Keeping up with modern life

Black clawed crab on a piling:

Half out of the water, about 1 inch across.

Black clawed crab in hand, pretending to be a clump of dirty styrofoam:

And holding a piece of the styrofoam as a sample.

I had gone back to the dock to see if I could find another of those mystery critters, and get a better look at it, but didn't see any. But these little crabs were foraging at the water line on most of the pilings.

The one I caught rolled himself into this tight little ball. Every time I turned him over, he immediately flipped over and closed down again. In his environment; dirty barnacles on tarry wood, and all encrusted with styrofoam bits; he blends right in. I took several photos of clumps of styrofoam, thinking they were crabs. And I probably saw crabs that I thought were styrofoam, too.

It makes me wonder; what, in unpolluted waters, do these crabs imitate? Or is this a behaviour they have picked up in response to the stresses of life under the docks?