Check it out. This is the same species of sand-sifting crab sold on this site. Better hope yours doesn't get this big! And I thought the 3" on in my tank was huge!
When me and my brother used to go surf fishing down in Florida we used to dig those up out of the sand to fish with, we called them sand flea's. Of course they were much much smaller than that. about an inch long maybe alittle larger, but the whitting loved them.