So I just started an aquarium with tropical fish for the first time in some 15 years. I have various sorts of community fish in there, including three swordtails. One is male and has been in the tank for months. The other two have been in the tank for about three weeks and are female. At least, one is. The other was female when I bought her. The next week, I noticed that her anal fin was starting to look pointy. This week, the sword on her tail started to grow out. I figured ‘she’ was a late-blooming male. Then today I looked again, and she definitely has that male anal fin, and definitely has the start of a sword, and now she looks pregnant, with the dark spot and the bvelly curve and everything.
I’ve done some research online, and found varying opinions. Some sites say that swordtails can and do change sex. Some say that they don’t, but that they are known for having late-developing males. What’s the straight dope on sex-changing swordtails? And can my fish be both semi-male and pregnant?
I used to keep swordtails many years ago. I can attest that they do in fact change sex. I had a females that gave birth, and later transformed into males. I never saw it happen the other way around.
Sex in fish is not an XY type thing like in mammals.