How a thumb-sized climate migrant with a giant crab claw is disrupting the Northeast's Great Marsh ecosystem

Nine years ago, I stood on the muddy banks of the Great Marsh, a salt marsh an hour north of Boston, and pulled a thumb-sized crab with an absurdly large claw out of a burrow. I was looking at a fiddler crab—a species that wasn't supposed to be north of Cape Cod, let alone north of Boston.