal fin of males transforms into the gonopodium.
In young animals, therefore, the sex cannot be distinguished on the basis of the gonopodium.
The female’s anal fin is wide and resembles a triangle.
The fin rays are usually clearly visible. The fin sticks out from the body like a sail.
The male’s remodeled anal fin looks like the fin is always folded.
Black Molly Male – Gonopodium in a Circle Source: Image on Wikimedia Commons License: CC Attr. SA 3.0 Author: Darkmax
The gonopodium is in the shape of a spike or dart and is more attached to the body.
In most species, the gonopodium is a type of tube through which the male’s semen slides.
In the so-called Goodeids, the highland medaka, the gonopodium forms a kind of open chute over which the seed slides.
In species like the Black Mollies, the gonopodium can be easily identified. In other species there is a dominant male.
All other males color like the females and pinch their fins in such a way that the gonopodium is difficult to discern.
These include Limia species in which only one alpha male will fully develop in a flock.
Some livebearers do not have a gonopodium. Mexican highland medaka e.g. B. have only one kind of gutter.
Platys (left male; right female)
Photo: Yvonne Gieseking
Male with clearly visible gonopodium: