Sex ratios: livebearers and killifish

[Innesfan]

Volumes have been written about skewed sex ratios among some species and aquarium strains of killifish and livebearer fry, and theories have been proffered about the effects of temperature, ph and other factors in determining gender. Despite this, some species and strains seem not to have read the texts and continue to throw skewed gender ratios. By way of example, my Fundulopanchax filamentosus produce a pretty consistent 50%-50% sex ratio, but my Aphyosemion sp. Lobaye, kept under the exact same conditions produce 90%-10% males. I've had some livebearers do the same, especially some wild forms.

Some hobby veterans have used a ploy to address this by raising the fry 2 or 3 to a bowl until gender can be determined. In many, if not most cases, this produced the desired result of ensuring both genders would be produced with one of each gender in each bowl. I have had great success with this method.

Problem is, it eats up a lot of space and increases the water-change workload. So my question is, has anyone ever read an article or paper anywhere that stipulated when gender is set, before we humans can determine it? In other words, if I can determine the gender of my Aphyosemion fry at 4 weeks, have the fish made up their minds as to what gender they are at 2 weeks? 3 weeks? 5 days? To know this, would greatly improve the workload/space issue by consolidating many small tanks into one larger one earlier on.

 

[emeraldking]

Well, I myself andf lots of other serious livebearer breeders work with the method of regulating the water temperature during the whole pregnancy to influence the sex ratio of newborns. Although, it's not a 100% guarantee that this is a solid method but it does work most of the time.

 

[GaryE]

I have battled this with my killies. I think there is no one answer. As usual, it goes species by species.

The 2 killies in a container trick has failed miserably here. I'm completely unconvinced, after even starting with 2 eggs in the dish and raising the resulting fish together. It solved nothing, repeatedly.

With one species, and I mean one because I haven't been able to do this with others, I have some sort of working trick. With Aphyosemion zygaima, starting in 1992, I got 37 males to one female. I had 140 ppm water, with an unchanging pH of 7.4. When I softened the water with snow or rainwater, I got more females. Never many more. The average temperature was 20.

Then I moved. Keeping my Aphyosemions at the same temperature as the previous house, but in mid 6 pH with water ranging from 65 to 100 ppm, I got all female bunches! I moved pH around, because I didn't have the seriously buffered water of the first house. I could change pH. It didn't seem to matter, over several generations. That fishroom averaged 20 degrees as well.

Then I moved again. My water reads a lot like the second house, but plants grow differently and I suspect the parts per million are different parts. My tap is from a blackwater lake. Here, I get a majority of female zygaima, but if I harden the water from the get go, can get more males. The wild card is that the fishroom is also warmer, running at 22 for 10 months and 24 in high summer.

I now get an average of one male to four females with zygaima. But, with the very genetically close A. ogoense and A. ottogartneri, this didn't work. In the first two houses, I got close to 50/50. Now, I get 5 female ottogartneri to every male, and lost the ogoense because I couldn't get any males. I have aphyosemion escherichi which I caught myself in very soft, pH 6.6 water at 22c. I have one male, and 30 females. So far, not one of the fish I've raised has been male, and I'm worried. I keep churning them out, hoping.

Diet, maybe?

We know that gender is determined by environment, after hatching. Beyond that, we'll seek a magic bullet answer, and probably not find it. I have learned how to manipulate gender with one Aphyosemion species, but I can't do the same thing with even their closest relatives. Go figure.

I think this may be a reason why I haven't seen any papers on when it's determined. The variables...

 

[Innesfan]

I think this may be a reason why I haven't seen any papers on when it's determined. The variables...

I feared this would be the answer.

The 2-in-a-bowl method worked for me with the Rivulus, Fundulopanchax and Aphyosemion species that were throwing skewed ratios, and one population of exceptionally gorgeous wild Belize swordtails that I got from the late great Joanne Norton, who warned me she was getting skewed sex ratios. She sent me 24 juves that wound up 23-1 male. I bred the one pair and raised some 3-to-a-bowl and got 60-40 males, a decided improvement. So if Dr. Joanne Norton--who cracked the angelfish genetic code and did such extraordinary work with livebearers--couldn't crack the sex ratio problem, well, we're in good company.

 

[GaryE]

It isn't just killies and livebearers. I have an interesting thing going with my Paranochromis brevirostris, a dwarf Cichlid I like more and more . My wild caught pair are still spawning sometimes, and I have two F-1 pairs guarding eggs or larvae as I write. If all goes well, I will have a good bunch to add to the 25 or so I am already raising.
The fish keeps falling out of the hobby because broods are all males. I managed to get 3 females from my first group of 15, but I don't understand why. It's a great success, but I'm not feeling successful in any grounded way. I have to see what these 25 juvies from 2 sets of parents and 3 spawnings give, plus what these new spawns produce. Then I have to figure out why.
Combined formal-scientific and hobbyist-scientific research into Pelvicachromis from not incredibly far away from them suggested 26 degrees celsius was the magic bullet for 50/50 ratios with that group. I bred a fair number of Pelvicachromis kribensis (for general readers, not the kribensis of the hobby, which is P pulcher. Someone importing for the hobby screwed up an identification way way back...) and the temperature worked for me. But before I moved here, I was playing with other temperatures, with P kribensis Moliwe, and wasn't getting terrible ratios. At 23c, I was getting a slight excess of females, but due to an illness in the family and a long run as the world's worst nurse, I lost the species after only 2 spawns. I'm going to look again this summer to try to get them via Europe - with the geopolitical situation I won't be buying anything from just south of Canada for a long time.

But the brevirostris ruin the scenario, because they came from 22 degree water. They suffer visibly at anything above 25, so the 26 degree marker can't be the target. My water is slightly harder than the capture stream's, and I can't match its oxygen levels in a glass box. That water was rolling. I'm just going to try to produce an enormous (to me) pile of them and see what sorts out. It's how I did the Aphyosemion zygaima and how I'm trying the A. escherichi mentioned higher up the thread. You have to have both sexes to be able to figure it out over several generations.

I have friends and family who once had a tank with a few ornamental fish, and wonder how I could have retained an interest in fish from my childhood until now. There are so many puzzles like this!

And I am jealous of your connection to Dr. Joanne Norton. That is one deeply respected name. There is almost no information about her online. It's disturbing, given what she did.