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
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).
II. HABITAT AND DISTRIBUTION
Regional Occurrence & Habitat
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).
The mudflat fiddler can be found
throughout the IRL, usually on muddy sediments near mangroves.
III. LIFE HISTORY AND
Age, Size, Lifespan:
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).
Although fiddler crabs are territorial,
the species is quite social and lives in large groups. Abundance
estimates are limited to U. 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 U. pugilator
and U. thayeri, the mudflat fiddler regenerated limbs
and hardened its carapace more quickly after ecdysis when inhabiting
waters with heavy metal pollutants.
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. 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 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 U. 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).
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 U. 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).
IV. PHYSICAL TOLERANCES
The mudflat fiddler is found in warm temperate
waters, but most populations are located at tropical and subtropical
latitudes. The documented thermal tolerance for U. rapax
ranges from 7 to 44 °C (Vernberg 1959, Vernberg & Tashian
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).
V. COMMUNITY ECOLOGY
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. 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
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
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 U. 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.
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).
Crustaceans are commonly hosts to a variety
of parasitic organisms. Parasites that infect U. 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).
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 U. 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. For extensive lists of other species found throughout
the ecosystems in which U. rapax occurs, please refer
to the “Habitats of the IRL” link at the left of this page.
VI. SPECIAL STATUS
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
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Report by: LH Sweat, Smithsonian Marine Station
at Fort Pierce
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