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The newly spawned eggs contain mostly highly molecularous components such as
fats, proteins, etc. Even in distilled water the osmotic pressure inside the egg
cannot be more than slightly larger than the osmotic pressure outside the egg-membrane.
As the membrane is permeable (see later) to oxygen molecules, water molecules etc.,
the water pressure may adjust itself between the inner and the outer liquid. As the
embryo develops and feeds on the yolk it gives away waste-products and if at least
some of these are not able to penetrate the membrane, the salinity inside the egg
will increase and as will the osmotic pressure. If now the salinity of the water
in which the eggs are kept is higher than normal to the species (compared with
its natural waters) the inner pressure will not increase to that value and this
will "burst" the membrane. Therefore the poor fry needs to make a higher osmotic
pressure, and this it forms by "eating his own tissues" by degenerating and decomposing
himself until the salinity reach the right value. But when eggs are kept in water of
lower salinity than normal to the species, the fry will come out too early (Aph.
petersi, Procatopus spec.). If eggs with "resting fry" are kept in water of certain
salinity, they may be hatched only by putting the eggs into distilled water. Then
they often will hatch in a few minutes. As an experiment I once hatched out many
half developed fry of Aplocheilus lineatus by putting them into
distilled water. Together with my younger brother, 9 years ago, I was
breeding the Aplocheilichthys macrophthalmus ("lampeye") in our normal
tap-water, which is alkaline (alkalinity about 5.0, that is about 14 German
degrees of temporary hardness) and of a salinity of about 300 ppm measured
(electrical) as sodium chloride. We had good and fertile eggs, but always
they turned out to be "resting fry", which died inside eggs after a few
weeks. Then we tried to hatch them by putting the eggs into distilled water,
and always this gave good fry in a few minutes.
I do not believe in this simple theory any more. The problems indeed are not so
simple. The hatching of "resting fry" by putting ripe eggs into distilled water
(or any water with much lower conductivity than the water, where the eggs were
stored) more seems to be a "dynamic phenomenon" than a "static one" and do not
support the above mentioned theory, that will say (if the membrane is permeable
to the simple salts in water also) that the water molecules (when putting eggs
from water with higher salinity into water with lower salinity) penetrate the
egg quicker than the various salts inside the egg can get out. This certainly
will increase the inner pressure and this may burst the membrane. But... when
I bred the Aplocheilichthys macrophthalmus in rainwater, there was no
"resting fry" and eggs hatched in due time. Certainly they always did this.
Here again the "theory of osmosis" could be used, but after all, the
rainwater not only had much lower salinity (conductivity) but also was
slightly acid, whereas the tap water was rather alkaline indeed. Therefore I
think we should concentrate the study of the properties of the egg-membrane,
that will say its permeability to oxygen (resting eggs) and various simple
salts (resting fry) under various conditions and in particular in dependence
of the pH of the water. Many years ago I read some paper prepared by Ruttner
(German biologist or water chemist?). He wrote that the permeability of
cell-membranes in aquatic vegetations to a very great degree was under
influence of the pH of the water.
"Resting fry" also may be hatched by adding "dry food" to the water. The
fry often will hatch long time before the water gets cloudy, that will say within
the first 1-8 hours. We do not know what acts in this type of artificial hatching.
Perhaps certain bacteria? Perhaps certain enzymes? However the method works very
well indeed and has always proved to be superior to the "osmotic method". I found
that "resting fry" of species which commonly are not considered as "annual fishes"
always could be hatched using distilled water or pure rainwater (if they have been
stored in water of higher salinity) whereas ripe eggs of the "annuals" very often
would not hatch by this method, but came to good hatchings when I used the "dry food
method". "They may have been very hungry, and came out as soon they saw the good dry
food" as Jack (Scheidnass) wrote me once.
Foersch found, that "resting fry" of Cynolebias bellottii lived in eggs for
up to a year. Also resting fry of other "real annuals" seems to be able to live
inside eggs in fully developed state for that long time or even more. But fry of
the "common killies" certainly will not be able to survive such a long resting period.
If you keep eggs with "resting fry" in water, you may inspect the circulation of the
bold-elements and by the speed of circulation you may judge the deepness of the dormant
life of the embryo. Very often you will find the circulation to be so slow, that there
seems to be no circulation at all. However, if you wait a little you will see the sudden
results of the few heart-pulsations.
No doubt the state of "resting fry" gives the species a chance to survive in
nature, when the natural waters dry up for some short time. You may force the eggs
of most killies into the state of "resting fry" simply by drying up the eggs and
keep them in moist air. Very many species have been shipped as live eggs in this
state. Most eggs are tough enough to stand the drying out in perlon or in peat.
But the state of "resting fry" is not the only resting state in the development
of eggs within the killies. If you breed the so-called "annual fishes" within killies.
The Cynolebias, the Pterolebias and the Nothobranchius and after spawning wash out the
eggs and place them only with little coarse peat or without any peat in small containers
on shallow water, you will find that not all the eggs will show an embryo or trace of an
embryo after 2-3 weeks.
