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Species Name: Uca pugilator
Common Name:      Atlantic Sand Fiddler Crab

I.  TAXONOMY

Kingdom Phylum/Division: Class: Order: Family: Genus:
Animalia Arthropoda Malacostraca Decapoda Ocypodidae Uca


Male Atlantic sand fiddler crab, Uca pugilator. Photo L. Holly Sweat, Smithsonian Marine Station at Fort Pierce.  



Photos of the female (top) and male (bottom) U. pugilator. Photo courtesy of Bjorn Tunberg, Smithsonian Marine Station at Fort Pierce.

 

Species Name:
Uca pugilator Bosc 1802

Common Name:
Atlantic Sand Fiddler

Species Description:
The Atlantic sand fiddler, Uca pugilator, is one of approximately 97 species belonging to the family Ocypodidae (Rosenberg 2001). Members of this family are characterized by a thick, squarish body and herding behavior (Ruppert & Fox 1988). Male crabs also bear one greatly enlarged pincer, either right or left, for combat and mating rituals; whereas, the claws of females are roughly equal in size. Fiddler crabs share many common morphological characteristics and behaviors, but identification of species is usually easily achieved through examination of body color and claw structure. Body color of 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 like some similar species. 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.

Potentially Misidentified Species:
Several other species of fiddlers occupy the estuarine habitats of the IRL, including: the saltpan fiddler, U. burgersi; the redjointed fiddler, U. minax;


the Atlantic marsh fiddler, U. pugnax; the mudflat fiddler and its subspecies, U. rapax and U. rapax rapax; the longfinger fiddler, U. speciosa; and the Atlantic mangrove fiddler, U. thayeri. The palm, or interior surface, of the major claw in all these species is rougher than that of U. pugilator.

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 (movable top part of the closable claw) 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 carapace of the mudflat fiddler, U. rapax, is about 2.1 cm long and light tan in color (Kaplan 1988). The color of the major claw is similar, with a darker lower palm and finger. The center of the palm is almost smooth, but still bears small granules. This species inhabits mud banks near mangroves and mouths of streams from Florida to South America. Crane (1975) defines the Daytona Beach area on the east coast of Florida as the northern limit for U rapax. The subspecies U. rapax rapax is very similar in appearance (see Crane 1975 for diagnostic characteristics).

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).

 

II. HABITAT AND DISTRIBUTION

Regional Occurrence & Habitat Preference:
Populations of U. pugilator inhabit the shores from Massachusetts to Florida (Kaplan 1988), the Gulf of Mexico from Florida to Texas, and the Bahamas (Crane 1975). The Atlantic sand fiddler is found on muddy to sandy soils, but is usually more prominent in sandier areas containing scattered shells and stones (Crane 1975). Individuals also occupy areas around mangroves and in salt marshes, among stands of the cordgrass, Spartina alterniflora (Brodie et al. 2005).

IRL Distribution:
The Atlantic sand fiddler is located throughout the IRL, mostly on sandy beaches near mangroves and salt marshes.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:

The maximum carapace width for U. pugilator is approximately 2.5 cm, but most individuals collected in the field measure up to 1.4 cm and 2.1 cm for carapace length and width, respectively (Crane 1975). The major claw in males is much larger than the body, with a maximum length of 4.1 cm (Gosner 1978) and up to 3.5 cm in most specimens collected in the field (Crane 1975). Little information is reported for the maximum age and average lifespan of U. pugilator. However, the lifespan in a similar species, U. rapax, is only about 1.4 years (Koch et al. 2005).

Abundance:
Although fiddler crabs are territorial, the species is quite social and lives in large groups. When U. pugilator was first described by Louis Bosc in 1802, he observed that “thousands or even millions” covered the beaches of the Carolinas (Crane 1975). Today, those numbers have declined as a result of pollution and habitat degradation, but relatively large populations can still be found. Little information exists for abundance estimates of U. pugilator in the IRL, but field densities in South Carolina populations have reached up to 75 m-2 (Pratt & McLain 2006). Studies on fiddlers in North Carolina revealed that females are more abundant than males (Colby & Fonseca 1984), a pattern that likely exists for populations in other locations based on courtship and mating behaviors (see below).

