The maximum length of the carapace in U. rapax is about 2.1 cm (Crane 1975, Kaplan 1988), although most specimens collected are between 7 and 18 cm (Koch et al. 2005). The major claw in male fiddlers is must larger than the carapace, with a maximum length of 6.3 cm (Crane 1975). As in other Uca species, the lifespan of the mudflat fiddler is relatively short, lasting only about 1.4 years (Koch et al. 2005).
Several other species of fiddlers occupy the estuarine habitats of the IRL, including: the Atlantic sand fiddler, U. pugilator; the saltpan fiddler, U. burgersi; the redjointed fiddler, U. minax; the Atlantic marsh fiddler, U. pugnax; the mudflat fiddler subspecies, U. rapax rapax; the longfinger fiddler, U. speciosa; and the Atlantic mangrove fiddler, U. thayeri. The Atlantic sand fiddler is mostly white to yellowish white, becoming paler during courtship (Crane 1975). Displaying males have a characteristic pink or purple patch on the middle of the carapace, which is often mottled brown in non-displaying males. The major cheliped (appendage bearing the major claw) of the male is yellowish white, often with pale orange at the base of the claw. The minor claw is white, and the eyestalks are buff to grayish white, never green. Many tubercles or bumps cover the outer surfaces of the claw. However, the oblique ridge of tubercles common in several fiddler crab species is absent in U. pugilator. Most populations of U. pugilator inhabit sandy shores from Massachusetts to Florida (Kaplan 1988), the Gulf of Mexico from Florida to Texas, and the Bahamas (Crane 1975).The saltpan fiddler is small, with a carapace length of about 1.2 cm (Kaplan 1988). The body is dark mottled brown, with red or pink on the carapace and red on the major claw. Walking legs are usually brown or striped with gray, and the palm of the major claw bears large tubercles. Most populations of U. burgersi are found in mud or muddy sand around mangroves or near the mouths of streams from eastern Florida to South America.The redjointed fiddler is large, with a carapace width reaching 2.3 cm (Kaplan 1988). It is aptly named for the red bands present on the joints of the appendages. The large claw bears many tubercles, which diminish to granules toward the bottom, and the upper finger curves down below the tip of the lower (Kaplan 1988). This species prefers muddy sediments around Spartina marshes, from brackish to nearly freshwater, in Massachusetts to northern Florida and Louisiana.The Atlantic marsh fiddler, U. pugnax, has a carapace approximately 1.2 cm long (Kaplan 1988). The body is usually brown or yellowish with a row of tubercles on the palm of the major claw (Ruppert & Fox 1988). This species is most abundant in muddy areas of salt marshes from Massachusetts to eastern Florida (Kaplan 1988).The longfinger fiddler, U. speciosa, has a small carapace length of about 1.1 cm (Kaplan 1988). Its color is seasonally variable, but usually remains darker than the characteristic brilliant white of the major claw. The palm bears a slightly curved row of large tubercles. These crabs inhabit muddy areas, mostly around mangroves from Florida to Cuba.The Atlantic mangrove fiddler, U. thayeri, has a carapace measuring about 1.9 cm in length (Kaplan 1988). The carapace and major claw are both brown to orange-brown (Crane 1975, Kaplan 1988), and both fingers of the claw are bent down (Ruppert & Fox 1988). This species is found on mud banks of estuaries and streams near mangroves, from Florida to South America. Females often build tall mud chimneys at the entrance to their burrows during breeding season (Crane 1975, Kaplan 1988).The subspecies U. rapax rapax is very similar in appearance to the mudflat fiddler (see Crane 1975 for diagnostic characteristics).Regional Occurrence & Habitat Preference: The mudflat fiddler is found in warm temperate to tropical coasts of the Gulf of Mexico and the western Atlantic Ocean from the Daytona Beach area on the east coast of Florida to São Paulo, Brazil (Crane 1975). Most populations inhabit mud or muddy sand banks around mangroves, river deltas, and near the mouths of streams and rivers. In some Brazilian mangrove forests, U. rapax is restricted to the high intertidal zone (Koch et al. 2005), and is most commonly found in medium-grained sand (Bezerra et al. 2006).