My Sex-Changing Swordtail!

[ginny]

Hi everyone! I just wanted to share something cool. At my LFS, they had a tank full of about 10 female and 3 male koi swordtails, most of which were missing an eye (someone put a ghost knifefish in by mistake and it ate a bunch of their eyes!). I really wanted a male, but the store wanted to keep the males to try to get them to breed. So, I got 2 females: one with two eyes, and he gave me a one-eyed for free!
 
About a month and a half later, I noticed something that definitely wasn't there before: a gonopodium on my one-eyed sword! I thought it may just be a weird-shaped anal fin, since I was positive it was female, but lo and behold, it's starting to grow a sword and it's definitely got male sex organs. I told my LFS and they said that swordtails can change their sex. I never knew that! I've read on other forums about people saying they've had them change before and some people were skeptical. So, I thought I'd show off my proof. How can you be 100% sure it's the same fish from before? Well, it's got a prety obvious tell
 
Check it out!
 
Before:

 
After:

(sorry this picture is kind of blurry, my iphone takes terrible fish pics...)
 

 

[LyraGuppi]

Sometimes young livebearers will take a while to develop, but sometimes they'll "hold back" when other males are in the vicinity (to prevent fighting).

 

[Byron]

Agree.  This is not so much an actual sex reversal, but as LyraGuppi said more of a late development.  Generally it occurs in young females, up to the age of five months.  It can also occur in old females that are no longer fertile.  I'm not aware of males becoming females, just "females" actually being males that develop late.  And of course, it can be induced through the injection of hormones, etc, in laboratory conditions.
 
Something very similar is common among species in the cichlid genus Apistogramma.  I experienced this first hand with a group of five wild caught A. kleei (now classified as A. bitaeniata) back in 1985.  In my aquarium, I assumed I had one male and four females, going by appearance and behaviours of the five fish.  The male spawned with a female and produced fertile offspring all of which I regularly removed to rear in another tank.  After a couple years, the male died, and to my surprise another appeared from the four assumed females, and proceeded to spawn with a female.  I subsequently learned this is the male's way of avoiding conflict with the more dominant male, not actual sex reversal.
 
Byron.

 

[eaglesaquarium]

Agree with Lyra and Byron.