Molting & Limb Regeneration:
Like other arthropods, fiddler crabs must molt in order to grow larger. This process, known as “ecdysis”, occurs most frequently in fast-growing juveniles and slows during adulthood. During ecdysis, the hard exoskeleton is shed in one piece, exposing the new, soft underlying skeleton. Water is pumped into the body to expand the size of the new exoskeleton before it hardens (eg. Guyselman 1953). Molting is not only used for growth, but also to regenerate missing limbs. During combat or to escape from predators, fiddler crabs autotomize or cast off limbs at a predetermined point (Weis 1977), usually at the base of all walking legs (Hopkins 2001). New limbs grow in a folded position within a layer of the cuticle, unfolding and expanding during the molting process. Ecdysis is triggered and accelerated by multiple autonomy and removal of the eyestalks (Abramowitz & Abramowitz 1940, Hopkins 1982). Molting under these circumctances may not result in growth, and the overall size of the crab may even decrease as energy is used to regenerate several missing limbs (Hopkins 1982). A single molt in some individuals is often enough to completely regenerate a missing limb (Hopkins 2001), but other crabs may require several molts before an appendage is restored to its original size. Several factors affect the frequency and success of molting and limb regeneration, including food availability, temperature and pollution. For example, the presence of methylmercury in polluted waters can partially or fully inhibit regeneration of limbs in both temperate and tropical fiddler crabs (Weis 1977).

Reproduction:
Fiddler crabs are social organisms that engage in elaborate mating displays before copulation. Males use their large claw to attract mates through a series of waving motions and acoustic drumming, also used to ward off potential competitors. Waving displays are often characteristic of a certain species, but usually occur at the mouth of the burrow in all crabs. In U. pugilator, the large claw makes a loop as it is brought up, pausing slightly before moving to the side and down in front of the crab (Crane 1975). The carapace rises with each wave and the small claw makes a roughly corresponding motion. In high intensity displays, a series of 4 to 5 waves is completed before the claw is lowered to its resting position. Often, acoustic drumming and other sounds are produced by the claws and legs to attract females (Crane 1975). Atlantic fiddler crabs court and mate both during the day and at night. In daylight, waving displays by males are likely most important; whereas, acoustic signals predominate during nocturnal courtship. Once the male has attracted a mate, she usually follows him into the burrow for copulation, and it has been suggested that underground mating in U. pugilator results in more viable eggs (Salmon 1987). The resulting fertilized eggs are carried in a clump, often called a sponge, on the abdomen of the female until hatching. Ovigerous, or egg-bearing, females were seen from May through August in North Carolina populations, most measuring over 1.0 cm in carapace length (Colby & Fonseca 1984).

Embryology:
Females release larvae into the water column once they are fully developed, usually during large nocturnal ebb tides (eg. Christy 1989). The purpose of this behavior is most likely to transport larvae offshore, away from abundant estuarine predators. Planktonic larvae develop through a series of five zoeal stages (Christy 1989), feeding mostly on smaller zooplankton. The final larval stage (postlarva) is the demersal, or bottom-associated, megalopa. As the larvae travel back toward the estuary, they metamorphose into megalopae and look for settlement cues such as the presence of other members of the same species (conspecifics) and the appropriate sediment type, before settling to the bottom and undergoing their final metamorphosis to a juvenile crab (eg. O’Connor 1993). Studies have shown that U. pugilator megalopae have the ability to delay metamorphosis for a limited time until a suitable habitat is found (Christy 1989, O’Connor 1991).

 

IV. PHYSICAL TOLERANCES

Temperature:
The Atlantic sand fiddler is one of only a few species that have large populations extending into temperate zones (Crane 1975). Because of this expanded range, U. pugilator is well adapted to subfreezing temperatures, regularly hibernating in their burrows during cold weather. These crabs are also adapted to warmer temperatures and high light levels, changing color presumably as a thermoregulatory mechanism (Silbiger & Munguia 2008, Wilkens & Fingerman 1965). In as little as five minutes after sun exposure (Silbiger & Munguia 2008), crabs can become pale, reflecting more light rays and remaining an average of 2 °C cooler than dark crabs (Wilkens & Fingerman 1965). The upper thermal tolerances for U. pugilator vary with humidity, at approximately 42 °C and 46 °C for saturated and dry air, respectively. The discrepancy between these temperatures is likely the result of higher transpiration rates for crabs in dry air helping to maintain a cooler core body temperature (Wilkens & Fingerman 1965).

