While looking around a nearby beach at low tide for living Northern Moon Snails, I came across a slew of Hermit Crabs re-purposing the shells of the species I was seeking. Instead of growing protective carapaces of their own, these crafty crustaceans prefer the rental model, trading in their mobile homes for a larger version when their personal needs and shell availability coincide.
According to Kenn and Kimberly Kaufman's Field Guide to Nature of New England (2012), there are at least 5 species of Hermit Crabs in our area. The individuals shown here are Long-clawed Hermit Crabs* (Pagurus longicarpus). This species has a roughly cylindrical right claw which is larger and longer than the left.
*Thanks to Aaron Hunt (an editor at the Bug Guide) for confirming my tentative identification.