By Donna M. Recktenwalt
It’s a problem that many killikeepers encounter from time to time: badly skewed sex ratios in a batch of fry.
What causes these apparent shifts from the norm? And more importantly, what can we do about them?
Even the experts admit that there is much about fish embryology and development that they do not know, including the time that gender is determined. “Are killie eggs ‘differentiated’ as to gender when they are laid?” asks one hobbyist. Is the spawning water a factor? Are fertilization rates higher under certain conditions?
“Apparently … [the determination of sex] can happen at various times in the [development] process, depending on environmental conditions,” says Brian Watters. Environmental conditions, including population density and water temperature, pH, and hardness may all have their effects.
In Apistogramma species, studies have linked sex ratios to temperature, especially during the first month. Cynolebias may react similarly. It appears that pH matters more to fish from narrow temperature ranges, such as rainforest Aphyosemions.
“I don’t think that the answer to skewed sex ratios lies in a single simple factor such as pH (or temperature),” says Brian Watters. “While I have … not conducted controlled experiments, … one observation that I have been able to make consistently … is that if I raise a large batch of Nothos (or South American annuals, for that matter) under fairly crowded conditions, I invariably get a large dominance of males. In contrast, smaller batches raised under less cramped conditions usually (but not always) produce more even sex ratios.”
Lee Harper got contrasting sex ratios in different Diapteron species under similar conditions. Lee uses unaltered tap water with a pH that drifts naturally toward the acid in his tanks. Makeup water is at pH 7.4, but the tanks tend to be 6.0 to 6.5. “For Rivulus xiphidius … I lowered the hardness by about 1/2 by the addition of rainwater. This … produced more females than males.”
Foreign breeders, too, have observed the phenomena of skewed sex ratios. In “Soderjanie i razvedenie aquariumnih rib” (Russian, 1991) A. S. Polonskii wrote: “The temperature and chemical composition of the water can affect sex ratio in the fry of Cyprinodontidae. For example, at 22-25蚓 most of the fry will be females; if the temperature is not constant, most of the fry will be males. At the same pH =6.0, Epiplatys dageti will have more female offspring (>90%) in the soft water (dH about 5 degrees); in the hard water (dH 24) about 90% of the fry will be males. Aphyosemion gabunense in the acidic water (pH 5.0) will give more females, at higher pH=6.5 more males.”
“There has [also] been some recent work looking at various chemicals that mimic female hormones,” says Andrew Broome. These have been shown to cause sex ratio skewing in salmonids and may even be responsible for a decreased sperm count in humans. “The males that do appear are increasingly ‘female-like,’ [with] some even producing proteins that are normally only associated with egg production.” The suspect chemicals apparently come from a variety of sources and seem effective at very low concentrations.
Others, too, have linked skewed sex ratios to pH. Supposedly Dr. Joanne Norton did some experiments that showed that platies’ sex was a function of pH.
But sometimes there is no apparent cause for what we see in our tanks.
“I have … two maturing colonies of Aphyosemion (Diapteron) abacinum and georgiae,” Lee Harper wrote a year ago. “Each is about 30 or so members. The abacinum are almost all males; the georgiae all look like females at the same age and size. They were hatched and raised under close to identical conditions-water, temperature, etc. In this case it seems to be genetically controlled. The parents in both cases were young pairs.”
“Certain species of Rivulus … [regularly] throw predominantly one sex,” reports John Boylan, who got skewed sex ratios ranging from 4:1 (male to female), to as much as 20:1 among the various species he keeps. R. agilae showed a 10:1 ratio at room temperature; which reversed to 1:10 when the eggs and fry (until sexable) were kept at 75蚌 (24蚓) or higher. R. sp. Isle of Pines showed a consistent 4:1 ratio unaffected by changing water parameters, from soft/acidic to hard/alkaline at a constant temperature. “Interestingly, some of my other Rivulus species breed with even sex ratios.”
What to do if faced with a skewed sex ratio in a batch of fry? Generally there’s nothing you can do. But on the next batch, try changing the pH of the water; raising or lowering the water temperature; changing the water more often; or giving the fry more room.
There’s no guarantee that any of these will work for you, but it might be worth a try.
Additional Killie-related Reading:
Bitter, F. JAKA 22(3): 96-102 (May 1989)
Boylan, John, “Sex Ratios with Rivulus,” AquaTropica, Vol. 1, No. 4.
Chroke, Sharno. JAKA 17(6): 232-233 (Nov 1984)
De Boer, R. JAKA 23(5): 155-157 (Sep 1990)
Morenski, R. JAKA 17(1): 10-12 (Jan 1984)
Wilson, Mike. JAKA 18(1): 31-39 (Jan 1985)
Tropical Fish Hobbyist, January 1997.
Additional Non-killie Reading:
Conover, D.O. & B.E. Kynard, 1981. “Environmental Sex Determination: Interaction between Temperature &
Genotype in a Fish.” Science 213: 577-579.
Conover, D.O. and D.A. Van Voorhees, 1990. “Evolution of a Balanced Sex Ratio by Frequency-dependent
Selection in Fish.” Science 250: 1556-1558 (Dec 14, 1990).
Rubin, D.A., 1985. “Effect of pH on Sex Ratio in Cichlids and a Poeciliid (Teleostei).” Copeia 233-235, 1985.
Marx, Jean, 1995. “Tracing how the Sexes Develop.” Science 269 (29 Sep 1995) 1822-1824. (Fruit flies and
nematodes)
Sullivan, J.A. & R.J. Schultz, 1986. “Genetic and Environmental Basis of Variable Sex Ratios in Laboratory Strains
of Poeciliopsis lucida. ” Evolution 40, 152-8.
— G.C.K.A. Newsletter, December 1998