Male fiddler crabs have an enlarged major claw that they use to attract a mate and discourage rivals. (Patrick Coin/Flickr)
Fiddler crabs are small crustaceans with a distinctive enlarged claw. They live on beaches, mud flats and marshes throughout the Chesapeake Bay.
Three species of fiddler crabs can be found in the Chesapeake Bay region:
Red-jointed fiddler crab, Uca minax
Marsh fiddler crab, Uca pugnax
Sand fiddler crab, Uca pugilator
Vary in color from tan to brown. Sand fiddler crabs are lighter than the other two species.
Males have one enlarged claw that can grow to 1.5-2 inches long. Females’ claws are equal size.
Carapace (shell) is squared with rounded rear edges
The red-jointed fiddler crab’s carapace has a groove behind each eye
Male marsh fiddler crabs have a royal blue spot on the center of the carapace
Male sand fiddler crabs have a blue or purplish carapace
Four pairs of walking legs
Carapace varies in size from less than an inch wide to 1.5 inches wide. The marsh fiddler crab is the smallest fiddler crab in the Bay region and the red-jointed fiddler crab is the largest.
Generally found in marshes, beaches and mud flats
Red-jointed and marsh fiddler crabs live in muddy areas in marshes. Red-jointed fiddler crabs are more tolerant of low salinities. Marsh fiddler crabs are found between the low and high tide lines, while red-jointed fiddler crabs dig burrows above the high-tide line.
Sand fiddler crabs live on sandy, salt and brackish beaches and flats. Unlike the other two fiddler crab species, sand fiddler crabs cannot survive in muddy areas because they can only feed over sediment particles of a certain size.
Fiddler crabs create tiny, sometimes elaborate burrows for mating, sleeping, refuge, and “hibernating” during winter. Burrows can be 2 feet deep.
Very active during the day
Return to their burrows at night and during high tide, plugging the entrance with mud or sand
Found throughout the Chesapeake Bay region
Red-jointed fiddler crabs range into the upper Bay, while marsh and sand fiddler crabs are generally restricted to the middle and lower Bay
Eat algae, bacteria and decaying marsh plants
Feed by sifting through sand or mud for food particles
Often eat in a puddle of water to help separate food from sand
Females have an advantage over males for finding and eating food because both their claws are small. The adult male’s major claw makes feeding more difficult.
Reproduction and Life Cycle:
Fiddler crabs mate every two weeks in summer
Males dig, maintain and defend a tidy, cylindrical burrow
To find a female partner, males stand next to their burrow while females walk past. The males wave their major claw to attract a female’s attention.
If a female is interested, she will stare at a male for a short period of time. The male then runs toward the female and runs back to his burrow. He repeats this motion several times until the female either moves on or follows him. If the female follows, the male drums the edge of the burrow with his claw, then leads the female inside, plugs the entrance and returns to the female to mate.
Females incubate their egg sponge for two weeks before returning to the surface. They release the eggs into the water, where they hatch and develop into juveniles.
Molts once or twice per year. During the soft-shelled period, fiddler crabs remain in or near their burrow.
The red-jointed fiddler crab is the most common and widespread fiddler crab in the Bay region
You can identify a fiddler crab species by examining its enlarged claw: the sand fiddler crab’s claw is smooth on the underside. If the claw is not smooth on the underside and had red joints, it is a red-jointed fiddler crab. Otherwise, it is a marsh fiddler crab.
Fiddler crabs breathe oxygen from the atmosphere
The male’s enlarged major claw is not for fighting predators, but primarily to attract a mate and discourage rivals
If a male loses its major claw, the remaining claw grows to the same size as the lost claw. The new claw that grows becomes the smaller claw.
Although the male’s major claw is large, fiddler crabs are not especially dangerous to handle. But be careful – the claws can break off easily.