fiddler crab in Australia Fiddle crabs are small crabs. The largest one are only about five centimeters (two inches). They reside along sea beaches and brackish intertidal mud flats, lagoons, swamps, and various other types of brackish or salt-water wetlands. Like all crabs, fiddler crabs shed their shells as they grow. If they lose a leg or claws during a growth cycle, a new one grows when they molt. If a male loses a large fiddle claw, a same size one grows back on the same side after the next molt. Newly molted crabs are very vulnerable to predator attack because of their soft shells. At that time they generally stay in their burrows until the new shell hardens. [Source: Wikipedia]
Fiddler crabs are seen by the hundreds in mud flats. They make slurping noises as they take in mud, extract organic material and eject little balls. They rarely venture more than a meter or two from their burrow. Somehow in their brains they count their steps and use triangulation to figure out where they are in case they have to make a run for it to the relative safety of their burrows. [Source: Douglas Fox Natural History, April 2004]
Fiddler crabs generally play a positive rose in the ecosystems of the environments where they live. In some places they help to stimulate the turnover of nutrients in the soil. They are sensitive to contaminants which makes them good environmental indicators especially for insecticides. Many larger predators eat them and thus they are important to local food webs. Blue crab, rails, egrets, herons and raccoons are among the animals that feed on the,. Most species of fiddler crab have no special status on the International Union for Conservation of Nature (IUCN) Red List or with the Convention on the International Trade in Endangered Species (CITES). [Source: Bethany Fisher, Animal Diversity Web (ADW)]
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
Fiddler Crab Characteristics
All fiddler crabs are similar in shape. They have a smooth carapace (shell that covers their min body) and a square-shaped body. Their eyes are located found at the ends of two long and slender, movable eyestalks that emerge from the center of the carapace. As is the case with all true crabs, they are decapods — animals with ten limbs;. The limbs include their claws, which are also called pincers or chelipeds. Even though fiddler crabs breathe in oxygen they have gills. The gills must remain wet to function. [Source: Chris Patterson, Animal Diversity Web (ADW) /=]
Female fiddler crabs have two pincers that are the same size. Males have one pincer like the female’s. The other is very large and conspicuously colored pink, red, blue, purple or white. The large claw looks fearsome but actually they are virtually useless in catching prey and defending the crab from predators, Its primary purpose is to attract mates. The large pincer can either be on the right side or left side. It is at least four times larger than the other and can make up to 30 percent of the male's mass.
Male and female red-jointed fiddler crabs are both reddish brown with a spot of gray color at the front. The claws of this crab has red joints. They have eight walking legs that are either olive or grayish brown in color. Near the center of the shell, called the carapace, there is a H-shaped depression. Behind the eye there are horizontal depressions. The eyes Red-jointed fiddler crabs have are compound. This means they are attached by eye stalks. The male can either be right or left clawed, meaning the large claw can appear on the right or left side of the crab. [Source: Bethany Fisher, Animal Diversity Web (ADW) /=]
Fiddler crabs can regenerate lost limbs. Regeneration takes place in two growth stages — basal and proecdysial. The first stage of growth — basal growth — results from an increase in the number of cells due to mitotic division of the blastemal cells. The second stage — proecdysial growth — is occurs as a result of increases in cell size due to protein synthesis and water uptake. It usually takes several months for complete full regeneration to take place and new pincers are usually not as large nor as strong as the original. [Source: Chris Patterson, Animal Diversity Web (ADW)]
Fiddler Crab Vision
Fiddler crabs have compound eyes located on stalks that emerge from the head. Each eye is composed of 10,000 ommatidia, the individual eyes that make up compound eyes. Most are on the stalks rather than at the end of the stalk. Even where there is a clear division of shapes fiddle crabs can only make out objects only about two percent as well as humans can. [Source: Douglas Fox, Natural History, April 2004]
Fiddler crab anatomy
Describing fiddler crab vision Douglas Fox wrote in natural History magazine wrote: “A fiddler crabs eyes are mounted on stalks that point straight up and they command a panoramic 360-degree view. The mudflat comprises the entire outer edge of the visual field, and the arching sky dominates the middle...Unlike human vision the crabs vison is sharpest around the edges. That’s a reasonable emphasis. After all, the outer edge is where other members of the species are scuttling about: both rival animals looking to steal one’s precious burrow and females in the market for a mate. But in the great round center of the crabs visual field there is nothing but sky — and the occasional bird swopping in for a crabmeat cocktail.”
