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Fiddler crab Uca sp. Family Ocypodidae

Fiddler crabs are common on Chek Jawa at low tide. Some are found on the sandy shores, others on the portions of the mud flats that are above the water at low tide. They are often found in groups.

Fiddler Rhythms: The male fiddler crab has one huge pincer, often highlighted in a bright colour that contrasts with his shell. The enlarged pincer may be as large and as heavy as the rest of the crab's body! This enormous pincer is not used to hunt or crush food. It is too small to effectively fend off most predators. Instead, it is used to attract females and to intimidate rival males. The male waves his large pincer in a style and rhythm unique to his species in order to attract the ladies. Fiddler crabs got their name for this behaviour, which resembles a musician playing on his fiddle.

Looking out: The eyes of a Fiddler crab are mounted on long stalk giving it a good all-round view of the air and the horizon. This helps it detect predators from afar while at the same time still being able to identify potential mates and rivals. When the crab scuttles back into its burrow, the eyestalks fold down into grooves along the body. Fiddler crabs are highly sensitive to movement and will scurry off at the slightest hint of danger.

Colourful costumes: Fiddler crabs can change colours. Males often brighten up during mating season. Sometimes, they appear different at night and during the day. This makes it challenging to identify the different species of Fiddler crabs by their colours alone.

Breathing air: Fiddler crabs cannot swim and prefer to breathe air. So at high tide, they hide in their burrows, plugging the entrance with a ball of sand to trap some air inside. However, they need water to keep their gill chambers wet as well as to process their food. They absorb water from the wet sand through hairs on their legs.

Fiddler food: Fiddler crabs eat the thin coating of detritus on sand grains. They scoop sand to their mouthparts with tiny feeding pincers that are spoon shaped and fringed with hairs. The bristle-like mouthparts scrape the sand grains clean of any edible titbits. A male fiddler crab cannot feed with his huge pincer and has only one much smaller feeding pincer. Females, however, have two feeding pincers and can thus feed much faster.

Fiddler babies: When a male Fiddler crab succeeds in persuading a female to mate with him, they retire into his burrow. The female may remain there until the eggs hatch. The eggs hatch into free-swimming larvae that drift with the plankton, changing into yet another form before settling down and developing into Fiddler crabs.

Role in the ecosystem: Fiddler crabs are eaten by many animals higher up in the food chain. The Kingfisher is among the birds that might snack on them.

Fiddler at Work
Male fiddler crab
waving his large pincer

Female fiddler crab
With two small pincers

quick facts
Body width to about 2-3cm wide, common

Classification:
OrderDecapoda
Class Malacostraca
Subphylum Crustacea
Phylum Arthropoda

See also...
Other crabs of the Family Ocypodidae on Chek Jawa
Soldier crab (Dotilla myctiroides)
Sand bubbler crab (Scopimera sp.)

Links
Atlantic Marsh Fiddler Crab (Uca Pugnax) on the Pelotes Island Nature Preserve website: fact sheet for kids with link to a colouring page for a fiddler crab and a video clip of a fiddler crab.
Three Common Florida Fiddler Crabs (Uca spp.) on the Public Health Entomology Research and Education Center website: fact sheet and photos and diagrams of fiddler crabs, how they eat, mate and more.
Fiddler Crab on the ACE Basin page of the US National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center website: fact sheet on some fiddler crabs of the US.
Fiddler crab on the Chesapeake Bay Program website: fact sheet on some fiddler crabs of the US.
Crab eyes on the Australia Advances website: research on how the fiddler crab's eyes help it find its burrow quickly can help us build better robots that can navigate efficiently.
Fiddler crabs see predators above the horizon by Eleanor Lawrence on the Nature Science Update website: how fiddler eyes help them to tell mates and rival crabs from dangerous predators from which they must flee.
Family Ocypodidae by Stefano Cannicci on the East African Mangroves site of the Natural History Museum of Florence: facts and photos on ghost crabs, fiddler crabs and soldier crabs.

Other references

Barnes, Robert D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore Science Centre, Singapore. 160 pp. online version
Ng, Peter K. L. & N. Sivasothi, 1999. A Guide to the Mangroves of Singapore II (Animal Diversity). Singapore Science Centre. 168 pp. online version
Lim, S., P. Ng, L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life and Times of Labrador Beach. Division of Biology, School of Science, Nanyang Technological University & Department of Zoology, the National University of Singapore. 160 pp.
Davey, Keith, 1998. A Photographic Guide to Seashore Life of Australia. New Holland, Australia.144 pp.
Mastaller, Michael, 1997. Mangroves: The Forgotten Forest Between Land and Sea. Tropical Press, Malaysia, 200 pp.
Morten, Brian & John Morten, 1983. The Sea Shore Ecology of Hong Kong. Hong Kong University Press. 350 pp.

a companion website to the chek jawa guidebook website©ria tan 2003 www.wildsingapore.com

a companion website to the chek jawa guidebook
website©ria tan 2003 www.wildsingapore.com