Salinity:
The Atlantic sand fiddler is most common in higher salinity environments. However, this species can tolerate a wide range of salinities. In high salinity waters, it is considered a better hyporegulator than many other Uca species (Green et al. 1959, Thurman 2005). This ability enables crabs to maintain a more constant salt concentration in their body fluids, even when exposed to air (Thurman 2003). In field studies, U. pugilator has been found in waters ranging from 0.2 to 36.2 ppt (Godley & Brodie 2007, Thurman 2005).


V. COMMUNITY ECOLOGY

Burrowing Behavior:
Fiddler crabs are known for digging burrows in muddy and/or sandy sediments of sheltered estuarine habitats. These tunnels are used for mating, to escape extreme temperatures and flooding, and as a refuge from predators. The burrows are generally located in the intertidal zone, have only one opening and are usually L-shaped (Ruppert & Barnes 1994). The depth of burrows can be as much as 60 cm (Gosner 1978), but most North American species dig no deeper than 36 cm (Ruppert & Barnes 1994). As the crab excavates the burrow during low tide, it transports sediment to the surface by carrying it in the legs of one side, rolling it into small balls and forming a pile at the entrance of the hole (Ruppert & Barnes 1994). When the tide comes in, most crabs retreat into their burrows, placing a sediment plug at the entrance to keep water from inundating the tunnel. Burrowing behavior differs somewhat according to species. In U. pugilator, the male excavates and defends the burrow, especially during the breeding season (Pratt & McLain 2006).

Territoriality:
Male fiddler crabs use their enlarged claw not only to attract females, but also in territory disputes with other crabs (Pratt & McLain 2006, Ruppert & Barnes 1994, Ruppert & Fox 1988). Individual territories are located around a single, centralized burrow. The areas are likely dependent on crab density and species, but have been measured at about 100 cm2 for U. pugilator (Pratt & McLain 2006). Combat among males of this species ranges from no contact to use of the major claw to push, grip or flip the opponent (Pratt & McLain 2006). Territoriality varies, and males are most aggressive toward intruders attempting burrow take-overs and similarly-sized male neighbors that may threaten mating success.

Trophic Mode:
Although they are occasionally cannibalistic, the majority of the fiddler crab diet consists of detritus, bacteria and algae on and in the sediments (Gosner 1978). The small claws transfer sediment to the mouthparts, where food is separated from sand and other unwanted particles. Food is swallowed and the mouthparts roll the remaining sand into tiny balls that are placed back on the ground. These balls are much smaller than those created during the excavation of burrows (Ruppert & Fox 1988). Mouthparts in many fiddlers are specialized for a specific size range of sediment particles, and this adaptation is partly responsible for the habitat and distribution of species. Crabs also wander while feeding and some species move as far as 50 m away from their burrows (Ruppert & Fox 1988).

Predators:
Predators of fiddler crabs include birds, fishes, turtles, and mammals such as otters and raccoons (Colby & Fonseca 1984, Crane 1975, Ruppert & Fox 1988), in addition to being occasionally cannibalized by other fiddlers (Gosner 1978). Crabs reduce predation risk by fleeing into their burrows, and some studies speculate that U. pugilator changes to a sandy color in part to camouflage its carapace from predators (Dawkins 1971). Larvae of Uca spp. are preyed upon by a variety of pelagic and benthic organisms, and are cannibalized by adult fiddler crabs in captive populations (O’Connor 1990).

Associated Species:
Many species of fungi are obligate associates of arthropod hosts (Mattson 1988). A fungus belonging to the genus Enterobryus has been discovered in the hindgut of U. pugilator. The fungus is not parasitic (Hibbits 1978, Lichtwardt 1976), and it has been suggested that the species may even provide necessary chemical compounds, such as amino acids, to the crab (Williams & Lichtwardt 1972). In addition to obligate associations, Atlantic fiddler crabs are found alongside several organisms common to mangroves and salt marshes. For extensive lists of other species found throughout the ecosystems in which U. pugilator occurs, please refer to the “Habitats of the IRL” link at the left of this page.

