03-07-2011, 06:40 PM
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It's totally normal sexual development, not a sex change
This happens with platys, mollies, guppies, and so far as I know all livebearers. It is NOT a sex change, and the males are not infertile, at least not for that reason. Any given fish may be infertile, of course, but not for that reason. In fact, some male platys, known as "early maturing males," are clearly male at around 1/2" and they ARE often infertile. Some are fertile, but they usually remain smaller, and are less likely to mate than their larger, later developing competitors.
These fish are all born looking female. Sometime later, usually at least 6 weeks to two months, the anal fins of the males start to differentiate. What actually happens is that the middle of the fin starts to grow longer, and over the next week or two, the elongated part becomes longer, the top part contracts, and you end up with a male gonopodium.
As for the "infertile" male someone mentioned: My guess is that you've probably had fry that you never saw. All livebearers will eat their fry, and the fry of other livebearers, unless separated soon after birth, or provided with LOTS of hiding places the adults can't reach.
If you want to keep the babies, separate the mother when she looks ready to burst and has developed a distinctly darker area behind her belly. Her sides may also appear lighter than normal, due to skin stretching. You can use a breeder net in the same tank, but I would recommend a seperate tank, because then the young will also have room to grow out some. Once you see babies, put her back in the main tank. Don't put the babies back in until they are at least 1/2 to 3/4 inch long, or longer if you have fish bigger than the parents in the tank. Remember, however, that livebearers are quite prolific. Platys can have 20 to 40 fry every month; guppies can have up to 100 every 3 to 4 weeks. So think about your overall capacity for fish, and/or ways to give away or sell them before deciding to let your fry grow up. You'll be overrun otherwise.