Ha, I bet that title has everyone looking! Seriously though, late last year I collected a small school of gambusia from a local lake. They were recently introduced as I know they were not present before. I had 10 fish, 7 females and 3 males of which two females and one male died leaving 5 females and 2 males. Over the weekend as I was cleaning the tank, I noticed I still have 7 fish but they all seem to be female! Are my male gambusia changing sex? What's going on here? And one other odd observation. While several of the females were heavily gravid, I have never seen fry. Not one. Any thoughts?

Dean

Usually one hears reports of females becoming masculinized rather than the other way around. As to gravid females and no fry, I dunno.

Could the ones you thought were males maybe have been small sickly females? If a female's anal fin is folded up (due to ill health) it might look sorta male-ish. Some males develop a gonopodium later than others, and look like females for quite awhile (swordtails especially) but I've never heard of any livebearer becoming female after it developed a gonopodium.

You've had your gambusia since late last year but have never seen any fry? They're probably all getting eaten. There's one explanation down.

Next explanation. There are two types of sex changes: sexual repression and an actual gender switch.

The first is sexual repression, as exhibited by Xiphophorus montezumae (swordtails with really long swords). When a batch of a dozen fry is raised by itself in a tank, the first male to reach maturity will subdue all of the others so that it appears that there is only one, maybe two males. But in reality there's an even number of males and females. If the dominant male is removed, the other males will race to be the first to show gender. Again, one male or maybe two will predominate.

The second type is actual sex change. Xiphophorus hellerii (swordtails) can change gender from female to male. The female hormones overwhelm the male hormones during development, youth, and reproductive age. But if a female swordtail lives long enough, she'll hit menopause and her female hormones will decrease. At this point, she will grow a gonopodium and be, for all intents and purposes, a male. I've heard this from others but never witnessed it myself. I've also never heard whether or not she/he is fertile, although some have claimed she/he is (but I don't believe them because it is possible for swordtails to packet sperm, much as guppies do).

Some species are also provoked to change gender in the absence of a dominant individual. When the alpha male clownfish dies, the alpha female in the colony becomes male and takes over his role. Or so I've heard.

I'm not familiar with gambusia in particular. It's really weird that they would have a gonopodium and then lose it. Honestly, I don't believe you. Was it a fully differentiated gonopodium?

I've heard of this happening in the wild where prescription pills are dumped down drains and make their way into waterways and create transgendred fish, mainly the Detroit River.

The specific term is "intersex". Paper mill effluent can do it, as well as substances in treated sewage such as ethinyl estradiol, a primary component of most birth control pills.

I had a supposed wild variety and the run of the mill variety of swordtail change sex on me before. A pair I was breeding pumped out almost entirely females and after a while of growing up I guess the environmental pressures righted themselves and suddenly I had a better ratio of male to female. This wasn't that early on either, maybe a moth or two into development.

I had a supposed wild variety and the run of the mill variety of swordtail change sex on me before. A pair I was breeding pumped out almost entirely females and after a while of growing up I guess the environmental pressures righted themselves and suddenly I had a better ratio of male to female. This wasn't that early on either, maybe a moth or two into development.

You're saying that your fry all started out with triangular anal fins and that at a month or two of age, some of the fry's anal fins developed into gonopodiums?
That's how livebearers are. Mollies, guppies, platies, swordtails. All males are born with triangular anal fins which then gradually become slightly pointed, more pointed, flatter, closer to the body, and eventually become the fully developed gonopodium. That is the norm.

Some species of livebearer (Xiphophorus montezumae is a good example) can take years to sexually mature. They intentionally delay the transition of their anal fin into a gonopodium. If there's a dominant male, the repressed male will continue to carry a triangular anal fin indefinitely until the dominant male's presence is gone and he is free to develop. It's a survival strategy. If the smaller male pretends to be female, he doesn't get beaten up by the dominant male. Some Xiphophorus hellerii males (the pet store swordtail) get killed by the dominant male because in a small fish tank space they can't escape, and the dominant male sees the smaller male as a threat to his right to the females. So the X. montezumae technique of gender repression is a good thing; it keeps the smaller male alive. Then he has time to get stronger and grow larger, and when the dominant male gets offed by his own overdominance and general alpha male tomfoolery, the less dominant male is ready and able to quickly morph his anal fin and snatch the position of lead male (and lead reproducer in the colony) for himself.

A paragraph-long side note:
That being said, I have heard of true gender inequality in batches of fry. The Xiphophorus Stock Center in Texas has been inbreeding species of fish for 80+ fish generations. They maintain lines of identical fish for research purposes, so that if you order a fish in 1980 or if you order a fish today, you get the same fish. They're so inbred that they're all basically genetically the same. Anyway, I heard them say once that one of their lines produced nothing but female fry for a couple batches in a row, and that they were starting to get distraught. Eventually they got a male or two and were able to propagate the line again, but they were freaking out for a while there. They never concluded what was wrong. Maybe one gender of fry was being killed in the womb by a pathogen. There are lots of pathogens that influence baby gender ratios, like Toxoplasmosis gondii does in humans.
Sorry for being off topic, but I thought it was kind of interesting.

Edited by Okiimiru, 15 September 2010 - 09:36 AM.

Also, I would like to see some pictures of the gambusia in question.

The specific term is "intersex". Paper mill effluent can do it, as well as substances in treated sewage such as ethinyl estradiol, a primary component of most birth control pills.

True. And sad.
Estrogen contamination in water is actually the world's leading cause of human male infertility, by the way. It's a big issue in Asia. That's why GloFish® were originally developed; they would selectively flouresce in the presence of estrogen contamination and indicate to people that the water wasn't safe.

Edited by Okiimiru, 15 September 2010 - 09:39 AM.