Scientists at Australia National University in Canberra studying fiddler crabs have developed a “crab camera” that mimic the vision of a fiddler crabs, giving a sky-centered “donut view” of the world.
Fiddler Crab Behavior
In the world of the fiddler crab most everything on land level are other crabs and things that come from the sky are predators. If a dummy is placed next to a crab the crab treats it as another crab and either ignores it or tries to fight with it or mate with it. If you wave a dummy over their heads from the sky they immediately run for cover to their burrows. In a controlled laboratory setting, they follow a constant circadian rhythm that mimics the ebb and flow of the tides: they turn dark during the day and light at night.
Fiddler crabs are colonial. They often live together in large clusters. Territorial fighting often occurs between the males. Usually they are defending their burrows. Despite their fighting, they travel in herds of thousands when feeding. [Source: Chris Patterson, Animal Diversity Web (ADW)]
Fiddler crabs communicate by a sequence of waves and gestures. This is most pronounced in the mating season when the male vigorously waves his large cheliped around to attract the attention of females. The movement of the smaller claw from ground to mouth during feeding looks a bit like the crab is playing the larger claw like a fiddle — hence their name.
Fiddler Crabs and Their Burrows
Most fiddler crabs dig burrows. Red-jointed fiddler crabs dig burrows in the sand or mud that are two to five centimeters in diameter and have various depths. The maximum depth of the burrow is 60 centimeters (two feet). The entrance to the burrow can be plugged up with mud if the crab feels threatened.
Red-jointed fiddler crabs dig temporary burrows to protect themselves from predators during feeding. When these crabs are threatened by a predator the crabs either quickly dig a burrow or run to the nearest burrow to hide. They dig temporary burrows to protect themselves from predators during feeding. In places with cold winters, the crabs bury themselves in a burrow to "hibernate". [Source: Bethany Fisher, Animal Diversity Web (ADW) /=]
A fiddle crab life's revolves around its burrow. Douglas Fox wrote in Natural History: “A crab’s most precious resource is its burrow. That’s where the animal hunkers down at hide tide, hides from birds, mates. And other crabs that leave the safety of their own burrows in search of a larger or better-positioned burrow are often the biggest threat. When a crab ventures even a few crab steps from its burrow to slurp some mud, other crabs are constantly trying to steal its burrow, forcing it to dart back time and time again to defend its home.” [Source: Douglas Fox Natural History, April 2004]
The burrows are also important for mating. The breeding period of Red-jointed fiddler crabs is every two weeks in the summer. During that time the males often have two burrows. One they live in and one that is for mating. The latter is small and round. When females return to their burrow after eating males stand at the edge of their mating burrows, waving their large claws in the air. If a female is attracted by the way a male waves his claw she will stop in front of the burrow. /=\
When the male sees this he will wave the claw more vigorously. After this the male will run from his burrow to the female and back again. This is to show her the location of the burrow. If she approves of the burrow she will go to the edge of it and wait. The male runs into his burrow and drums both claws against the side and the female will feel the vibrations and enter. The male then leads the female to the back of the burrow and the returns to the entrance to plug it up with mud. Locked in the burrow mating occurs and two weeks later the female returns to the surface and releases her eggs into the ocean to develop. /=\
Fiddler Crab Food and Feeding Behavior
Fiddler crabs live in burrows (holes) and pick up food with their pincers that deliver it to a set of hair-fringed blades that scissor back and forth in front of their mouths. One set of hairs sorts out grains of sand and mud. Another set moves potentially edible material to the mouth. Inedible material collects at the bottom of the mouth and is coalesced into a pellet that is removed with the pincer. [Source: Douglas Fox Natural History, April 2004]
Fiddler crabs are omnivorous. They eat algae and decaying vegetation and occasionally eat other fiddler crabs. Red-jointed fiddler crabs sift through sediment and extract any nutrients that may be there. After they have gotten all the nutrients out they leave behind small pellets of unusable sediment. They prefer to eat in puddles of water so they can separate the food from the garbage. The large claw of the male keeps him form eating properly. He has to eat twice as much and twice as fast as the female crab to obtain the same nutrients. The main food source is a plant called cordgrass [Source: Bethany Fisher, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web: Sand Fiddlers ingest particles of sand or mud and they use their mouthparts to scrape food materials from the sediment, and then deposit the sediment back down on the ground as a "feeding pellet." The actual method of consumption occurs when the scooped mud is put in their mouths and the entrapped detritus is filtered out using specialized brush-like mouthparts. Water is pumped from their gills into their mouths to float the detritus free of the mud. The food material consists of decaying organic matter or unicellular plants such as algae. The chelipeds are used for picking up the small amounts of sediment not for crushing things or for a grip. Because of one enlarged claw the males cannot eat as fast as females so they have to eat twice as fast. [Source: Chris Patterson, Animal Diversity Web (ADW)]
Fiddler Crab Mating
During the mating season fiddle crab males wave their large claws around, sometimes accompanied by a little dance, to signal females that he is ready to mate. Responsive females follow the male to his hole. The dance and style of fiddling varies from species to species. Some stand as high as they can and wave their claws back and forth. Other hold their claws still and jump up and down.
