Size Matters

For fiddler crabs, it’s all about the claw. 

On a breezy early-summer afternoon, with dark storm clouds piling up in the distance, the shores of tidal Indian Field Creek near Yorktown are alive with motion; amid the spindly marsh grasses, diminutive fiddler crabs scuttle and scurry. 

At the tread of a footstep or the shadow of an intruder, they dart in retreat to one of the small holes that dot the ground, barely bigger than the diameter of a dime—entryways to burrows that might tunnel as far as a couple of feet down into the marsh. But just as quickly, once the alarm has passed, the crabs emerge again and resume their activity.

Among these small crabs, some are possessed of one very large—indeed an almost comically oversized—front claw, like an Arnold Schwarzenegger arm affixed to a toddler. And they seem to be engaging, with these steroid-caliber claws, in an odd sort of calisthenics or crustacean disco move, repeatedly thrusting their claws into the air.

This, however, is another kind of dance. These Travoltas of the salt marsh are males in search of mates and the gesturing claw is the crab equivalent of a “Heyyyy, girl,” to passing females. It’s “look at my claw and how I wave it,” says David Johnson, a marine ecologist and assistant professor at the Virginia Institute of Marine Science—and yes, it is the size, and themoves, that matter here.

However much The Claw might be the secret to the male crabs’ success with the ladies, lugging about an appendage as big as your body, notes Johnson, comes at a cost. “It’s like having an upright bass you have to carry around and wave all day,” he says. It takes a lot of energy to do that, and yet the claw isn’t good for much else. The crab can’t eat with it (fiddlers feed by sifting through the sand or mud for organic materials). He can’t dig his burrow with it. And though he can use it to spar with fellow males, it does him no good in fending off predators. Yet it’s essential enough that, according to a Chesapeake Bay Foundation field guide, if he should lose his big claw, the other front claw will grow large to replace it (and a new, small claw will grow where the first was lost). 

There are three species of fiddler crab in Virginia’s Chesapeake Bay region—the red-jointed, the marsh and the sand fiddler crab—and each species has its own particular wave. If a female likes what she sees, the pair will withdraw to his burrow, after which, in due course, she will become gravid with eggs—thousands of them, like tiny grains of quinoa. When the eggs are fully incubated, she’ll release them into the water at high tide, and within as few as 15 minutes they’ll hatch into minute larvae that “look nothing like fiddler crabs,” says Johnson. They will then spend weeks to months floating with the tides, molting through stages until they reach their adult form and return to the land as small fiddlers.

Though the larvae at sea have the ability to move up and down in the water column and thus are not subject entirely to random chance, nevertheless, they won’t necessarily come ashore again where they were hatched. They do, however, require water temperatures no colder than 63 degrees, Johnson says—and thus, the northward creep of fiddler populations is an indicator of warming seas. The crabs’ northern range, notes Johnson, used to be Cape Cod, but for several years he’s found them in the marshes of northeastern Massachusetts.

Their presence can also be a sign of sea-level rise. “If fiddler crabs are moving into your yard, that’s telling you that you need to move out—your yard is about to be marsh,” says Johnson.

Yet fiddler crabs may also play a surprisingly important role in maintaining the health of salt marshes, which serve as an important buffer against coastline erosion and flooding. 

Bethany Williams, a graduate student in Johnson’s laboratory, has been studying how the presence of fiddlers can have a positive effect on the growth of marsh grasses, which in turn are important for stabilizing marshes.

Marshes grow upward every year, and “the more grass you have, the more dirt will settle out of the water at high tide, which helps the marsh build,” explains Williams. 

Fiddler crabs act like the earthworms of the marshes; their burrows help aerate the soil and allow nutrients (including the crabs’ own droppings, or “biodeposits”) to reach the plant roots when water washes in with the tide. 

With fiddler-crab populations still healthy and abundant, then, they may serve as a small army with a big job—preventing erosion and maintaining the strength of our coastlines.

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