Gulf mud fiddler crabs live in coastal marsh regions of the northern Gulf of Mexico, from Western Central Florida to Texas. They are the most common fiddler crab species found in salt marshes of Mississippi and Alabama. ("Fiddler Crabs Of the Northern Gulf Coast", 2013; Mouton and Felder, 1996)
Gulf mud fiddler crabs create cylindrical burrows in marshy areas and sand or mud flats. They burrow more actively when few other species are present. This species is semi-terrestrial and prefers moderately salty marshes (they are tolerant of water salinity of 5-35 ppt, with larger individuals found more often in environments at the lower end of this range) with smaller-grained substrate and lower clay content. (Crane, 1975; "Fiddler Crabs Of the Northern Gulf Coast", 2013; Levin and Talley, 2002; Mouton and Felder, 1996; Thurman, 1979; Thurman, 1982)
Gulf mud fiddler crabs have four pairs of walking legs and a fifth (most anterior) pair of legs that are modified into chelipeds (clawed limbs). Body color is generally dull gray, becoming more green-blue towards the anterior of the carapace and eyestalks. Their color differs from day to night: during the day, these crabs are darker as their melanophores (dark pigment spots) are open and leucophores (white pigment spots) are closed, while the reverse is true at night, making them lighter in color. Crabs in this genus have carapaces that are narrow anteriorly, but the front of this species is comparatively broad. Their upper orbital margins are typically composed of two sharp edges, creating an eyebrow that is strongly inclined and nearly vertical. These crabs can be distinguished from other species by a pubescence (covering of soft down/short hairs) on the ventral side of their first and second pairs of walking legs and males' extremely large major cheliped. They can be distinguished from crabs in their most closely related genus, g. Ocypode, by their smaller eyes on longer stalks, longer antennae, and shorter legs. (Crane, 1975; Rothschild, 2004; Thurman, 1979)
Although sexes are similar in size, these fiddler crabs are sexually dimorphic, with the males having one small cheliped and one very large cheliped, which is used during courtship. Female gulf fiddler crabs have two small chelipeds and a tubercle (projection) at their gonopore. Average carapace length of a female is 12mm and the average width is 16mm; for males the average carapace is 13mm long and 21mm wide. (Crane, 1975; Thurman, 1979)
Female gulf mud fiddler crabs lay thousands (reports of over 19,000 per clutch) of very small eggs (averaging 25 mm in diameter). Clutches, known as a "sponges," are incubated by females for two weeks, after which eggs are released into the water where they hatch. Fiddler crabs begin their lives as planktonic larvae, completing five zoeal larval stages, increasing in size after each molt. They reach a final larval stage (megalop), at which point young look like very small crabs, but continue to live in the water, on the bottom. Megalops have more mouth parts than previous zoeal stages and do not go through numerous molts, instead finding a shallow area to hide. At this point, they lose their swimming capabilities. Males begin to develop their characteristically large cheliped after their third molt and, if they lose their larger chelipad during combat, their smaller one will develop further to replace it. Juveniles typically reach sexual maturity by August, when they begin to hibernate for colder months. They will begin to reproduce during the following April when warm weather returns. (Crane, 1975; "Fiddler Crabs Of the Northern Gulf Coast", 2013; Hyman, 1922; Mouton and Felder, 1995; Wenner, 1985)
Gulf mud fiddler crabs live in large social groups consisting of about 20 individuals, making partners easier to find. Males wait near their burrow entrances and wave their larger chelipeds, using a weak jerky movement repeated 8-15 times to signal females. Males fight with each other for partners, creating dominance hierarchies when they "win" females. Unlike many crustaceans, fiddler crab females do not need to have recently molted in order to mate. An interested female will stare at a male, at which point he will move toward her then toward his burrow a number of times until she follows him. A male will then drum on his burrow's edge, lead the female in, then plug the burrow's entrance and return to her to mate. She will remain in his burrow to incubate her sponge (egg clutch) before releasing it into the water. This species is polygynandrous, and females may lay multiple egg clutches during a breeding season. (Crane, 1975; Hyman, 1922; "Fiddler Crab (Uca rapax)", 2012)
Females develop sexually (ovaries increase in size) during the winter and spring seasons after they reach adulthood, with a peak reproductive period during summer months (May to August). During reproductive periods, females follow males into their burrows, where they mate. On average, 19,000-20,000 eggs are laid per clutch, though there are records of clutches as large as 45,000. Eggs vary in color from red to gray. After a two week incubation period, eggs are released into the water, but it is difficult to determine how many embryos grow into viable offspring. (Mouton and Felder, 1995; Salmon and Atsaides, 1968; Thurman, 1979; deRivera and Vehrencamp, 2001)
Female gulf mud fiddler crabs exhibit parental care by carrying eggs clutches (sponges) on their genital appendages at night and remaining in a burrow with the eggs during the day. They aerates their eggs by standing in the water and jerking their appendages back and forth, a behavior that is continued until the eggs are ready to be released and hatch. Males exhibit no paternal investment following fertilization. (Hyman, 1922)