These eggs could be called by the name of "resting eggs" (Foersch). To my opinion
these eggs represent a very interesting object for simple "home-made" research. Under
the above mentioned conditions, that is to say, a good supply on oxygen to the eggs
(shallow water, no peat and at least only little and coarse peat) I never found resting
eggs in any of my Aphyosemion (10 different species) except the "sjoestedti", a species
which also in other respects differs from the other aquarium-kept species. But in the 4
"species" of Nothobranchius (palmquisti, guntheri, melanospilus and kuntae) there
always were at least some "resting eggs" in batches of eggs which were kept in water
(or dried more or less hard). The same happens in Cynolebias (belottii, nigripinnis
and whitei) and "Cynopoecilus" (ladigesi and melanotaenia) and Pterolebias (longipinnis
and peruensis). There were no "resting eggs" in the various crosses within Aphyosemion
(except "sjoe" male to "cog" female) out of 50 eggs, only one egg did not catch fungus
and this sole egg stayed transparent (with some trace of "embryo" (or better a formless
mass of cells) for some months). In this genus, until now I have had 16 different types
of crosses which showed development of an embryo in the eggs. But in the 6 (7) types of
crosses with the Nothobranchius (I kept last year) only 3 (4) there were only "resting
eggs" which did not develop any embryo ("traces of embryo" in some eggs) within the months
they were under inspection. After some months the yolk in these eggs decomposed and fell
into more and more oil drops. None of the eggs from crossings in this genus gave any
hybrid, which grew up to maturity, but hatching was observed in 3 combinations.
N. palmquisti male to N. guntheri female seems to be the most promising combination
which no doubt would give you live-able fry. But the "tuberculosis" took mine when
they were about the show color.
The Cynolebias crossings easily gave eggs, but may get fungus. Until now only
3 combinations gave eggs that stayed transparent after 3 weeks, but none have showed
the smallest trace of development of an embryo.
The cross Cynolebias (Cynopoecilus) ladigesi male to C.
melanotaenia female (250 eggs) gave resting eggs as well as eggs with
normal development. Here is the data of this cross (8 different batches, each
10-15 eggs have now been mailed as experiments, so I do not have the exact
data for all eggs). - First cross with only one male to one (later two)
females gave during one week only 16 eggs. Of these only one did not catch fungus. 12 days after spawning
there was a small embryo in this egg. After 23 days there was black pigmentation.
As the fry did not hatch within 55 days after spawning, the sole egg was dried up
in peat.
- Next spawning had two small male "ladigesi" and 4 small females "melantoaenia".
Spawning in fine mud with only little coarse peat (in order to prevent the eggs to clot)
21-23 C. Water type the usual one (300 ppm NaCl, pH about 6.8)
- 30 March. I lifted out 15
eggs with big (nearly hatchable) embryo. These eggs were dried in peat (hatched 11
April, 12 fry came out, all fine). 10 eggs were reserved for experiments (water
filled ampulles, no air, sealed) in order to try to force them into the state of
"resting eggs".
- On 03 April, 4 batches with total 60 eggs (mostly without any
embryo) were shipped to "egg-friends".
- 04 April, eggs counted, there were still
169 (169, 60, 15, 10 = 254). 11 April the 169 eggs were sorted: 52 with big,
pigmented embryo, 117 with no embryo or only a transparent embryo.
- 15 April
another 60 eggs (4 batches of 15) were shipped to friends, in the few days
(11-15 April) many more of the "transparent eggs" have got black pigmentation.
Today there are (16 April) only 46 transparent eggs in the bowl, together with
20 with ore or less pigmentation. The fry that hatched on 11 April are just as
fine and sound as the pure "melanotania" I hatched at the same time in order to
have something to compare to. Of course, these two broods are not mixed, but
have their own tanks.
But the state of "resting eggs" is not the same as found in the above
mentioned species within Cynolebias, Pterolebias and Nothobranchius. Something
points in the direction that newly spawned eggs of many other killies may be
forced into the state of "resting eggs" by reducing of the oxygen concentration
in the medium, in which the eggs are stored (water, peat, etc.). When I recall
my first fumbling steps in the breeding of "annual fishes" (1955, Aphyosemion
"filamentosum") the thing that was most interesting was the fact that eggs of
this semi-annual fish did not develop, if they stayed in the fine mud I used in
my spawning tanks. As these eggs so easily are washed out of the mud, because
they do not adhere to peat, I again and again found transparent eggs in the mud
in tanks where no "filamentosum" had been for months. I these eggs after washing
were stored on shallow water without any peat, some of them got fungus, but all
the rest developed a livable embryo within the coming two-three weeks. The embryos
turned into the state of "resting fry" when they had finished their development
and only after some months they hatched out as feeble fry, most of which were
able to live. Rarely was I able to force them out by using distilled water, but
dry food always was effective. In 1956 I washed out the bottom mud of a tank where
no killies had been kept for at least 5 months. There were hundreds of transparent
eggs; none showed any trace of embryo. After drying up of this mud, no fry hatched,
so possibly these eggs were too old. Unfortunately I did not search for eggs in
this mud after the first watering.
