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
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
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
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
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).
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
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.
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
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
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).
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
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.
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).
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 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. 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).
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
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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