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

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David Samuel Johnson, Virginia Institute of Marine Science

(THE CONVERSATION) 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.

As it turned out, the marsh I was standing in would never be the same. I was witnessing climate change in action.

The Great Marsh is on the Gulf of Maine, the piece of the Atlantic that extends approximately from Cape Cod, Massachusetts, to Nova Scotia, Canada. The marshes along the gulf are critical breeding sites for many bird species. But the water there is warming faster than almost anywhere else on the planet. And with warming water comes warm-water species.

Maryland blue crab and black sea bass, both southern species, are now being caught in Maine lobster traps. And fiddler crabs, whose charismatic...