All those little "pebbles" are hermit crabs! They have one large claw. At first I thought they were very large bugs. When I approached them, they would scurry into their holes in the sand, and drop in feet first, holding the claw aloft like a flag!It reminded me of the Cheshire Cat in Alice in Wonderland:
"the crab disappeared quite slowly, starting with the legs, and ending with the claw, which lingered for quite some time after the rest of him had gone."