Fiddler Crabs
Of the Northern Gulf Coast
Fiddler crabs
are small semi-terrestrial crabs that inhabit tidal marshes and the adjacent sand and mud flats. They are recognized by the square shape of their bodies and by the male fiddler's oversized claw. Fiddlers dig cylindrical burrows where they take shelter from predators, hot sun, winter cold, and high tides. Fiddler crabs typically live in colonies; if you find one, you're likely to find dozens nearby.
Fiddler crabs are named for the distinctive waving movements made by the males with their enlarged claw during courtship. The motion resembles a musician playing a fiddle or violin. Each species has a distinct waving motion. The motion of the feeding claws on both males and females also resembles a fiddling motion. In Peru, the motions are compared to sewing and fiddlers are called maestro-sastres or master tailors. Their names in Japan (Siho Maneki) and in Brazil (Chama Marés) interprets the fiddler's motions as beckoning for the tide to return.
The video below shows typical male fiddler crab waving behavior. Additional video links are provided at the end of this page.
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Male Gulf Mud Fiddler Displaying, Uca longisignalis
Video by Don Abrams
Taxonomy
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Five species of fiddler crabs are found on the Mississippi coast. Photographs are provided in the table below. Differences in species are often subtle. A taxonomy guide to northern Gulf coast fiddler crabs is provided here.
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Uca panacea
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Uca spinicarpa
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Uca minax
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Uca longisignalis
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Uca rapax
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Two additional species of fiddlers occur on the northern Gulf coast.
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Uca pugilator
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Uca speciosa
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Photos from "Guide to Common Tidal Marsh Invertebrates of the Northeastern Gulf of Mexico," Richard W. Heard. |
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Similar Species
Fiddler crabs are closely related to the ghost crab, Ocypode quadrata. Ghost crabs are larger and very light in color. Ghost crabs prefer open sandy beaches rather than tidal marshes, and they're much larger than fiddlers.
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Photo by Chris Snyder |
Fiddler crabs also resemble crabs in the family Grapsidae, commonly known as marsh crabs, friendly crabs, shore crabs, square backed crabs, or wharf crabs. Male Grapsidae crabs lack the enlarged claw of male fiddlers.
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Photo by Robert Lord Zimlich |
Anatomy & Physiology
Original image credit: By Christopher Thomas [CC-BY-SA-3.0 via Wikimedia Commons"
Range and Habitat
Fiddlers live in in the inter-tidal zone of marshes and the adjacent mud and sand flats. They always live close to water, with particular species adapted to a wide range of salinity. The species described here exist along the entire northern coast of the Gulf of Mexico.
Feeding
Fiddlers are deposit feeders and scavengers, ingesting a variety of food including detritus (decayed organic matter), nematodes (small worms with unsegmented cylindrical bodies), bacteria, fungi, algae, and decaying animal matter. The females have both their small claws adapted for feeding. The male cannot use his large claw for feeding; therefore, he must use the small claw much more rapidly in order to eat the same amount as a female during the same period of time. Despite this feeding disadvantage, male fiddlers are usually larger than females.
| Female | Male |
| Gulf Mud Fiddler crabs feeding, Uca longisignalis Videos by Don Abrams |
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Fiddler crabs feed on the falling tide and are most active at low tide when the greatest area of exposed mud flats exists. As the tide rises, the fiddlers retreat to their burrows and seal the entrance with a mud ball plug. Fiddlers may range as far as 50 yards from their burrows while feeding.
When feeding, the sand fiddler takes sandy material into its mouth (buccal cavity) and uses specialized spoon-shaped setae on its maxillipeds to scour the algae and other organic matter off the sand grains. As the sand grains are cleaned, the crab forms the grains into "feeding balls" or "pseudo-feces" which are discarded. Often large concentrations of pseudo-feces occur around the openings of the burrows of sand fiddlers. Similar balls, usually larger, are formed with material that the crab brings to the surface when excavating a burrow.
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| Fiddler crab burrow. Note the larger excavation balls at the lower right portion of the photo. The smaller balls are feeding balls. |
Field of fiddler crab excavations and feeding activity show the effect of fiddlers on their habitat. |
| Click either image for a larger view. Photos by Don Abrams. | |
One study found that the Gulf mud fiddler crab takes in approximately 0.4 g of material in six hours. Foods eaten have been estimated to be 33% diatoms, 25% fungi, 20% vascular plants, and 20% unknown material. (Grimes, Huish, and Kerby, September 1989; Ringold, 1979)
Place in the Food Web
Fiddler crabs make up an important part of the diet of many coastal animals, including blue crabs, mud crabs, purple marsh crabs, red drum, diamondback terrapins, willets, gulls, rails, herons, rice rats,
and raccoons.
