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Re: Sex Ratio "Experiment"
Hi there Folks, eh;
Recently, I wrote an article for JAKA concerning skewed sex ratios of a
species of South American annuals that may be of some interest. It is lengthy.If
you are not interested, please delete now.
TTFN, eh
Jim
A Controlled Experiment Concerning
Skewed Sex Ratios in Simpsonichthys
By Jim Robinson CKA
For the last 4 years since I first obtained Simpsonichthys flavicaudatus Hellner
2, I have had severe difficulties with biased sex ratios. It has been so skewed
in favour of females that I have nearly lost them 3 times. The ratio I was
getting was approximately 1:25, male to female. Fortunately, I had found a single
male out of a spawn to continue the line. Consequently, I have been unable to
distribute this magnificent animal into the hobby. However, because of the huge
amount of females, the males did not last long. They literally spawned themselves
to death and wasted away. I guess they died happy, though.
About a 2 years ago there was a discussion on one of the killie mailing lists
about this exact problem in Nothobranchius only there was an overabundance of
males. One respondent stated that he had placed 2 fry of a species into
individual tanks and grew them in this manner. He stated that he ended up with a
higher percentage of females than he was previously getting.
I decided to try the same thing in the hopes of a more balanced sex ratio with
the Simpsonichthys flavicaudatus Hellner 2. The biggest problem for me, though,
was to get enough eggs in order to get fair test results. Finally, in the winter
of 1998 I ended up with 5 groups consisting of 1 male and 25 females. I felt that
this would be sufficient to do my experiment.
The theory behind all this, I'm assuming, is that the males, as they are
developing, produce hormones which prevent more males from developing. Hence,
when an entire spawn is grown together, the first male to develop produces a
hormone causing the other fish in the spawn to become females. This can only
happen because the sex of a newly hatched egg laying fry is not determined for up
to a month. After this time the sex has been established and all the fry can be
put back together. In this experiment, I gave the fry 6 weeks in isolation just
to be sure.
A similar situation to this can be found with guppies. Although the sex of a
guppy is determinate at birth, guppy breeders still attempt to separate males and
females as soon as possible. Apparently, the males give off a hormone which
stunts the growth of the females so the separation is necessary. I still remember
trying to separate guppies at 1 day old using a magnifying glass and a
flashlight. This is one of the reasons I am now working with killies and not
guppies!
Other experiments have been performed concerning sex ratios of fish. B.E. Kynard
noted that with the Atlantic silverside (Menida menida) sex determination of the
fry was determined under the control of both genotype and temperature during a
specific period of larval development. Uwe Romer, in his experiments with
Apistogramma spp. found that higher temperatures usually resulted in a higher
percentage of males and that higher pH values resulted in more females.
Both of these experiments dealt with the variable of temperature. Romer's
experiment also dealt with pH. In the following experiment these variables have
been controlled, along with all other foreseeable variables so that only the
number of fry per container varied. Thus, hopefully, only this single variable
would be tested.
The experiment itself was conducted from March 15 to May 1, 1999. Eggs from the 5
numbered groups, which were all collected at the same time, were all hatched at
the same time using the same water. Fry from each group were separated into 750
ml. and 1750 ml. containers. Five fry were placed in each of the 750 ml
containers and 10 fry were placed in each of the 1750 ml. containers. In
addition, half of the fry from each spawn were placed together in a 20 l. tank
with the same water. Any fry from each spawn not included in the experiment were
also placed together in a 20 l. tank. These were also included at the end of the
experiment as a mixed control group. The temperature in all containers was kept
at a constant 24 C.(75 F) Exactly half the water was changed every other day
using the same water at the same temperature. Every effort was made to keep all
of the variables controlled. These variables included:
1/ same collection date;
2/ same hatch date;
3/ exact fry numbers;
4/ same water;
5/ same amounts of water;
6/ same water temperature;
7/ same amount of water in each change;
8/ same amounts of food;
9/ the spawning adults were all kept in the same sized tanks at constant
temperature in a system that automatically changed water.
To simplify:
Group 1a) -4 (750 ml) containers with 5 fry each
b) -2 (1500 ml) containers with 10 fry each
c) -1 (20 l) aquarium with 40 fry in each
Group 2a) -2 (750 ml) containers with 5 fry each
b)- 1 (1500 ml) containers with 10 fry each
c) -1 (20 l) aquarium with 20 fry in each
Group 3a) -2 (750 ml) containers with 5 fry each
b) -1 (1500 ml) containers with 10 fry each
c) -1 (20 l) aquarium with 20 fry in each
Group 4a) -6 (750 ml) containers with 5 fry each
b)- 3(1500 ml) containers with 10 fry each
c) -1 (20 l) aquarium with 60 fry in each
Group 5a) -4 (750 ml) containers with 5 fry each
b) -2 (1500 ml) containers with 10 fry each
c) -1 (20 l) aquarium with 40 fry in each
All others -1 (20 l) aquarium with 52 fry
On May 2, all of the fry in the 750 ml containers from each of the numbered
groups were placed into a 20 l tank to complete their growth. The fry in the 1500
ml tanks were also placed in 20 l tanks. The fry already in 20 l tanks were also
moved to other 20 l tanks. To maintain identical conditions all tanks were placed
on a central system which does automatic water changes and is held at an exact
temperature of 25 C.(77 F).
The results were fascinating! With all of the extra attention these 'wee
beasties' received, not even 1 single fry was lost. Absolutely zero mortality!
