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Species Name: Uca thayeri
Common Name:      Atlantic Mangrove Fiddler Crab

I.  TAXONOMY

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


Male Atlantic mangrove fiddler crab, Uca thayeri. Photo courtesy of Bjorn Tunberg, Smithsonian Marine Station at Fort Pierce.

 

Species Name:
Uca thayeri M. J. Rathbun 1900

Common Name:
Atlantic Mangrove Fiddler
Mangrove Fiddler


Species Description:
The Atlantic mangrove fiddler, Uca thayeri, 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 (Negreiros-Fransozo et al. 2003), 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. The carapace and major claw of U. thayeri are both brown to orange-brown (Crane 1975, Kaplan 1988), and both fingers of the claw are bent down (Ruppert & Fox 1988).

Potentially Misidentified Species:
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 and its subspecies, U. rapax and U. rapax rapax; and the longfinger fiddler, U. speciosa.

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 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.

 

II. HABITAT AND DISTRIBUTION

Regional Occurrence & Habitat Preference:
This species is found in the lower intertidal zone of deep mud banks near mangroves in the western Atlantic Ocean from Florida to Brazil (Costa et al. 2006, Crane 1975, Warner 1969). Females often build tall mud chimneys at the entrance to their burrows during breeding season (Crane 1975, Kaplan 1988). See “Burrowing Behavior” below for more information.

IRL Distribution:
The Atlantic mangrove fiddler occurs throughout the IRL, but is mostly found on muddy sediment around mangroves in the lower intertidal zone.

 

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:

The Atlantic mangrove fiddler has a maximum carapace length of about 1.9 cm (Kaplan 1988). Little information is reported for the maximum age and average lifespan of U. thayeri. 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 larger groups than other brachyuran crabs (Teal 1958). Densities recorded for U. thayeri populations in the field range between 4 and 15 individuals m-2 in Florida (Miller & Vernberg 1968, Weaver & Salmon 2002) and 16 m-2 in Trinidad (von Hagen 1973).

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 (da Silva Castiglioni et al. 2007). 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 circumstances 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. The presence of methylmercury in polluted waters can partially or fully inhibit regeneration of limbs in both temperate and tropical fiddler crabs (Weis 1977). When compared to U. pugilator and U. rapax, U. thayeri exhibited slower regeneration of limbs and hardening of the carapace after ecdysis when inhabiting waters with heavy metal pollutants. For example, the Atlantic mangrove fiddler also regenerated slower in cooler temperatures and ceased regeneration when placed in large groups.

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. thayeri, the large claw makes a near vertical loop, jerking 2-4 times as it is brought up (Crane 1975). Individual waves last about 0.5 s, comprising larger displays that can occur over several seconds to many minutes. Often, acoustic drumming and other sounds are produced by the claws and legs to attract females (Crane 1975). Atlantic mangrove 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, although aboveground mating has also been occasionally observed for this species (Salmon 1987). The resulting fertilized eggs are carried in a clump, often called a sponge, on the abdomen of the female until hatching. Reproduction in U. thayeri is seasonal, with ovigerous, or egg-bearing, females present during the warmer months (Costa & Negreiros-Fransozo 2003). Though seasonal, reproductive periods in fiddler crabs are usually wide, and females can spawn several egg masses per year (Costa & Negreiros-Fransozo 2003). Most ovigerous females studied in the field ranged between about 1.5 and 2.3 cm in carapace width. Ovigerous U. thayeri also build characteristic chimneys on their burrows. See “Burrowing Behavior” for more information.

Embryology:
Female U. thayeri can carry approximately 31,000 eggs at one time, incubating them for 12-14 days at summer temperatures (DeCoursey 1979, Christy 1982). During this time, most of the yolk is consumed, and larvae hatch with almost no remaining yolk (Christopher et al. 2008). Crabs release larvae into the water column once they are fully developed, usually during large nocturnal ebb tides (eg. Christy 1989), although the hatching rhythms for U. thayeri are most tuned to tidal flows and can occur during both day and night (Weaver & Salmon 2002). 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).

