The fiddler crab, an important detritovore in the salt marsh community.
The fiddler crab, an important detritovore in the salt marsh community.
E. Zabel, courtesy of URI

Fiddler Crabs
Sand Fiddler Crab (Uca pugnax), Marsh Fiddler Crab (Uca pugilator),
Mud Fiddler Crab (Uca minar)

Color: Male fiddler crabs have one large claw and are brighter in color than females, having a purple grey or blue carapace with irregular markings of black or brown. The females have equal-sized claws and generally have a much more subdued coloration on their carapaces.
Size: Carapace is up to 1 inch long. Major claw on the male can be up to 2 inches long.
Habitat: Mud, sand, or salt marsh, near the high-tide line.
Seasonal appearance: All year; hibernate in burrows during the winter months.

Description

Fiddler crabs are named because of the extreme difference in the size of the claws of the male, with the larger claw resembling a fiddle. Several types of fiddler crabs are common to Rhode Island. All fiddler crabs are similar in shape, having a smooth carapace and a square-shaped body. The eyes are located at the end of two long and slender, movable eyestalks located in the center of the carapace.

The state also hosts a similar species called the marsh crab (Sesarma spp.). The marsh crab has two small, equal-sized claws and a square back with eyestalks on the outer corners of the carapace. Marsh crabs burrow with fiddler crabs, and although they are herbivores, they sometimes prey on the fiddlers.

Life History and Behavior

Fiddler crabs are the little crabs found living in burrows near the water's edge. The large second claw of the male fiddler crab is known as a secondary sexual characteristic and is used to attract a mate during the breeding season as well as to protect territories. The male crab will stand by the entrance to the burrow waving the larger claw in an effort to attract a female. Fiddler crabs are colonial, often living together in large clusters. Territorial fighting occurs between the males, and they will go to extremes to defend their burrows.

Despite their fighting, they travel in groups of thousands when feeding. They live in long, slanting burrows up to three feet long that they dig with their walking legs. The crab will plug the entrance when the tide rises and will emerge from the burrow after a receding tide to feed. They feed on decaying plant material present in salt marsh mud and sediment. They remain in their burrows throughout the winter months.

The fiddler crab can stay out of the water in damp ground for months at a time. They have gills for breathing in the water, but they also have a primitive lung, which enables them to live on land.

Special Notes


Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.