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Cory breeding logistics

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    Did anyone here try raising large amount of cory fry? I may be wrong about the estimates and staining results, and have no fry yet, but it seems that I have about 400 fertile eggs out of about 600 deposited….probably to hatch in 2-3 days. How to deal efficiently with this is not obvious… I don’t even understand if the space requirements are prohibitive or not.



    Hi Mikev

    About 200-250 C. aeneus at once, but that’s some time ago. The problem is not the space for the fry. The smaller the tank, the better they get food. I did it with two 30 lt tanks. Two tanks, half risk. But what is desired: a lot of time to feed – at least twice a day – and to change the water. With that dense population waterchanges are decisive. The system of small tanks hung up in the main tank won’t work with large quantities of fry I think. But I never tried that.

    With seperate tanks you have the opportunity to use Methylenblau.

    But be aware that with growth you need one or two larger tanks.

    Cheers Charles



    Thanks Charles! — then this may be doable. Yes, definitely more than one tank, random fry tank disasters do happen…. I guess 2-3 10g (40lt) tanks then.

    The fish incidentally is indeed C.aeneus — but it is the long-finned version which is not too common yet. And it took some effort and a long wait to get them spawn…so I want to raise all I can.



    Not as bad as I feared, but I do have 100+ babies now and it will probably increase yet. I guess 2 5g tanks for now…. donno what I got myself into.

    (I will need to re-examine how staining works on these eggs… quite a lot of them cleared the stain but failed to develop anyway).



    160 or so at the end.

    Before I forgot: http://www.micropress-inc.com/fishpic/snailsandeggs.jpg

    does anyone know the exact nature of the white core in eggs — seen in the above photo. Such eggs seem never fertile, but just what is going on in there?



    Not sure why this happens, but I do get the same thing in infertile Rivulus eggs.



    And I’ve seen it in pleco (Stirusoma) eggs too… so here are 3 unrelated types of eggs with the same problem.

    I really only want to know if white core is caused by infertile (non-developing) egg going bad OR white core is a disease (fungus?) that kills the embryo.



    Don’t really know anything about breeding or raising corys, but it seems to me that another possible cause of dead eggs like this is an embryonic lethal mutation. I have seen these occasionally in in wild caught and farm raised danios as well as lab strains. Animals that when reproducing produce a lot of eggs, can afford carry recessive lethal mutations because there are so many other survivors. Mutations that become apparent at later times can be bred or eliminated based on the breeder’s desires, but embryonic mutations are often overlooked.

    The white core of the egg is just the remains of the egg or embryo after it dies, if nothing (like fungus or bacteria) eats it. The embryo becomes opaque and reflects light (or blocks light from below) where it would have been transparent before.

    Once the unfertilized eggs are either counted or eliminated, deaths due to embryonic mutations would most commonly be about 25% (for a recessive) of the remaining eggs. Finding something like this kind of ratio is a clear indication of a genetic cause. Breeding experiments could confirm it. Once the embryos die, they will often look like unfertilized eggs.

    Recessives like this would most frequently be found when breeding siblings together, or less frequently in fish related to each other more distantly.

    Your picture does not have the fluffy look of fungus that I have seen on eggs which would be extending out side of the shell.

    If you can get a good look at the developing embryos before they die, you may be able to see some characteristic flaw in their morphology (or maybe not). A magnifying glass or a dissecting microscope would be good, but your camera might be able to show real big defects is they exist. Especially if you can get the embryos out of the shells (possibly with small pointy tweezers), not sure how easy that might be with cory eggs.



    Very interesting, Bill!

    I wish I took a careful count but roughly 25% white core eggs seems correct. Approximately 600 eggs total, resulting in 160 hatches, and about a third of bad eggs having white core (there were more eggs that did not have white core but still failed the stain test). And the fish is definitely highly inbred (LF strain, maintained by sibling breedings). It might be interesting to see if this can be corrected by crossing with a wild caught (LF gene is dominant making this easier).

    checking this against other cory species I bred: I do not recall any white cores in wild caught or nearly wild caught fish. The problem was observed with c.venezualian and c.panda (both originating from farm strains and thus likely inbred)… I’ll try to determine the ratio for c.venezualian when they spawn next.. but for c.panda it does not correlate well with your embryonic mutations theory, I’m certain that the ratio is way above 25% (last spawn — 2 days ago — was 12 eggs, 9 white cores and 3 fertile to hatch tomorrow probably)… I should have kept running count but i think it is about 2/3 white cores.

    It seems like more data is needed.

    Thank you!

    (I doubt I can figure out the problem with my camera…I actually tried, just to see if there is an embryo in white-core eggs or not, and cannot see anything.)


    kim m

    The eggs with white core is usually referrred to as infertile among Corybreeders. I do it too :)

    So far I have bred 62 species of Corydoradinae and it gives a clear picture; as mentioned above, species that produce a lot of eggs give a lot of white-cored ones too. It is my experience. I see this mainly with species from the aeneus-group and the elegans-group.
    It has nothing to do with inbreeding I believe, as I have several rare species that has been bred for genereations from an original stock of just a handful of specimens that give very good fertility and hatch rate.