A group of a dozen or so male fiddler crabs may surround a female and wave their large claws, seemingly in unison. The female then selects one of the males and goes down his hole to mate. Why did she chose one and not the others when they all seem to be acting the same. Studies have shown that the victor often begins his fiddling a fraction of a second earlier than the others.
Atlantic fiddler crabs breed approximately every two weeks for most of the summer. Reproduction occurs in burrows similar to the ones they live in only they are larger and better-maintained. According to Animal Diversity Web (ADW) The process of reproduction begins by males standing at the edge of their burrows, usually lining up with other males only centimeters away. Then, as a female approaches the male crab of her choosing, the male waves his large cheliped vigorously. If the female is still interested, the male then runs toward her, back to the burrow, back to her, and back to the burrow again to show which burrow is his. The male finally drums his large cheliped against the sides of the burrow until she enters. Upon her entrance the male crab seals the burrow with a mud cap. [Source: Chris Patterson, Animal Diversity Web (ADW) /=]
The two crabs mate. The female places all of her fertilized eggs, which number a quarter million or more, on her abdominal flap, where the eggs form a small spongy cluster. Two weeks later after incubating her eggs the female returns to the land surface and release her eggs into into the nearest tidal creek during high tide. The eggs hatch. During the next several months the young fiddler crabs undergo metamorphosis and change into their final form. These new adult crabs return to the land, where they live out the rest of their lives. /=\
Fiddler Crab Species
Thera are more than 100 species of fiddler crabs and they make up 11 of the 13 genera in the Ocypodidae crab family. These were formerly members of the genus Uca. In 2016, most of the subgenera of Uca were elevated to genus rank, and the fiddler crabs now occupy 11 genera making up the subfamilies Gelasiminae and Ucinae. [Source: Wikipedia]
Red-jointed fiddler crabs (Scientific name: Minuca minax, formerly Uca mina) are also known as brackish water fiddlers. Found along the eastern shore of North America from Cape Cod to Texas, they inhabit muddy or sandy beaches that are uncovered at low tide. They are the largest of the three Chesapeake Bay species of fiddler crabs and are the most common and widespread ones there. These crabs prefer areas of low salinity and can survive in freshwater for at least three weeks. Once a year adults molt their shell. If the male ever loses the large claw, the remaining one grows to a large size and a new smaller claw replaces the lost one.
Sand fiddler crabs (Scientific name: Leptuca pugilator, formerly Uca pugilator) are another one of the three fiddler crab species residing on U.S. east coast. They are found from Cape Cod to Texas, with the exception of Florida south of St. Augustine. They favor strongly brackish to saltwater salinities and primarily live in low marshes, which have sediments that are covered by water during high tides, with lots of saltmarsh cordgrass. Atlantic fiddler crabs live in burrows that they dig for themselves. The burrow can be closed with a mud cap. During low tide the crabs leave their holes and search for food, but never stray too far from their burrow unless they are males courting females or trying to drive off rivals. [Source: Chris Patterson, Animal Diversity Web (ADW) /=]
The Sand fiddler crab are approximately 3.8 centimeters 1.5 inches) wide and 2.5 centimeters (1.0 inch) long. Males are more brightly in colored, having a purple grey or blue carapace with irregular markings of black or brown. The females have much more subdued coloration on their carapaces.
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023