Fishermen find fiddler crabs an effective bait, especially for sheepshead, pompano, and both red and black drums.
The sensitivity of fiddlers to pollutants, heavy metals, and pesticides makes them a good indicator of the health of an ecosystem. In building their burrows, fiddlers provide a beneficial effect on the marsh by aerating the soil and bringing organic matter to the surface. As they sift though the sands while feeding, they aerate the substrate and prevent anaerobic conditions (decay without oxygen present).
Behavior
The male fiddler crab uses his large claw in courtship and in combat against other males during territorial disputes. Courting males wave their large claws in rhythmic patterns that are unique to each species of Uca. The Panacea sand fiddler male uses it to produce a rapid, drumming sound. The sound apparently is caused by the claw striking either the edge of the fiddler's carapace or the ground.
Fiddlers take refuge from predators and high tides in their burrows. As the tide rises, they seal the entrance with a pellet of mud and wait out the high water while breathing trapped air.
Fiddler crabs recognize their own burrows, but when threatened they will escape into any convenient burrow. Fiddlers may also burrow into the sand to escape from predators. When the danger has passed, they emerge and resume their normal behavior.
Fiddler crabs move sideways, never forward or backward.
A territorial dispute between two Gulf mud fiddler crabs from adjacent burrows, Uca longisignalis.
Video by Don Abrams
Life Cycle & Reproduction
The male fiddler digs, maintains and defends a tidy, cylindrical burrow that is L-shaped with a single opening. The burrows are usually no more than about 14 inches deep, but sometimes as deep as 24 inches.
To find a female partner, the male stands next to his burrow, often near other males standing next to their own burrows, while females walk past. The male waves his major claw to attract the female's attention.
If a female is interested, she will stare at the male for a short period of time. The male then runs toward the female and runs back to his burrow, repeating this motion several times until the female either moves on or follows him to the burrow. If the female follows, the male partly enters the burrow and drums the edge with his claw, then leads the female inside, plugs the entrance and returns to the female to mate.
Unlike most other crabs, fiddlers don't have to wait to mate until the female has recently molted and her exoskelton is still soft. The female incubates her sponge, or eggs, for two weeks, then returns to the surface to release them into the water, where they hatch and develop into juveniles. The number of eggs per clutch varies by species and with the size of the individual crab, typically 10,000 to 40,000 for northern Gulf Coast fiddlers.
From April to October, Mississippi coast fiddler crabs can produce larvae every two weeks. Their release is usually timed to the highest tides, probably so that the ebb flow takes them to deeper water. From November to March, low temperature inhibits reproduction.
The planktonic larvae go through two to five zoeal stages before becoming bottom-dwelling megalopa and then finally metamorphosing into a juvenile crab as they return to the marshes. Depending on the species, the process from egg to adult takes a few weeks to months. Fiddlers typically have a life span of one to two years.
Interesting Facts About Fiddlers
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Fiddler crab colonies can include thousands of individual crabs.
Photo by Cade Pogue - The male's large claw is primarily for mating and territorial displays, rather than combat. Either the left claw or the right claw of the male crab may be the larger claw.
- A male fiddler's large claw may be as much as two-thirds of its total body weight.
- Fiddler crabs change their pigmentation, shifting from dark during daytime to light at night.
- Fiddler crabs reabsorb their shells rather than shedding them as they grow.
- Fiddler crabs are poor swimmers and generally avoid the water as adults.
- Unlike blue crabs and other marine crabs, fiddlers breathe oxygen from the atmosphere.
- If a male fiddler crab loses it's larger claw, the remaining claw grows larger and the new claw is regenerated as the smaller claw.
Resources and References
Websites & Publications
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| Photo by Don Abrams |
- "Guide to Common Tidal Marsh Invertebrates of the Northeastern Gulf of Mexico," Richard W. Heard, Mississippi Alabama Sea Grant Consortium, MASGP-79-004, 82 pages.
- Fiddler Crabs, South Carolina Department of Natural Resources
- Scientific papers on fiddler crabs, BioStor.org, 30 papers on various topics
- Fiddler Crab Info Website - many broken links, but still a useful source
- Fiddler Crab References List - list of 2000+ citations, last updated 2007
Fiddler Crab Videos on Youtube
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- Fiddler crab (Uca pugilator) waving
- Fiddler crab excavating his burrow and feeding
- Wakulla Beach, Florida, crabs in marsh grass
- Feeding balls and fiddler crab feeding
- Crab feeding and forming sand balls (Sand Bubbler Crab, not a Gulf coast fiddler, but its feeding behavior is similar.)