 

IV. PHYSICAL TOLERANCES

Temperature:
The Atlantic mangrove fiddler is found in warm temperate waters, but most populations are located at tropical and subtropical latitudes. Little information exists on the thermal tolerances of the species or the water temperatures in which they are usually found, but studies have documented populations at air temperatures ranging from about 21 to 26 °C (Costa & Negreiros-Fransozo 2003).

Salinity:
The Atlantic mangrove fiddler is best equipped physiologically for low to moderately saline habitats (Thurman 2005). However, this species can tolerate a wide range of salinities. Individuals have been found in waters ranging from 12.9 to 35.1 ppt (Thurman 2005), but a broader salinity range likely exists for the species as a whole.


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. thayeri, the males and females excavate burrows, but ovigerous females build tall chimneys of mud at the mouths of their tunnels (Costa et al. 2006, Pratt & McLain 2006, Salmon 1987). These chimneys or funnels can extend up to 10 cm high, and females will rebuild the chimney usually within 24 hours when it is disturbed (Salmon 1987). The burrow entrances of non-ovigerous females are usually flush with the surrounding sediment, while males build a small lip around their holes.

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. Most studies on territoriality have been conducted for a similar species, U. pugilator. Crab density likely plays an important role in territory size, but have been measured at about 100 cm2 for some populations (Pratt & McLain 2006). Combat among males 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. Ovigerous U. thayeri females may also defend their burrows during the mating season (Salmon 1987).

Trophic Mode:
Although they are occasionally cannibalistic, the majority of the diet in fiddler crabs 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. The Atlantic mangrove fiddler is commonly found in fine to very fine sediments, where it uses feathery bristles, called setae, to clean adhered detritus and other particles from single grains of sand (Bezerra et al. 2006, Maitland 1990, Miller 1961).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 or hiding between marsh grasses and mangrove roots. 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).

Parasites:
Crustaceans are commonly hosts to a variety of parasitic organisms. Parasites that infect U. thayeri include trematodes such as: Probolocoryphe lanceolata in the hepatopancreas , Maritrema prosthometra in the thoracic musculature, and Gynaecotyla adunca in the antennal gland (Smith et al. 2007).

Associated Species:
Atlantic mangrove fiddler crabs are found alongside several organisms common to mangroves and other muddy estuarine areas. For extensive lists of other species found throughout the ecosystems in which U. thayeri 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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Bezerra, LEA, Dias, CB, Santana, GX & H Matthews-Cascon. 2006. Spatial distribution of fiddler crabs (genus Uca) in a tropical mangrove of northeast Brazil. Sci. Mar. 70: 759-766.

Christopher, CE, Salmon, M & RB Forward, Jr. 2008. Is the hatching clock of fiddler crab larvae (Uca thayeri) phenotypically plastic? J. Crust. Biol. 28: 328-333.

Christy, JH. 1982. Adaptive significance of semilunar cycles of larval release in fiddler crabs. Biol. Bull. 163: 251-263.

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.

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.

da Silva Castiglioni, D & ML Negreiros-Fransozo. 2006. Physiologic sexual maturity of the fiddler crab Uca rapax (Smith, 1870) (Crustacea, Ocypodidae) from two mangroves in Ubatuba, Brazil. Braz. Archiv. Biol. Technol. 49: 239-248.

da Silva Castiglioni, D, Negreiros-Fransozo, ML & RCF Cardoso. 2007. Breeding season and molt cycle of the fiddler crab Uca Rapax (Brachyura, Ocypodidae) in a subtropical estuary, Brazil, South America. Gulf Carib. Res. 19: 11-20.

DeCoursey, PJ. 1979. Egg-hatching rhythms in three species of fiddler crabs. 399-406. In: Naylor, E & RG Hartnoll, eds. Cyclic phenomena in marine plants and animals. Proc. 13th Europ. Mar. Biol. Symp. Pergamon, Oxford.

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