Background

At low tide, the marshes and sand flats of the Bay teem with animals foraging for food. One of the most abundant–and apparently busiest–creatures is the fiddler crab, a small, close cousin to the ghost crab. Male fiddlers are easy to identify by their enlarged fiddler claws, which they wave or saw back and forth in a "fiddling" motion–not to fight predators, but to attract mates and discourage rivals. Female fiddlers do not develop a large claw, so identifying their species must usually be done based on their close proximity to a male fiddler.
In addition to their distinctive claws, fiddlers are known for their interesting habit of creating tidy and sometimes elaborate burrows for mating, sleeping and "hibernating" during winter, and temporary burrows for refuge from predators during feeding times.

Fiddlers are small, semi-terrestrial crabs belonging the genus Uca. Currently around 100 species and subspecies have been identified, but three chief species are found in the Chesapeake Bay:

1. Red-jointed fiddler, or Uca minax. This is the most common and widespread fiddler in the Bay, and the largest of the three Bay species. Red-jointed fiddlers prefer brackish waters of low- to mid-salinity. It is nearly as large as its close relative, the ghost crab. The fiddler claw of this species has bright red joints. It prefers muddy habitats, and digs its burrow above the high-tide line, then creates a mud ledge to shade the entrance to its burrow.
2. Marsh or mud fiddler. Uca pugnax also is a widespread species but is not as tolerant of low salinities as the red-jointed fiddler. It is the smallest Bay fiddler; its body is less than an inch wide.
3. Sand fiddler, or Uca pugilator. This is a fairly small species that prefers sandy habitats and does not generally survive in silty mud. Sand fiddlers are lighter-colored than the other two species; to make a definite identification, turn over the male's fiddler claw–the underside of the sand fiddler's claw is smooth.

Fiddlers are very active during daylight hours, foraging, mating and digging burrows. At low tide they emerge from their burrows and roam over the flats in search of algae or decaying vegetation. Females have the advantage when it comes to finding and eating food, because both their claws are small and dexterous. Unlike other crabs, which use their claws or chelipeds to crush food or to grasp objects, the fiddlers' claws are used to pick up sediment, which they scrape with their mouths for food particles derived from organic matter and unicellular plants.

The small claws help the females sift through the sediment, which they then redeposit on the ground in the form of pellets after removing nutrients. Fiddlers often eat in a puddle of water to help separate the food from the sediment. The adult male's large claw inhibits the entire feeding process, so it must generally eat twice as much or twice as fast in order to obtain the same nutrition.

The three Bay species also help to maintain larger marsh ecological processes in consuming detritus from cordgrass and other plants. They contribute to the aeration of soil around marsh grasses, and thus support their growth, and stimulate the turnover of important nutrients in the soil. Fiddlers additionally play an important role in the food web by providing food for many other large predators, including blue crabs, colonial waterbirds such as egrets and herons and small mammals such as raccoons.

Habitat and Life Cycle

Fiddlers dig temporary burrows to protect themselves from predators during feeding, but they dig more permanent burrows for mating and sleeping, which often are tidier and more elaborate, and which they defend. These "homes" are about 1/2-inch wide and descend into the mud or sand between a foot and three feet deep, often ending in a space like a tiny room. The burrows may connect with others through tunnels. When feeding, fiddlers may skip into the nearest available burrow to escape predation, or dig a quick temporary burrow. All burrows provide shade during the hottest parts of the day and are refuges during high tide.

Fiddlers will roll up a small amount of mud and plug up their burrow's opening during high tide, trapping a small pocket of air. Unlike blue crabs, fiddlers obtain their oxygen from the atmosphere. They have gills, however, which must be kept wet to remain functional. In winter fiddlers stay in their burrows in a form of hibernation, and the following spring reappear from their submerged homes on the flats in droves.

Fiddlers exhibit many interesting forms of behavior–but there may be none more interesting than the mating ritual. In summer, fiddlers tend to breed about every two weeks. During this period the male fiddler digs, maintains and defends a tidy cylindrical burrow. When looking for a mate, he stands near the edge of the burrow, often alongside a string of other males and their (similarly well-maintained) burrows, while the females, returning from foraging, walk past. The male waves his large fiddler claw until he attracts the attention of an interested female, who then stares at him for a short period.. The male resumes his claw-waving, and if the female remains receptive, the male runs toward her, then runs back to his burrow, and repeats this motion several times until she either moves on or follows him to the burrow. The male then partly enters the burrow and drums the edge with his claw. If the female decides to enter the burrow, the male leads her to the terminal "room", returns to the opening to plug up the entrance with sand or mud, and descends to the female again to mate. The female incubates her eggs (or "sponge") for two weeks and returns to the surface to release her eggs in the water, where they will hatch and develop into juvenile crabs.

Other facts about fiddlers

• Challenges from other fiddler males rarely result in direct combat, since a fight could severely damage both crabs.
• Fiddlers can be either right- or left-"clawed."
• Adult fiddlers molt once or twice a year. During its soft-shelled period it remains near or in its burrow.
• If an adult male loses its fiddler claw, the remaining claw grows to the same size as the lost claw, the claw it regenerates becomes the smaller claw.
• Fiddlers have compound eyes, a little like a fly's, attached to eye stalks. A recent study conducted by John Layne at the Duke University Laboratory in Beaufort, NC, indicates that for the fiddler, the visible horizon is the animal's main point of reference, and fiddlers seem to respond not to the shape or size of a particular predator but to where the object or thing appears in relation to its visual horizon. If it "sees" an object above the horizon, it treats it as a predator and responds accordingly with defensive behavior. If the thing appears at or beneath the horizon, it is either treated as the same species or ignored. (Reported in online issue of nature; research written up by John Layne in 1998 issue of Journal of Experimental Biology.)

Middle fiddler image courtesy: Jeff Shields - VIMS

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Last modified: 12/11/03