A unicorn in the marsh

A fiddler crab, tee hee
A rare species of fiddler crab

In the Great Marsh, he should be a myth, like a unicorn. But there he is. Like a nervous cuckoo clock in the mud, he pops out two or three times before he completely shows himself. A male fiddler crab (Uca pugnax), with a blue burnish to his shell and the characteristic obscenely large claw. I am astonished to see him. Astonished because I am probably the first person to see a fiddler crab pop out of the Great Marsh mud.

The Great Marsh stretches like a verdant yawn in the Gulf of Maine, which is the whole of the ocean from Nova Scotia to Cape Cod. Here the salty waters are chilled by the Labrador Current, a river of sub-arctic seawater that is born between Canada and Greenland and plunges south like a fist. The warmer waters from the south are deflected off the flexed arm of Cape Cod and kept out of the Gulf of Maine by the colder waters of the Labrador Current. As my New Englander friends like to brag, particularly when the snow is deep and the wind biting, this is a place only for the hardiest of souls.

Ovigerous (egg-bearing) fiddler crab, Uca pugnax, caught in a Rowley, Massachusetts, salt marsh
Ovigerous (egg-bearing) fiddler crab, Uca pugnax, caught in a Rowley, Massachusetts, salt marsh. Credit: Ashley Bulseco-McKim

Fiddler crabs are a warm-water species preferring the temperate waters south of Cape Cod. I am of the same opinion. I can snorkel in swim trunks in Buzzards Bay in July, which is on the south side of Cape Cod, but when I try to do the same on Cape Anne and its rocky shores, it’s like an ice cream headache for my entire body. I, too, am a warm-water species.

I search the mudbanks for other crabs, but find only him. Maybe he is a unicorn. An accident of currents and luck. Then I search other tidal creeks. Jericho, West, Clubhead, Nelson. Though their numbers are low, I find more fiddlers. Their burrows, only as wide as my thumbnail, perforate high in the mudbanks near the hairline of the Spartina grass. Many burrows are abandoned. Active burrows are identified by what look like chocolate ice-cream sprinkles – what we locally call ‘jimmies’ – scattered around the burrow entrance. These are fecal pellets from a recently fed crab. A quick probe with my finger (a very scientific technique, I assure you) confirms the fiddler’s residency.

I search beyond the Rowley marshes and find more fiddlers. Directly behind J.T. Farnhams’ in Essex. Off Atlantic Avenue in Gloucester. Chubb’s Point in Manchester-by-the-Sea. Next to the Endicott Square Shopping Plaza in Danvers. Massachusetts indeed has a new resident. So does New Hampshire as the fiddlers have journeyed as far north as Hampton.

Salt marsh fiddler crab, Uca pugnax, Manchester-by-the-Sea, Massachusetts
Salt marsh fiddler crab, Uca pugnax, Manchester-by-the-Sea, Massachusetts. Credit: Jon Whitcomb

How can a warm-water crab invade the province of the cold, a place where it has never before been able to survive? The answer lies in the fact that the province of the cold is becoming warmer. As the climate warms, so too does the ocean as the overburdened atmosphere gives some of its heat to the ocean to hold, an arrangement as old as the Earth. In the summers of 2012 and 2013 the Gulf of Maine waters warmed to 68 degrees Fahrenheit from the typical and chillier 63 degrees of years prior.

Those five degrees may not mean much to those of us who can adjust thermostats, but those five degrees are significant to an animal that uses the environment to regulate its temperature. For fiddler crabs, those five degrees are the difference between scuttling across the marsh and an arctic death.

Fiddler crabs did not arrive in the Great Marsh in the form that we recognize them, with their snapping claws and rounded bodies. They instead arrived as larvae carried by the currents and tides. The larvae are mostly heads, translucent triangles with Pinocchio noses and topped with a single spine, with legs and tails dangling. For fiddler-crab larvae, 64 is the key that unlocks the Gulf of Maine. Below that number in degrees Fahrenheit and these drifting triangles do not metamorphose into the crabs that make thumbnail sized burrows in the marsh. But give that number a degree or five, and those once-thought unicorns of the Great Marsh become a reality.

And so, borne on the currents of climate change, the fiddlers have made a surreptitious arrival to the Gulf of Maine and the Great Marsh.

The consequences of this incipient colony are unknown. As burrowers, fiddler crabs are engineers that re-work the soil and the marsh chemistry. If the effect is positive or negative is not yet known. I can only give you the answer that is common given by scientists that can be utterly frustrating:

It depends.

Too many crabs and the marsh grass cannot establish, which may ultimately lead to marsh loss. Only a few crabs and the marsh grass grows better. What I can tell you for certain is that with the arrival of these new colonists, just as when we arrived in the 1600’s, the Gulf of Maine and the Great Marsh will be changed forever.

Already we have seen marine species such as lobster, flounder and hake shift northward as a result of climate change. Some, however, still consider climate change a myth, like the unicorn. But sometimes seeing is believing. Like a fiddler crab in the Great Marsh.

The science is here: Fiddler on the Roof: A northern range extension for the marsh fiddler crab Uca pugnax

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5 thoughts on “A unicorn in the marsh

  1. I just love that you told me this, I was surprised and a little skeptical, and then I fell in a crack in the marsh, and when I looked up, there was one of the little buggers, waving their big claw at me.

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