Why Do Some Female Swordtails Turn Male?

[keithp]

When I was younger I used to have a tank full of swordtails, and I noticed a few of the largest females eventually turned male, even after they had been pregnant. Why does this happen, and can they mate once they turn male?

 

[LauraFrog]

As far as I know, it's due to the reduction of female hormone production in old females. They stop becoming pregnant and may grow a gonopodium and a sword. However, they never actually change to being male. They still have an ovary and no testes, and are obviously infertile.

 

[keithp]

Oh ok so they look male but have female reproductive organs, and are infertile. That's really interesting. Any other tropical fish do this?

That reminds me a little of an ocean fish (a type of rock bass I think) that in the school is 1 male and all females, but when the male gets old or dies the biggest female in the school turns male and changes appearance and reproduces as a male.

 

[LauraFrog]

It's different because it's an actual sex change... a lot of fish do this, especially marines and secondary freshwater or estuarine fish, usually big ones. That's why they have to set size limits on fishing, to make sure people don't inadvertently take out all of the fish of one sex.
Other livebearers have been heard of doing this but it seems to be most common in swordtails of the common livebearers at least. The fish are still female, but there are not enough female hormones to allow them to continue becoming pregnant, and so they develop male external sex organs. One theory is that it's a decoy so that males will think they are another male and will not harass them to mate, but it's just a theory, no research has been done as far as I know.

 

[Colin_T]

Hi Laura
I have to disagree with you about female swordtails not being able to breed once they have changed sex. In the absence of male fish, a dominant female swordtail (and other livebearers) will often change sex and become a fully functional male to keep the species alive. However, the majority of fry will be female and only a very small percent turn out to be males.

 

[5teady_2012]

Hi Laura
I have to disagree with you about female swordtails not being able to breed once they have changed sex. In the absence of male fish, a dominant female swordtail (and other livebearers) will often change sex and become a fully functional male to keep the species alive. However, the majority of fry will be female and only a very small percent turn out to be males.

100% agree with what Colin has put.

 

[Colin_T]

hey 5teady has popped in for a visit. Long time no see, how you been?

 

[Ludwig Venter]

Just took some photos of my snails, & this female sword turning into a male happened to be in the pic.... see the developing gonopodium... Used to have 1 male and 4 females, now have 1 female and 4 males..... (all delivered babies previously... would not have believed if I did'nt see it for meself!

 

[Colin_T]

it's pretty bizarre isn't it. You end up with super sized males. I first saw it years ago when a big female green swordtail changed. I ended up with a male about 4inches long (body size) with a 2inch swordtail filament. It was about 3 times the size of the normal males.

 

[Ludwig Venter]

it's pretty bizarre isn't it. You end up with super sized males. I first saw it years ago when a big female green swordtail changed. I ended up with a male about 4inches long (body size) with a 2inch swordtail filament. It was about 3 times the size of the normal males.

Yea... mine just happens to be in the same pic with the only remaining female.... They are a lot bigger than the original male.... you can actually also see the start of a sword developing in the tail.....

 

[spikejack]

I had a large female sword until she decided to change to male. I had a few female swords in the tank at the time. I tried a few times to introduce another male into my large tank but none survived that long. All the time the sex change sword was in the tank my swords never bred

 

[Stephanie_fish_lover]

I have two swordtails, and I thought I bought two females, but then one of them got pregnant (I got them back in March of this year) she gave birth to 17 babies, none survived unfortunately. I did notice that after a while one of my females is slimmer then the other and darts around way more then the other, everyone I ask that knows swordtails say she is actually a he. I then looked up if it was possible for them to "life finds a way" on people and sure enough they can its rare but it happens. My question is, is Frienderman (I did not name it my friend did) actually a male or is it a she that just never fattened up? Mergwen my female is a nice fat happy fish. this would help clear up much confusion on my life and help me in my choice of getting more of them to add to my tank.

 

[Byron]

There is another possibility here, and that is that the female was impregnated before you acquired the fish.  Unless the fish are separated almost immediately after birth, this can easily occur.  And once impregnated, a female can deliver several broods without a male being present.
 
With only two swordtails, if one is male and one female, it will be pretty obvious; the male will continually harass the female.
 
Byron.

 

[Rowie6]

This has just occurred to one of my Blue female Swords,The male jumped out of the tank around six weeks ago and i noticed the start of a sword and noticed this fish was now a male a few days ago, and i am 100% sure that this fish has had fry before!!!!