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
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
The Atlantic sand fiddler is located
throughout the IRL, mostly on sandy beaches near mangroves and
III. LIFE HISTORY AND
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).
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).
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
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
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).
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
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 &
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
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 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
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
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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