Although there is little information available regarding life span for this species specifically, fiddler crabs in general are known to have life spans of 1-2 years, dependent largely on predation. (Grimes, 1989; "Fiddler Crabs Of the Northern Gulf Coast", 2013)
These crabs walk in a sideways motion, as do most crabs, moving back and forth from land to water. During low tides, burrows are dug using the walking legs. Males are known to make a tapping sound using their walking legs at night during low tides, although this species is largely diurnal. During high tides, these crabs will plug their burrows with mud, breathing trapped air until water levels retreat. Gulf mud fiddlers are territorial, guarding their individual burrows, although, when threatened, individuals are likely to run into any nearby burrow. Males establish social hierarchies through displays to, and combat for, females. Gulf mud fiddler crabs are sympatric with other Uca species (Uca minax, Uca spinicarpa and Uca vocator) and, when found with these species, tend to be smaller in size than when living in colonies made up only of conspecifics. (Crane, 1975; "Fiddler Crabs Of the Northern Gulf Coast", 2013; Pearse, 1914; Thurman, 1979)
These crabs rarely move farther than one or two meters from their burrows. (Crane, 1975)
Gulf mud fiddler crabs communicate with each other mainly through visual channels. Males communicate by waving their large chelipeds, which may be stimulated by female pheromones (sensed via their setae), physical touch of either sex, or visual cues (for example, another crab approaching). They also create noise and vibrations, using their larger chelipeds and, as part of their mating rituals, males "dance" for the females, running back and forth between their burrow and a female, also performing a "curtsey." These crabs communicate mainly for reproductive purposes, including creating dominance hierarchies. (Salmon and Atsaides, 1968; deRivera and Vehrencamp, 2001)
As larvae, gulf mud fiddler crabs filter feed, eating plankton, algae, decaying matter and smaller crab larvae. In their adult stages, they feed most actively as the tide is going out, eating detritus and algae in sand and along the shoreline. Females feed by scooping mud into their mouth with their spoon-like chelipeds, using both alternately, but males feed using only their smaller cheliped. They remove food matter from grains of sand and mud, forming inedible material into balls called pseudo-feces, which are discarded. (Hyman, 1922; Mouton and Felder, 1996; Pearse, 1914)
These fiddler crabs are prey to amphibians, birds, crustaceans, fishes, jellyfish, small mammals, and reptiles. Adults will hide in their burrows when threatened. (Anderson and Anderson, 2000; "Fiddler Crabs Of the Northern Gulf Coast", 2013; Hyman, 1922; Kruczynski and Ruth, 1997; "Uca longisignalis (Gulf marsh fiddler; tou-la-lou)", 2013; Overstreet and Heard, 1978)
These fiddler crabs are important to their ecosystems as biodegraders, cleaning coastlines by eating dead matter on beaches. They also aerate the substrate as they feed, providing oxygen to salt marsh plants. Gulf mud crabs also serve as an important food source for many species. (Anderson and Anderson, 2000; Hyman, 1922; Mouton and Felder, 1996; "Host-parasite database", 2013; Roa, et al., 2009; "Fiddler Crab (Uca rapax)", 2012)
Gulf mud fiddler crabs serve as intermediate hosts to a number of trematode and nematode parasites, some of whose terminal hosts are birds and mammals. (Anderson and Anderson, 2000; "Host-parasite database", 2013; Roa, et al., 2009)
Gulf mud fiddler crabs, along with many other species of fiddler crabs, are often found in the pet trade. Additionally, many researchers study fiddler crab burrowing behavior and reproductive habitats. (Asakura, 2009)
There are no known adverse effects of this species on humans. (Mouton and Felder, 1995)
This species has not been evaluated by IUCN and is not considered to be endangered or threatened. (IUCN, 2012)
This species has not been researched very well because Uca species are often grouped together as a whole. Some researchers have reported that gulf mud fiddler crabs are a subspecies of Uca rapax. (Crane, 1975)
Kelly Whelan (author), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
helps break down and decompose dead plants and/or animals
areas with salty water, usually in coastal marshes and estuaries.
an animal that mainly eats meat
flesh of dead animals.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.
an animal that mainly eats decomposed plants and/or animals
particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
parental care is carried out by females
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
An animal that eats mainly plants or parts of plants.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
fertilization takes place within the female's body
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
marshes are wetland areas often dominated by grasses and reeds.
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
an animal that mainly eats all kinds of things, including plants and animals
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
the business of buying and selling animals for people to keep in their homes as pets.
chemicals released into air or water that are detected by and responded to by other animals of the same species
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
an animal that mainly eats plankton
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.
mainly lives in oceans, seas, or other bodies of salt water.
an animal that mainly eats dead animals
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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