Sex ratios

[GaryofMontreal]

Hello,
If I posted this on a killie list, a lot of readers would groan and say they'd been where I was. It may be the same for livebearer keepers - let's see...

I started out a while back with a pair of Girardinus falcatus, directly from the wild in Cuba. They have been healthy, active and really appealing in my tanks - this is a fish I want to have around for as long as I can. Right now, I have 70-80 fish of varying sizes in a heavily planted 55 gallon. The temperature is ambient (25 today, ranging from 23 to 27 over a summer month), pH is slightly alkaline, hardness is high.
Not one fish from four broods is male yet. I know that gender is environmentally determined after the young are freeswimming, but what might the factors be with this Girardinus? I have manipulated killifish ratios with water hardness and/or temperature. Any suggestions for these little livebearers? My one male may be the only livebearer on the planet that doesn't annoy all the females in his tank - he's too shagged out.
Gary

[Melody]

Sex determination in Livebearers can vary in complexity and I'm not familiar with what is involved with Falcatus. In Swordtails, for example, the genetics involved with sex determination can be on several chromosomes which interact with each other and the results determine the sex. In Swordtail broods the females almost always outnumber males to one degree or another (depending on the strain), sometimes 10:1 but averaging 2-4.

I have heard of pH and temp impacting the ratios, but the impact was before birth. I've always wanted to get a few people involved in informal experiments but it hasn't happened yet.

I think it is likely that Falcatus males mature later. You can try putting a dozen or whatever into another tank and see if that prompts male development. You should at least see alpha males show up eventually. They apparently don't 'practice' when they're young like some livebearers do, instead they wait until they are sexually mature to chase females at all.

I don't pretend to know the species in anything but a general sense, so I'll learn along with you as the fish mature. :smile: Very interesting!

[blainep]

Good looking little fish Gary !

As far as sexing ratios, I've found in my tanks, females outnumber males nearly 10 to 1 sometimes. I'm not sure why, but I had started a temperature experiment to see if the ratios would change at all.
I never got to finish that experiment, but I'll be trying it out again once my tanks are cycled and settled again.

[Melody]

If you're going with temp's, keep an eye on the number of fry you get too. An Australian study found that for every degree the temp went up, the brood size went up by 3. I'd be interested in knowing how typical that is.:smile:

[GaryofMontreal]

There has been informal experimentation with killies. I have a species I've maintained, with one short disaster break, since 1992. At a hardness above 100 ppm, I was blessed with solid 35:1 male to female ratios, but down around 50ppm, it was 6 males to 4 females.
I have friends who manipulate other species using temps.

What really kills me (since Melody's posting leaves me hope of keeping my falcatus going) is that the four Heterandria bimaculata I managed to catch and to bring back from Guatemala are all absolutely, undeniably females. :no:

[Melody]

They couldn't be prehit? Or were they too young?

Eggers can be more easily manipulated environmentally because that's where the eggs are as the fry develop in them. You can even force albinoism with eggs.

Temp is tricky though - sometimes the temp's needed to impact sex ratios are so high they can kill the fish. All studies of this sort should be done within the acceptable temp ranges for the fish. With pH you have some room for error with most Livebearers, but I wouldn't play with it when working with a wild-caught fish. Are you matching their wild parameters? Maybe that could throw the ratios.

Sometimes ratios are thrown by predation too - males are smaller and more easily eaten. :wideeyed: Out of that many fish though, I would be very surprised if they're all females, making late development a feasible hope to have. That said, it may be a peculiarity with that specific species that I haven't heard about. They're often used in scientific studies (as are Gambusia) and I haven't seen any mention of gender bias so that's encouraging.

One thing is certain, if the ratios could be easily thrown then the farms would be doing it. No information that I've read in Aquaculture circles gives a method for doing this. I have a how-to guide for Swordtail aquaculture that says that is why they study ratios, but so far no sound method has been determined. There's big money in it, so you can bet it has been extensively studied.