Although they are not Paros, sex determination in Bettas is still quite a mystery. Many spawns are heavily skewed in one direction or the other (I myself have had spawns that are nearly 100% male or 100% female, and I’ve tried to isolate different variables like pH or temperature – haven’t found anything consistent yet!) It is thought that Bettas rely on environmental cues to determine sex, but the link has not been definitively proven.
Here is a research article that I found extremely interesting. I made an “abstract cartoon” and a summary breaking down the article. A link to the actual article is at the bottom of the page:
http://www.ingloriousbettas.com/sex-determination-in-bettas.html
This article was particularly interesting because females that had their ovaries surgically removed (dubbed “neomales”) were actually able to go on and father viable offspring, notably of both sexes! This argues against an XX/XY system of sex determination. Anecdotal evidence of females spontaneously switching to males are well known among Betta breeders (I in fact witnessed this first hand – a female that actually placed 2nd in her class at one show “switched” in transit to the next show, where s/he placed in a male class!)
I wonder if Paros have a similar sex determination system as Bettas, and if so, what can cause skewing towards one sex or the other. Peter, is it known what the approximate sex ratios typically are in the wild? I know this information would be very difficult to collect properly (ex: perhaps nesting males are more evenly distributed in discrete territories but females roam around in loose shoals, then there could be a collection bias in any one catch. This pattern of behavior is similar to nesting male bettas/roaming female bettas). Would be interesting to see if there are skewed sex ratios in the wild, and when they occur.
Cheers
Gianne