"She" is a "He"!

A bit of history: I bought 4 platies back in early May. 2 female Calicos, 1 male Mickey Mouse (MM) and 1 female MM. Turns out that female MM was a juvenile male, and there were agression issues with the older male, so I returned the juvenile to the store in exchange for a definite female.

Again, there was some agression, but the new female MM was handling it well, and I decided to just leave her to see if it all sorted out. Well, it didn't, but she did still hold her own...eating well, swimming around all the time, and hiding out just a little bit. I was perplexed as to why the male wasn't accepting another female...thought maybe it was because she looked a lot like the male that was replaced.

Well, now, three months later, her anal fin is changing. She is a he after all! I should have paid attention to how they were interacting...after all, the fish would know better than me! In regards to definitely buying a female, I'm thinking that the tank at the LFS had nothing but juvenile males, and the staff didn't know.

Edit: One question that I'm having trouble finding an answer to: At what age does a juvenile male platy start developing a gonopodium? Is there a difference in fin shape before the development starts?

I've been looking closely for 3 months now! :laugh: I should have paid attention to how the fish were interacting...that was the biggest clue. There was no sign of development until this last few days. Surprising since this fish was a fairly good size when I bought "her."

Oh well, when the babies in my other tank get bigger, I can mix the fish up a bit and separate those males.

Oriongirl, do you know what age this typically happens, or does it vary from fish to fish?

Interesting to watch the development though. The first thing I'm seeing is that the bottom edge of the fin is starting to enlongate a bit, and has developed verticle "stripes," or segmentation. Very hard to see, but visible if you look really closely. The rest of the fin seems to be separating/changing. If I didn't know that the gonopodium was developing, I would probably mistake it for a health issue like the start of fin rot or damage from fighting.