How is a livebearer’s offspring’s sex determined?

[kevy]

I always thought it was random, like in humans, but I have a batch of fry that all appear to be female. They are at lest three months old and definitely look female. The fact that they are all the same sex makes me believe some environmental condition determines it, like in alligators or crocodiles (I forget which but I know the temperature determines it at a certain age). If I were to guess I would say pH because the temperature in my tank was fluctuating a lot (heater problems) when the first couple batches were born.

So my question…

Does an environmental condition determine livebearer sex?

Maybe they are just too young to tell, or maybe its coincidence. Or maybe only the females survived to this age.

Any answers or leads to places I could find an answer would be appreciated.

 

[yhbae]

There are some articles that experiments with pH and concludes that pH does affect the sex ratio for some of the livebearers. Lower the pH, more male. Higher the pH, more female. There's natural tendency that you get more female than male in general.

I too, experienced a batch that comprised of 100% female only.

 

[suemack]

hi kevy - 2 things seem to affect the male/female ratio in livebearer fry

as yhbae says pH - high pH more females, lower pH more males - with a pH of 6.6 you seem to get equal numbers male/female

other factor is that the female fry seem to be a lot stronger and more of them survive

sue

 

[kevy]

I did some additional searching on the web found exactly what you two said. I also found out they don't know very much about swordtails (the fry I had in question).

"other factor is that the female fry seem to be a lot stronger and more of them survive"

I never thought about that and it makes a lot of sense.

Thanks a lot for your replies everyone. If you would like to read a little more about this, the link below is a good article about the sex ratio of swordtails.

swordtails

 

[yhbae]

hi kevy - 2 things seem to affect the male/female ratio in livebearer fry

as yhbae says pH - high pH more females, lower pH more males - with a pH of 6.6 you seem to get equal numbers male/female

other factor is that the female fry seem to be a lot stronger and more of them survive

sue

6.6 eh? That's good to know...

My tap water comes at KH=2. With heavy surface disturbance (--> low CO2), my pH is at 7.4. Hence I wasn't surprised when I saw alot more females than males.

I dunno if I want to mess with pH in this case as I'm sure platies don't like acidic water that much. (I want to see if I can get to 1:4 ratio for M:F)

 

[blue_fish]

yeah, you're right... about 3 months, they would most likely be feamales... but wait a little longer, those fan fins will begin to flop and become males

 

[paulthegreat]

As far as I have been able to conclude all livebearer fry(gup, mol, plat and swords) are born female. When the start to reach 65-80% of full síze a number of them will make the transformation to male. I have never seen any conclusive studies that specific enviromental factors are the cause. Most thermometers are not accuratly calibrated and the pH tests are not confirmed with a genuine scientific pH meter. I had thought it was something like temp or pH at birth but it more seems to be conditions and balances when they get to "teenage" stages. I have sold many Swordie juvies and never had one turn male but I have seen those that I have sold turn male at the pet stores with a ratio of about 1 male to 3 or 4 females. Personally I would say it is a bit random with a higher percentage of female.