    Just to be more clear about what I am talking about. The “white cores” I would just call dead eggs or embryos. They could be dead because they were not fertilized (which could be in any proportion) or they could die from a genetic defect after they are fertilized (the simplest thing to expect would be 1/4 to die from a genetic cause). I would expect some unfertilized eggs in almost any large egg clutch. If the unfertilized eggs (at least in zebraafish) die pretty young. If these dead eggs (due to lack of fertilization) are removed or counted, then the ratio of later dieing eggs may reveal a ratio indicating a genetic cause. some mutations can cause earlier deaths that could occur in the same time period as an unfertilized egg would die, but a close look at the developing egg might show something weird, indicating a mutation.

    Here is a link to a normal embryonic developmental sequence in zebrafish in photographs. I could not easily find one for corydoras but I would expect them to be pretty similar.

    What is the stain exclusion thing? I am not familiar with that. If it is dye does not get into a live embryo, than that should not stain a fertilized embryo which may die at a later time for genetic reasons.



    Staining: there are three outcomes I saw: (1) the egg rejects dye or clears it out quickly –> fertilized. (2) the egg is uniformly stained (3) the egg shows strong blue core and lighter blue around. Both (2) and (3) are not fertile, but it seems that the cause may be different. (3) is probably the case when a white core was present. I will take photos the next time i have a spawn from a “white core” species. So there are two types of “dead eggs”, and there are likely two causes.

    Case (1) does not guarantee that a fry would catch… it seems that for LF aeneus it is “probably”, but for c.venezualian (also aeneus group) about half of the fertile eggs do not hatch (and then some fry that does hatch looks weak and dies quickly).

    I’m not sure about the correlation between white cores (or unfertilized eggs) and the species being prolific… c.panda is not particularly prolific, at least in my case, but white core %age is very high… (and for c.panda white core seems indeed synonymous of dead eggs); c.pygmaeus is seems quite prolific but I never saw them producing white core eggs. Ditto for c.paleatus.

    Here is another view of white core eggs http://www.micropress-inc.com/fishpic/venez-fung-eggs.jpg. sorry for the horrible mess, I missed this spawn (did not expect this group to spawn, this is my backup venezualian group… it is a miracle I got a few babies out of this).

    Bill, how would you classify the case of high %age of weak fry (ones that die within a few days): genetic or not? I see this only with c.venezualians (I have not bred too many cories :( ), saw this also affecting at least a couple of rainbow species.



    Opps, goofed up the previous link, it actually has some mutant embryos in it. This is a better one:


    Does this make sense? Staining: sounds like you expose the eggs to something like methylene blue and see if it stains the egg inside the shell. Staining would indicate that the egg/embryo inside the shell is not keeping out the stain. This would happen if the cell membrane of the egg or the cell membranes of the cells of the embryo are dead. The cell membrane breaks down and dye molecules can get in.

    high %age of weak fry: does this mean fry showing up as weak at later stages of development?

    What are the indications of the fry’s weakness?

    Could be a genetic thing: some mutations (or possibly other non-genetic causes) can result in fry without swim bladders inflating correctly. They can have feeding problems making them weaker and will mostly be on the bottom. In zebrafish they are often bent in the middle of the body since the inflation of the swim bladder helps to straighten the body of the embryo as it develops. This also results in them twirling around when they try to swim. I have accidentally raised some of these fish, but they are delicate.

    I have also seen eggs developing in fungusy messes, have weirded out development. Not sure if it is due to the fungus using up the oxygen or the fungus putting out something which impedes development in some way, but many fish coming out of this kind of a situation will not do so well.

    I have also seen on occasion some fish not hatching out of their egg shells when they should. As a result of being confined in the egg shell for so long they can also con out bent. .



    Yes, much better link — thank you.

    high %age of weak fry: does this mean fry showing up as weak at later stages of development?

    What are the indications of the fry’s weakness?

    For cories: healthy fry would run around once in a while especially if disturbed (slight water motion), weak does not try to move and I cannot see much of fin flickering…tends to die within 2-3 days, before the yolk sac is consumed. With c.venezualian where “weak fry” seems to be very common, but I’m not determining them accurately… I’m losing more fry in early age than the number of weak I see.
    Sample numbers (previous hatch):
    35 eggs ==> 15 fertile eggs ==> 9 hatches (7 “normal”, 2 weak, other 6 eggs developed but failed to hatch, either embryo too weak, or died before hatching) ==> 4 1cm babies now. Not very impressive.

    I’ve seen “weak” fry in two rainbowfish species… there it is much more obvious, the fry tends to spin in place, cannot reach the surface (even if it is 0.5″ away), and dies within 2-3 days. In both cases weak fry is nearly 100%, making their breeding difficult, bordering on impossible.



    Mike, out of interest, which rainbowfish species?

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