Just monitor these guys for a while - they die, it's going to pollute your tank. Remember, you are taking these guys from the low 30s to the 70s. Quite a shock... They should be very slightly open (most people will think they are closed) when left undisturbed (enough to get the edge of a knife in them. Thump them, and they should squeeze closed - you'll never get a knife in them.

BTW, they are MUCH better eating than oysters - as much as I like oysters. A heck of a lot easier to open, too.