I'm thinking, after some cursory, spotty, and incomplete research, that this is a portly spider crab.

These aren't big crabs (4 inches across the back). They inhabit estuarine waters from Nova Scotia all the way down the North American eastern seaboard, and into the Gulf of Mexico. They are known for growing a 'garden' of sponges and seaweeds on their carapace, something like a chia crab. It's the hairs on their back that attract such growth. I should warn my brother-in-law. He's furry enough to run the risk of having all sorts of denizens getting a free ride on his back.

Photo source: Antonio Guerra[Image]