Why Daphnia magna's eggs stay long time in brood chamber and turn into white color until death without hatching?

You should look for pathogens or parasites.

For more details, refer to 

Smirnov, N. N., 2013. Physiology of the Cladocera. Academic Press, San Diego.

or

Ebert, D., 2005. Ecology, Epidemiology, and Evolution of Parasitism in Daphnia. Bethesda (MD): National Library of Medicine (US), National Center for Biotechnology Information.

I attach Dieter Ebert's book; it is Open Access.

Good success to your work,

Christian.

Couple of things to look for.  Are the guts of the Daphnia full of your media?  Suggests how well they are grazing and feeding.  I don't see much in the photo you provided.  And secondly, what stages are the egg/embryos in?   Smaller size with changes in reproduction suggests the production of males and ephippia?  Do the eggs have just two and resemble the carapace of the Daphnia?  Males would not have any eggs.  All would relate back to your media and inadequate nutrition causing the population to crash.  Eggs move through 5 distinct stages of development and a healthy population of Daphnia should represent all stages.  The Daphnia is your photo contains eggs in first stage of development - look at others to see eyes on embryos up through full development.  Also for D. magna that brood size looks quite low.  I am only counting 7 eggs.  I think the population is stressed.  

I think that environmental conditions are bad and that includes the quality of the food. If you're using algae are being cultivated in a nutritionally poor environment, poor algae are food for herbivorous and therefore its size will be small and their offspring will not develop.

Really thanks for your replies!

Normally, I feed Daphnia with 2ml purchased YTC and 1ml algea per 2L culture medium. Maybe the concentration of food is too high, so it increases the infection rate on Daphnia and causes oily medium.

So, I starved these stressed Daphnia for 24h to weaken paratisms (?). Then now it seem to be better a little bit. Recently, I try to feed daphnia with decreased food concentration: just 1ml YTC/2L, 1ml algea.

Hope it will work and my healthy daphnia will come back ^^

Thank you again

For me that 's a situation I've never experienced. Feeding D. magna with live microalgae (I used Selenastrum capricornutum) was quite productive. Newborn individuals grew large, produced lots of partenogenic eggs and hatching ocurred about 9 days life. While the concentration of algae was high enough to produce a greenish colour of the aquous medium, I coul d see the green content of the digestive tract and the same individual could reproduce by parthenogenesis a couple of times, The absence of food caused the production of two resistance encapsulated eggs on the dorsal chamber, which was - as theory states and I observed - caused by the induction of sexual reproduction (this paper states it nicelly:       http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0104545). Ther fact that the medium you are using turns greasy or oily could have something to do with a reaction of Chlorella to some component/s of the the medium. Did you try to raise only Chlorella in that medium and watch the outcome of it? Parthenogenic eggs are dark brown (as I see them, maybe its dark yellow!). So if they turn white, they somtehing in the diet is wrong. But daphinds are feeding, cause if not they would not have produces tose eggs at all. Definitely, I would try the medium with only Chlorella (no daphnids) and watch waht happens to the algae. 

There's anothet thing I forgot to tell. Try to use some air bubbling into the flasks. Tht's important  to increase CO2 production to algae, to increase O2 to daphnids and to keep algae and crustacean in suspension. That agitation promotes a the contact with food and consummers. Use aquarium air bubbling.  

Depending on the purpose of raising Daphnia, Chlorella and yeast are not natural foods.  Healthy eggs of all the species I have seen are usually green with a small dots of yellow lipid (sometime this is a larger dot).  Less healthy eggs are less green and lesser amounts of lipid.  I published my first paper many years ago on feeding Daphnia a multi-species strew of algal species and had great luck.  As for the eggs turning white, my guess is a fungus - I've seen this is resting eggs but not in parthenogenetic eggs.  The number of eggs per clutch and timing are also indicative of the health of your population - in a large species like magna I would expect at least 10 eggs/clutch every 48 hours.  Fewer eggs and longer inter-clutch intervals are signs of an unhealthy environment.  Under optimum conditions it is common to see as many as 50 or more offspring/clutch.  Some Australian species (even larger than magna) can have more than 100 offspring/clutch.

Dear all helpers,

I finally found out that my problem is in algea.

I tried to put 4 beakers as follows to check my troubles:

1/ only daphnia water

2/ daphnia water + YTC

3/ daphnia water + algea

4/ daphnia water +YTC +algea

then put all 4 beakers into chamber for 2 days. But no thing strange happen, so at that time I can not find the solution.

After that, I kept feeding old algea and changed new YTC but culture condition was still not good.

Then I kept feeding old YTC and changed new purchased algea (normally I use my available algea inoculated in my lab). Fortunately, daphnia population recovered strongly with many eggs/ clutch, and the babies after hatching swim activately.

Condition is still good until now.

So recently I  just try to overcome algea culture system while still use purchased algea for my daphnia's safety ^^

Thank you for your help!