How long did the crab survive? Or did they not last a day or so? All inverts (and fish) need to be acclimated to your tanks temp and salinity. I once checked the salinity of the tank water that came from the lfs before I acclimated to my tank. It was like 1.018 and my tank is around 1.025 that's a huge difference! Next time try the drip method, a piece of airline tube running from your display into the bag, literally a drip a second into the bag for about an hour or until the water has doubled in volume...