Knobs and bumps are not unknown among crabs, but a crab that is among the world’s bumpiest thrives on Orange County shores. 
It’s called, appropriately enough, the lumpy crab, and sometimes the rubble crab. Both common names get the point across: a shell so burdened with bumps it looks a bit like a pile of gravel.
That might be the point. This red-brown crab is often found under rocks, where it remains immobilized, often in small holes, unless poked and prodded. The bumps are likely a bit of evolutionary camouflage to protect the crab from predators.
The crab also can be recognized by the darkened tips of its pinching claws.
