Sex and Cichlids
No, it's not a daytime TV 'soap' but have I got your attention? Consider this then: All cichlidophiles know that when selecting a species group for breeding purposes, you pick some that are small and some that are large, anticipating that you'll have about an equal number of males and females. Afterall, it has long been known that the largest fish in a given brood will turn out to be male, while the smallest will end up being female. Ever wonder why that is?
The most obvious explanation is that males grow faster than females ... however, as it turns out, all things obvious are not necessarily true. Studies have shown that at least for some cichlids, it isn't sex that determines size but rather, it's the other way around; size is determined by sex. (1) Strange as it may sound to you, some cichlid individuals become males because they are growing faster and becoming larger than their congeners, while the smaller individuals are maturing as females.
How is this possible? Simply because sex is not determined in cichlids in the same way that it is in humans. Human sex determination is all in the genes. A female has two X chromosomes while a male has an X and Y chromosome. Thus the female always contributes an X to each offspring while the male has a 50 percent chance of contributing either an X or a Y. The sex of each offspring is entirely determined by which chromosome the male contributes.
Not so in fish, and as befits the vast diversity in fish, there is considerable environmental influence by such as pH, temperature, hormones and social conditions. For example, genetically female guppies will mature as males if raised in extreme temperatures. Ruben (1985) determined that sex ratios in broods of Pelvicachromis pulcher varied with pH. He determined that at 180 days post hatch, 96 percent of a given brood would be males when raised at pH 5.0 whereas at pH 6.9 only 20 percent became males. Similar results have been obtained by other researchers with Apistogramma cacatuoides.
Interestingly this is not an 'all or none' phenomenon and is very species dependent. Not even the high temperature, low pH experiments produced 100 percent males; The greatest differences were 99 percent males in Apistogramma diplotaenia at ph 4.5 and 29°C versus 4 percent males in A. caetei at ph 6.5 and 23° C. In the only African cichlid studied, Uwe Romer and W. Beisenherz (1966) determined that neither temperature nor pH had any influence at all on sex determination in Pseudocrenilabrus multicolor.
The summary results for all cichlids studied, indicate that in a group of fry, the largest fish in a given tank "decides" to become male, while the smallest "decide" to become females. All experiments show that sex determination in cichlids occurred somewhere between six months and one year and between 50 and 100 mm SL (standard length). The cichlids become one sex or the other not simply based on their absolute size, but rather based on their size with respect to the fish around them.
References:
1. Francis, R.C., and G.W. Barlow. 1993. Social control of primary sex differentiation in the Midas cichlid. Proc. Nat'l. Acad. Sci. USA 90:10673-10675
2. Ruben, D.A. 1985. Effect of pH on sex ratio in cichlids. Copeia 1985:233-235.
3. Romer, U, and W. Beisenherz. 1996. Environmental determination of sex in Apistogramma and two other freshwater fishes. J. Fish Biology. 48:714-725
No, it's not a daytime TV 'soap' but have I got your attention? Consider this then: All cichlidophiles know that when selecting a species group for breeding purposes, you pick some that are small and some that are large, anticipating that you'll have about an equal number of males and females. Afterall, it has long been known that the largest fish in a given brood will turn out to be male, while the smallest will end up being female. Ever wonder why that is?
The most obvious explanation is that males grow faster than females ... however, as it turns out, all things obvious are not necessarily true. Studies have shown that at least for some cichlids, it isn't sex that determines size but rather, it's the other way around; size is determined by sex. (1) Strange as it may sound to you, some cichlid individuals become males because they are growing faster and becoming larger than their congeners, while the smaller individuals are maturing as females.
How is this possible? Simply because sex is not determined in cichlids in the same way that it is in humans. Human sex determination is all in the genes. A female has two X chromosomes while a male has an X and Y chromosome. Thus the female always contributes an X to each offspring while the male has a 50 percent chance of contributing either an X or a Y. The sex of each offspring is entirely determined by which chromosome the male contributes.
Not so in fish, and as befits the vast diversity in fish, there is considerable environmental influence by such as pH, temperature, hormones and social conditions. For example, genetically female guppies will mature as males if raised in extreme temperatures. Ruben (1985) determined that sex ratios in broods of Pelvicachromis pulcher varied with pH. He determined that at 180 days post hatch, 96 percent of a given brood would be males when raised at pH 5.0 whereas at pH 6.9 only 20 percent became males. Similar results have been obtained by other researchers with Apistogramma cacatuoides.
Interestingly this is not an 'all or none' phenomenon and is very species dependent. Not even the high temperature, low pH experiments produced 100 percent males; The greatest differences were 99 percent males in Apistogramma diplotaenia at ph 4.5 and 29°C versus 4 percent males in A. caetei at ph 6.5 and 23° C. In the only African cichlid studied, Uwe Romer and W. Beisenherz (1966) determined that neither temperature nor pH had any influence at all on sex determination in Pseudocrenilabrus multicolor.
The summary results for all cichlids studied, indicate that in a group of fry, the largest fish in a given tank "decides" to become male, while the smallest "decide" to become females. All experiments show that sex determination in cichlids occurred somewhere between six months and one year and between 50 and 100 mm SL (standard length). The cichlids become one sex or the other not simply based on their absolute size, but rather based on their size with respect to the fish around them.
References:
1. Francis, R.C., and G.W. Barlow. 1993. Social control of primary sex differentiation in the Midas cichlid. Proc. Nat'l. Acad. Sci. USA 90:10673-10675
2. Ruben, D.A. 1985. Effect of pH on sex ratio in cichlids. Copeia 1985:233-235.
3. Romer, U, and W. Beisenherz. 1996. Environmental determination of sex in Apistogramma and two other freshwater fishes. J. Fish Biology. 48:714-725
