Even in nature the different species of the Mollienisia genus display a curious tendency to produce hybrids. It is thought that these fish are still in process of speciation and that they have yet to acquire sufficient specificity not to be able to mate with other species. Certain zoologists thing that the species of mollies have an unstable genetic plan. The sexual life of these fish is in fact filled with unusual events. Fertilization is internal, the male introducing its milt into the body of the female through a small anal canal which is modified into a 'gonopodium' or sexual organ. The female waits for a certain time after mating before spawning: The young are born already formed and leave the maternal body as utterly independent fish.
That is probably the main way to tell the difference between male and female. There is no size difference, and the only way to tell is to look behind the tail fin. If there is an organ that looks like a stick or straw, it is definitely a male. If it's only a fin, it's a female. The sex of the fry shouldn't be determined, because they all look like females in the beginning, and they either stay female, or change to male, time will tell. Females tend to choose mates that are smaller compared to themselves, and if you do plan to breed mollies, or any other live-bearing fish and want to in succession, always have more than one tank, so one can be a maternity ward, one can be a tank for babies, and so on. Babies need a tank that is at least 10 gallons, or they won't grow at a normal rate. Overcrowding is another problem since fish owners usually have numerous live-bearers of different species.
One hybrid molly is known which exists in the wild only in its female state. Each female in its turn produces only female offspring, and so on. IN order to reproduce, all females of this hybrid form must mate with a male Mollienisia sphenops or a male of the latipinna species but always produces female offspring.
