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It has long been rumored that female Swordtails can change gender when there are no males available. There are dozens of accounts online of this miracle sex change happening. Can Swordtails really change gender?
No, female Swordtails can’t change their gender. When there is a dominant male in a group of Swordtails, other less dominant males hold back their development, giving the appearance they are female. If the dominant male dies, one of the sub-dominant males quickly matures, giving the impression they have changed gender.
How Do You Sex Swordtails?
Sexing Swordtails is relatively easy when you are looking at a dominant male as the dominant males have a ‘sword’ at the end of their tail fin.
You can tell the sex of a less-dominant Swordtails by looking at the analfin. The analfin on female Swordtails is roughly triangular in shape whereas the analfin on the male is more pointed. The male analfin, known as a gonopodium, is used to transfer milt directly into the female during breeding.
The image below demonstrates the differences between male and female analfins.
How Many Males Can You Keep Together?
Male Swordtails are territorial. They will defend their small territory, and the females within it, against all other males. This aggression is usually reserved for other male Swordtails. To keep the aggression levels down, only one male Swordtail should be kept with a group of 3 or 4 females.
If you are buying a group of young Swordtails, purchase 7 or 8 and grow them on. Once you can sex the fish within the group, select a male, ideally one who has started to grow his sword and show his dominance and keep him with 3 or 4 of the females. Rehome or move the remaining fish to a new aquarium. The small harem of Swordtails you keep will hopefully then go on to form a good breeding group.
How Do Swordtails Breed?
Swordtails are livebearers. Unlike egg-laying fish, livebearers give birth to live, free-swimming, baby fish that are fully independent of their parents at birth.
With egg-laying fish, generally speaking, the female scatters or deposits the eggs and the male fertilizes them when they are in the water. With Livebearers, the male inserts his ‘milt’ into the female and she retains the eggs inside her body until they are developed (28 to 35 days typically)
Despite what you may have read elsewhere, female Swordtails can not change gender. When this appears to have happened in your aquarium, what you have actually witnessed is a sub-dominant male, maturing into a dominant male. The sword on the anal shows everyone that this male is now the dominant male who is overseeing the females in his harem.