A breeder of killies, who once had the Cynolebias (Cynopoecilus)
ladigesi in his tanks again and again, will find the small, so
characteristic eggs of this species, when he washes out his spawnings of
other species. The "ladigesi" is the "weed of the killie-breeder", but
fortunately the fry is so small, that it is not able to eat fry of the
species you want to raise. Again and again I hatch this species from peat,
where I never thought any "ladigesi" have been for years. By this way I
always have a stock of this species, even as many are fed to bigger killies
in order to keep down the stock. Even from a sample of peat which had been
dried, and certainly very hard, for an unbroken period of 450 days one fry of
this species came out and was found.
In order to control the theory of the "oxygen-concentration" in connection
with the state of "resting eggs" in killies others than annuals I now take from
most spawnings some eggs which are stored in small ampulles, about 10-20 in each
(the same ampulles which I used for shipping of eggs), without any air-bubble,
cork on and parafined at the "cork-end". These interesting experiments are recent
and should go on for some years before one may draw any conclusion from them, but
until now only very few eggs in some of the batches (never all in one batch) have
changed in a way that may be explained as the death of the egg. Here I have several
of these ampulles. 10 eggs C. ladigesi/melanotaenia cross, only 3
weeks in ampulle, all fine, no trace to spot of an embryo; 4 eggs of C.
melanotaenia, in ampulle for 11 weeks, all fine no trace of an embryo; 5
eggs same species, 9 weeks, fine and transparent.
From a sample of mud, spawned in Feb. 1957 at a local breeder of Cynolebias
nigripinnis in March 1959 after drying of the mud for about three weeks
hatched about 100 fry and washed out about 200-300 eggs, most of which were
transparent. 100 of these eggs, with or without any embryo, were at once put
into very fine (dirty) mud and stored in a glass totally filled with water
and airtight closed. These should stand for at least (?) year before they
once more are washed out and controlled. But I also sorted out some eggs at
various states of development in order to control how they would behave in
the small ampulles, where they could be controlled without opening of the
glass. They have now been in their ampulles for 6 weeks: 4 eggs with big
pigmented embryo; all dead and partly decomposed; 6 eggs with non-pigmented
embryo seems to be all right; also 6 eggs at the same state and 10 eggs
without any trace of an embryo.
The research on "resting eggs" may be of a certain interest to zoologists
and biologists. The breeders of killies however would no doubt be very interested,
if they were able to control this state of development. At present we are able to
store some species up to one year and even more by keeping the eggs in dry peat
(not completely dry indeed, but as moist as a good smoking-tobacco or a little drier).
In this way we are able to reduce the number of species which we keep "in water" as
fry or adults. During winter-time, when live food is scarce, we can "dry up" some of
the species, which will not take dry food and in spring when the live food shoals
again, we may raise new stocks from the dry peat. All my 4 "species" of Nothobranchius
have been sleeping this winter as eggs in peat in airtight glasses. Now all 4 species
are in the tanks again and (except N. palmquisti, which as usually
gave a lot of males) in pairs. But... if we could force eggs of any species
(indeed this is a very optimistic thought) into the state of "resting eggs"
and keep them in this state for many months (or even for years) this would be
real progress. However, we certainly do not know all the factors which
govern the state of "resting eggs" and much research have to be done on this
interesting subject. Perhaps we should try to find out the properties of the
membrane of the eggs, because if some factors could reduce the "pores" of the
membrane in a way that oxygen can not penetrate the membrane, we may be close
to the solution of the "resting eggs" in the Cynolebias and other real annual
fishes.
Foersch divided several batches of eggs from spawnings in equal portions
and found that the percentage of "resting eggs" differed in the two parts of the
same spawning. So the factors that control the percentage of resting eggs could
not only be some factors of inheritance.
Among aquarists who work on killies, there is a common belief, that repeated
drying and watering may start the development in the "resting eggs". Perhaps this
helps, but not at all in all cases (remember the 5 year egg of Cynolebias
bellottii at Dr. Foersch). Foersch also found that the temperature did
not influence on the "percentage" of "resting eggs" in different batches, but
that extreme low temperatures, that will say, temperatures close to the
freezing point of water, turned more eggs into the dormant life of "resting
eggs".
Foersch found that the batches of eggs gave the lowest "percentage" of resting
eggs, when he dried up the peat as soon after spawning as possible. This was with
Cynolebias bellottii. But in Cynolebias melanotaenia and in
Nothobranchius palmquisti, I found the contrary to be the case. But as I dry
up eggs much more than most other aquarists, my very sudden and "hard" drying
up may have stopped the development in eggs of the several batches of these
two species. In most of these breedings I divided the lot of eggs in two
portions. One was stored on shallow water in order to keep an eye on the
development. The dry peat was stored at the same temperature with lots of air
in the airtight plastic-boxes. When the eggs in the water after some weeks
showed a ripe fry, I watered the peat and found (in some cases) no
development in eggs or at least by far not so many eggs with embryos as is in
the eggs which were stored in water.
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