Well, I don't know about any specifically 'sand' crabs, but there are a couple of good ones in "Origami Sea Life" by Lang/Montroll (great book btw). Now you have me inspired-those front views are great.
Go to http://www.origami.com click on diagrams, write crab and you will get two answers, one of which is Blue Crab by Marc Kirschenbaum. That might do.