i don't know what large crystal you referring to, but the crabs can be removed if you have a certain deviceI await great things with this game. I already enjoy it as it is, so I know I will more so later on. Waiting for the update, seeing as I think I've done all the game has to offer. I have crabs blocking me now, and the large crystals too, which will probably be dropped after a certain update.
go to Dribble Village (NW), one of the house is a scientist that can craft a crab erase device if you help her with little errand
after you got the machine, erase 10 crab and go to Opal Harbor, there's bunch of crab at the beach, get an item they offer, give it to same scientist and you'll get another machine that this time will erase pig