The results can be summarized thusly:
Experiment # 1
Fry
Numbers Ratio
Group 1
a) 20 fry 7males 13 females
Approx. 1:2
b) 20 fry 5males 15 females
1:3
c) 40 fry 5males 35 females
Approx. 1:7
Group 2
a) 10 fry 4males 6 females
2:3
b) 10 fry 3males 7 females
Approx. 1:2
c) 20 fry 1males 19 females
1:19
Group 3
a) 20 fry 6males 14 females
Approx. 1:2
b) 20 fry 5males 15 females
1:3
c) 40 fry 3males 37 females
Approx. 1:4
Group 4
a) 30 fry 12males 18females
2:3
b) 30 fry 8males 22 females
Approx. 1:3
c) 60 fry 4males 56 females
Approx. 1:14
Group 5
a) 20 fry 8males 12 females
2:3
b) 20 fry 6males 12 females
1:2
c) 40 fry 6males 34 females
Approx. 1:6
Others
52 fry 4males 48 females
1:12
The average for all 5 groups kept in containers with 5 fry: a) 37 males-62
females Ratio:1:2
The average for all 5 groups kept in containers with 10 fry: b) 27 males-73
females
Ratio: 3:8
The average for all 5 groups kept in containers with half the fry c) 19 males-181
females
Ratio: 1:9
The average of all others: 4 males-48 females
Ratio: 1:12
As a point of interest, I repeated this same experiment with eggs from the same
numbered groups from May1 to June 15. The only difference was the age of the
original spawning groups of fish. It was important to do this second experiment
simply because there are some species of fish that produce more of a certain sex
when they are young and reverse this trend as they get older. The results were
almost identical!
Experiment # 2
Fry numbers
Ratio
Group 1
a) 20 7 males 13 females
Approx. 1:2
b) 20 5males 15 females
1:3
c) 40 4 males 36 females
1:8
Group 2
a) 10 4 males 6 females
2:3
b) 10 3 males 7 females
Approx. 1:2
c) 20 2 males 18 females
1:9
Groups 3
a) 20 6 males 14 females
Approx. 1:2
b) 20 6 males 14 females
1:3
c) 40 3 males 37 females
Approx. 1:12
Group 4
a) 30 12 males 18 females
2:3
b) 30 10 males 20 females
1:2
c) 60 6 males 54 females
Approx. 1:9
Group 5
a) 20 7 males13 females
Approx. 1:2
b) 20 5 males 15 females
1:3
c) 40 4 males 36 females
Approx. 1:9
All Others
23 1 males 22 females
1:22
The average for all 5 groups kept in containers with 5 fry: a) 36 males-63
females
Ratio: Approx. 3:5
The average for all 5 groups kept in containers with 10 fry: b) 29 males-71
females
Ratio: Approx. 3:7
The average for all 5 groups kept in containers with half the fry c) 19 males-181
females
Ratio: Approx. 1:9
The average of all others: 1 male - 22 females
Ratio: Exactly 1:22
It seems rather obvious from these results that, indeed, one can improve a
terrible sex ratio by limiting the number of fry placed together for the first
month of the fishes' life at least with this species. It does require a great
deal of work and a large number of tanks and containers. However, for some fish,
which have a history of skewed sex ratios, it may be the only way to ensure the
survival of that species. It actually appears from this experiment that, the
fewer the number of fry held in each container, the closer to equal the sex
ratio. To generalize that this will work for all species is extremely dangerous.
This experiment has proven only that sex ratios in this single species can be
altered. We may, by the fact that this is a South American annual species of the
genus Simpsonichthys, predict similar results for other Simpsonichthys species
and possibly for other South American annuals, but even this is somewhat risky.
However, based on these results, it would not be risky to say that this method
may just work in balancing sex ratios and that, at the very least, it is a good
starting point to try with other fish.
Obviously, the next step is to attempt this same experiment with Maratocoara
lacortei and Campellolebias dorsimaculatus. Both of these fish have also given me
considerable difficulty with skewed sex ratios. The difference being, I get far
more males than females. I wonder if this experiment will work in reverse so that
I get a larger number of females. Let's hope so, eh!
<soapbox mode on> Remember, the more fish we can breed in captivity, the less
fish we need to remove from their natural habitat. Also, at the rate
'civilization' is destroying the natural habitats, it is incumbent on each and
every one of us to attempt to breed and distribute to ensure the survival of all
species. A friend told me recently of a newly found species, Simpsonichthys
marginatus, which is not even listed.in 'A World of Killies' by Wildekamp or in
'Pearl Fishes' by Costa. Its natural habitat is gone. No more can be found. The
original habitat is now an enormous rice plantation and the species is, at the
very least, extirpated, if not extinct altogether. Collectors still can not find
any new locations for this amazing fish. "Now you see it...now you don't!"
<soapbox mode off>
schmidtcarney at ecr_net wrote:
> KI>>First, folks collect easy to collect data on
> KI>>hypothesized factors:
> KI>> mostly temp, pH, any others?
>
> KI>Perhaps you might want to consider age of parents also. Not sure this is
> KI>significant but on a number of species(in the Aphyosemion genus) I seem to
> KI>get more even ratios as time passed.
>
> Exclusively Killies members have told me that sex ratios for Epiplatys
> lamottei change as the male ages--if I remember correctly, mostly males
> when young and mostly females when old. Any ExK members out there that
> can tell more?
>
> Catherine
>
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