Minuca rapax (Smith, 1870) (redirected from: Uca rapax)
Family: Ocypodidae
Common names: Mudflat Fiddler
Synonyms: Gelasimus palustris H. Milne Edwards, 1852,  more...
Minuca rapax image
Minuca rapax  

Species Description: The mudflat fiddler, Minuca rapax, 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. The carapace of M. rapax is narrow between the eyes, and light tan (Kaplan 1988) or occasionally greenish blue in color (Crane 1975, Ruppert & Fox 1988). Eyestalks and the tips of the major claw in some specimens are also green to blue (Crane 1975). However, the claw in most individuals is gray to greenish yellow, sometimes with hints of orange and white finger tips. The center of the palm is almost smooth, but still bears small granules. Whitening of the carapace common to displaying males in other Minuca species is poorly developed in M. rapax, and females are similar in color to the less brilliant males.

Potentially Misidentified Species: Several other species of fiddlers occupy the estuarine habitats of the IRL, including: the Atlantic sand fiddler, Leptuca pugilator; the saltpan fiddler, Minuca burgersi; the redjointed fiddler, M. minax; the Atlantic marsh fiddler, M. pugnax; the mudflat fiddler subspecies, M. rapax rapax; the longfinger fiddler, L. speciosa; and the Atlantic mangrove fiddler, L. 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 L. pugilator. Most populations of L. 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 M. 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 grass marshes, from brackish to nearly freshwater, in Massachusetts to northern Florida and Louisiana.

The Atlantic marsh fiddler, M. 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). T

he longfinger fiddler, L. 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, L. 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 M. 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, M. 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).

IRL Distribution: The mudflat fiddler can be found throughout the IRL, usually on muddy sediments near mangroves.

Age, Size, Lifespan: The maximum length of the carapace in M. 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 Minuca species, the lifespan of the mudflat fiddler is relatively short, lasting 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. Abundance estimates are limited to M. rapax in the IRL, but populations in some Brazilian mangrove forests reach about 20 individuals per square meter (Koch et al. 2005).

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 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. 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). When compared to L. pugilator and L. thayeri, the mudflat fiddler regenerated limbs and hardened its carapace more quickly after ecdysis when inhabiting waters with heavy metal pollutants.

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 M. rapax, the large claw creates a weak circular, almost lateral movement, with a distinctive slow progression and jerking motion (Crane 1975). Jerking is always present, and the number of moves ranges from 8 to over 30, creating a display series lasting up to 13 seconds each.

Often, acoustic drumming and other sounds are produced by the claws and legs to attract females (Crane 1975). Mudflat 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 few to no reports of aboveground mating have been documented for in M. rapax (Crane 1975).

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 throughout the warmer months in Brazilian populations (da Silva Castiglioni et al. 2007), and individuals were considered mature at carapace widths of about 1.4 and 1.2 cm for males and females, respectively (da Silva Castiglioni & Negreiros-Fransozo 2006).

Embryology: Female mudflat fiddlers can carry 5,000 to 30,000 eggs at one time, each with a volume of about 0.01 mm³ (Figueiredo et al. 2008, Greenspan 1980). Crabs release larvae into the water column once they are fully developed, usually during large nocturnal ebb tides (eg. Christy 1989). Unlike some other crabs, studies on M. rapax in Panama found that the species only released larvae at night (Morgan & Christy 1994). The purpose of this behavior is most likely to transport larvae offshore, away from abundant estuarine predators, while reducing predation risk under low light conditions.

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).

Temperature: The mudflat fiddler is found in warm temperate waters, but most populations are located at tropical and subtropical latitudes. The documented thermal tolerance for M. rapax ranges from 7 to 44°C (Vernberg 1959, Vernberg & Tashian 1959).

Salinity: The mudflat fiddler is most common in brackish water and is best equipped physiologically for such environments (Thurman 2003, 2005). However, this species can tolerate a wide range of salinities, and individuals have been found in waters ranging from 2.5 to 35.8 ppt (Thurman 2005).

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 M. rapax, the male excavates and defends the burrow, especially during the breeding season (Pratt & McLain 2006). Burrows of some populations of mudflat fiddlers are most prevalent in the high intertidal zone (Koch et al. 2005).

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, L. 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.

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 (Genoni 1985, 1991; 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 mudflat fiddler is commonly found in medium sand, where it uses spoon-shaped 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). Studies on feeding behaviors in M. rapax document foraging excursions up to 2 m from the burrow, and crabs have advanced homing abilities that allow them to find their burrows regardless of orientation or obstacles (Layne et al. 2003).

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 occasionally being 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 Minuca 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 M. rapax 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). Mudflat fiddlers are also infected by the larvae of the spiny-headed acanthocephalan worm Arhythmorhynchus frassoni, which usually complete their life cycle as endoparasites in the digestive tract of vertebrates (Nickol et al. 2002).

Associated Species: Many species of fungi are obligate associates of arthropod hosts (Mattson 1988). Fungi belonging to the genera Enterobryus and Taeniella have been discovered in the hindgut of M. rapax. These species are not parasitic (Hibbits 1978, Lichtwardt 1976), and it has been suggested that they may even provide necessary chemical compounds, such as amino acids, to the crab (Williams & Lichtwardt 1972). In addition to obligate associations, mudflat fiddler crabs are found alongside several organisms common to mangroves and salt marshes. 

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.

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Minuca rapax image
Minuca rapax  
Minuca rapax image
Minuca rapax