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Live mysis are problematic for most of us and I have not been able to keep them alive long, other report that they eat each other as a fast pace. The refrigerator is not a likely choice as it is normally kept at 40 degrees and would be too cold for them to survive.
dang. I was thinking about trying to get a population of Halocardina rubra breeding. They are a small red brackish shrimp from Hawaii, about the size of mysis, and they tend to breed all on their own. I had 7 in a critter keeper with no filtration and a small piece of live rock, fed them some spirulina once a week and they turned into about 40 shrimp. They aren't cannibalistic and they are virtually impossible to kill. They can actually go a couple years without eating.
The Halocaridina are really great little shrimp but, if I remember correctly they only produce a couple of times a year and only have a small number of young. I fed them to the seahorses for over a year (in addition to other things) but could never keep a population going and the cost tripled when Hawaii started limiting who could sell them (my supplier had their own ponds and raised them). I am a little surprised you can find them now. This is another case of regulating a sustaining source but not regulating the actual problem. The reason for their lowered population is all the koi ponds (koi love them) and other fish that have been put into their natural habitat. Restricting selling them for food, especially from a supplier that grows them and maintains their own ponds is not going to help repopulate them.
I totally agree about Hawaii's regulations, they take ineffective approaches for a lot of their wildlife. My shrimp would go gravid about every 2-3 months and held about 6-7 eggs at a time, so it would really only work if you had several hundred and only harvested a few. I'm not sure if I can still get them, but my source sold lots of 500 for $20.
If you need to feed live (I don't know if they will eat frozen), there is a great product out for hatching them. It has become pricey but I still recommend getting two (I used to keep dwarf seahorses as well as tried to raise erectus hatchlings and had to hatch them daily) so that you can start one while the other is hatching. These are a modern take on an old method and hatch out fresh eggs in about 14 hours (longer for older eggs but I have never had it take more than 16 and always had some available even after 12). It only requires daily rinsing and filling, nothing else and works wonderfully.
This is a modern take on a very old method using a pie plate and floating cup. Since there is no agitation (and little mess!) your hatch rate may be lower than a pop bottle but I ran two at a time for multiple feedings and could easily feed a brood of seahorse babies.
If you keep it under a light (for warmth and attraction to the cup) you will get a decent hatch but you will need to remove them within 24 hours as the water gets stale. Unless you need mega amounts, with good eggs you will be satisfied with the results.