Fiddler crab

Text and photos by Peter Vankevich

From mid-to-late summer into early fall when walking Ocracoke Island’s sandy trails and along sound-side areas such as Molasses Creek, you may see fiddler crabs scurrying away. Living in terrestrial groups, they can be seen by the dozens or hundreds at any one time.

Fiddler crabs are found throughout the world’s coasts, mangroves and salt marshes and comprise about 100 species.

Away from the ocean, you will typically encounter the sand fiddler crab (Uca pugilator). This species ranges along the Atlantic coast from Cape Cod to the Gulf of Mexico. Small in size, they are about one and a half inches across.

Easily identifiable males have one enlarged left or right claw, called a cheliped, and a much smaller one. The females have two equal-sized small claws.

Their common name derives from the up-and-down feeding motion of the smaller claw as it passes food to its mouth across the larger claw thus appearing to play a fiddle.

Male fiddler crabs will use their large claw for defense and to fend off rival males. During courtship, standing by their burrows, they will raise the large claws up and down to attract females. Displaying males have a characteristic pink or purple patch in the middle of the carapace.

During molting, missing legs and claws can be regenerated. If the larger claw is lost, the smaller claw grows into a larger claw. Then a new, small claw develops.

Another interesting anatomical feature is their eyestalks–protrusions that extend their eyes up, providing a better field of view for scavenging and spotting predators.

Their colors—ranging from tan to brown—provide the perfect camouflage while against dry and wet sand.

Like their close cousins the ghost crabs, fiddlers will dig burrows along the marsh edges and use them for safety, incubation and hibernation. When the tide comes in, most crabs retreat into their burrows, placing a sand plug at the entrance to keep water from flooding the burrow.

When environmental conditions are right, sand fiddler crabs can be abundant. Females will lay as many as 2,000 eggs. During the two-week gestation period following fertilization, she will stay in her burrow, usually located along the marsh edges, then come out and lay her eggs in a receding tide.

A fiddler crab’s life span, if it survives predators and capture as bait, is only two years.

Omnivorous scavengers, fiddlers feed on decaying detritus, algae and fungus. They have an important beneficial niche in the ecology of the island, not the least of which is that they are a food source for the rails, herons, egrets and ibises.

When feeding, fiddlers will sift through the sands and aerate the substrate that in turn encourages the growth of marsh grasses.

But fiddler crabs are sensitive to pollution, which may drastically reduce their numbers, and contaminants such as polychlorinated biphenyls (PCBs) and insecticide-fertilizer mixtures which, when ingested, may be transferred to bird and fish predators.

Due in part to their small size, they do not have much commercial value, though they are sold as aquarium pets.  Fishermen will use them as bait to catch mollusk-feeding fish such as black drum, pompano and sheepshead.

When you encounter fiddler crabs, take a moment to observe their behavior to reflect on the complexity of their seemingly simple lives.

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