That is not all from one crab and they generally try not to kill the crabs. There is some debate on how many actually die with the companies bleeding them claiming almost none and some other people saying up to 60%, so the truth is probably in the middle in there.

As far as keeping them alive in a tank it's not to hard. I have been working with them for a few years now and we have some at work. They mostly eat shrimp here not fish food. We have raised them from eggs. One of the big issues with raising them in captivity for the long term is we still are not sure how large chunks of their lifecycle works. For example while we have some ideas we do not have definitive proof of how they develop their sex. We think there are genetic factors but if I remember correctly there was a paper published a year or so ago that said there might be a temperature Factor, somewhat like turtles.

Fun fact, they will not eat micro plastics. We had an undergrad doing an experiment to see if they would eat micro plastics and how it would pass through their system but we were never able to get them to even ingest any.