 

VI. SPECIAL STATUS

Special Status:
None

Ecological Importance:
The digging activity in fiddler crabs exists not only to create territorial burrows, but also to bring organic matter to the surface, stimulating microbial growth. Burrowing activity often increases when food is limited to create a more abundant nutrient source, but also results in the stimulated growth of nearby mangroves and Spartina plants (Genoni 1985, 1991) through increased soil aeration and more nutrient availability.

 

VII. REFERENCES & FURTHER READING

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Abramowitz, RK & AA Abramowitz. 1940. Moulting, growth, and survival after eyestalk removal in Uca pugilator. Biol. Bull. 78: 179-188.

Brodie, RJ, Behum, ME, Monroe, E, Glenn, N & JL Staton. 2005. Recruitment to adult habitats following marine planktonic development in the fiddler crabs, Uca pugilator, U. pugnax, and U. minax. Mar. Biol. 147: 105-111.

Brown, FA, Jr. & MI Sandeen. 1948. Responses of the chromatophores of the fiddler crab, Uca, to light and temperature. Physiol. Zöol. 21: 361-371.

Christy, JH. 1978. Adaptive significance of reproductive cycles in the fiddler crab Uca pugilator. Hypothesis. Science. 199: 453-455.

Christy, JH. 1989. Rapid development of megalopae of the fiddler crab Uca pugilator reared over sediment: implications for models of larval recruitment. Mar. Ecol. Prog. Ser. 57: 259-265.

Christy, JH & M Salmon. 1991. Comparative studies of reproductive behavior in mantis shrimps and fiddler crabs. Amer. Zool. 31: 329-337.

Colby, DR & MS Fonseca. 1984. Population dynamics, spatial dispersion and somatic growth of the sand fiddler crab Uca pugilator. Mar. Ecol. Prog. Ser. 16: 269-279.

Costa, TM, Silva, SMJ & ML Negreiros-Fransozo. 2006. Reproductive pattern comparison of Uca thayeri Rathbun, 1900 and U. uruguayensis Nobili, 1901 (Crustacea, Decapoda, Ocypodidae). Braz. Archiv. Biol. Technol. 49: 117-123.

Crane, J. 1975. Fiddler crabs of the world: Ocypodidae: Genus Uca. Princeton University Press. New York, NY. USA.

Dawkins, M. 1971. Perceptual changes in chicks: another look at the “search image” concept. Anim. Behav. 19: 566-574.

Fingerman, M & SW Fingerman. 1974. The effects of limb removal on the rates of ecdysis of eyed and eyestalkless fiddler crabs, Uca pugilator. Zool. Jahrb. Abt. Allg. Zool. Physiol. Tiere. 78: 301-309.

Genoni, GP. 1985. Food limitation in salt marsh fiddler crabs Uca rapax (Smith) (Decapoda: Ocypodidae). J. Exp. Mar. Biol. Ecol. 87: 97-110.

Genoni, GP. 1991. Increased burrowing by fiddler crabs Uca rapax (Smith) (Decapoda: Ocypodidae) in response to low food supply. J. Exp. Mar. Biol. Ecol. 147: 267-285.

Godley, J & R Brodie. 2007. Effect of summer storms on early life stages of Uca minax, U. pugnax and U. pugilator in North Inlet Estuary, South Carolina, USA. Mar. Ecol. Prog. Ser. 342: 197-204.

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Guyselman, JB. 1953. An analysis of the molting process in the fiddler crab, Uca pugilator. Biol. Bull. 104: 115-137.

Hibbits, J. 1978. Marine Eccrinales (Trichomycetes) found in crustaceans of the San Juan Archipelago, Washington. Syesis. 11: 213-261.

Hopkins, PM. 1982. Growth and regeneration patterns in the fiddler crab, Uca pugilator. Biol. Bull. 163: 301-319.

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Report by: LH Sweat, Smithsonian Marine Station at Fort Pierce
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