Fiddler crabs can be found on beaches and in brackish swamps around the world. They are also known as Calling crabs and can be one of any of the 100 semi-terrestrial crab species that make up the genus Uca.
In this guide, I will tell you how to keep Fiddler crabs in your home aquarium (paludarium). You will know some details about their life you will not see anywhere else.
Most fiddler crabs in the pet trade are sourced from the brackish coastal swamps in the southern U.S. The closest relative to the fiddler crab is the Ghost crab. Unlike land crabs, such as Hermit crabs, Fiddler crabs are partially terrestrial and spend only some of their time in the water.
Quick Notes about Fiddler crab
||Florida Fiddler Crab, Signal Crab, Calling Crab and Uca Crab|
|Scientific Name||Uca sp.|
|Tank size (minimal)||10 gallons (~45 liters)|
|Size||5 – 6 cm (~2 inches) across the leg span|
|Optimal Temperature||25 – 31°C (~75°F – 88°F)|
|Water type||Slightly Brackish (SG 1.005)|
|Optimal PH||7.2 – 8.2|
|Optimal GH||8 – 25|
|Optimal TDS||150 – 200 (100-300)|
|Nitrate||Less than 40 ppm|
|Life span||up to 2.5 years|
|Color Form||Brown, Grey, Black, Red, Yellow|
Natural Habitat of Fiddler Crabs
In the wild, Fiddler crabs prefer to live in the open mudflats under the shade of mangrove trees (usually between
10-30 cm below the surface in these mangrove habitats). They (and Sesarmid crabs) are the dominant invertebrates in mudflats of mangrove forest in the tropical and subtropical region.
These crabs create tunnels in the mudflats to escape from predators and avoid desiccation due to high temperatures. Therefore, the most important factors determining their presence are sediment texture, porosity, organic content, water content, and temperature.
Burrow excavation efficiently oxidized anoxic subsurface sediment layers. The oxidation effect of burrows enhance the decomposition rate of organic matter and nutrient cycling in the mangrove ecosystem.
Fiddler Crabs Behavior
Fiddler crabs are a very active and social crab species. Males are highly territorial and will fight each other, for burrows and space. Females are not that aggressive and usually do not fight.
They are a burrowing genus of crabs and readily construct dens in the substrate of their environment. These crabs also prefer to stay close to their burrows. Depending on the species, these burrows serve as a safe haven from predators, molting retreat, and even breeding area.
When Fiddler crabs are not active on the surface, they remain inside their burrows. When active on the surface, crabs periodically return to their burrows and emerge a few minutes later. This serves to avoid predation and physiological stress and to replenish water supplies.
Although it is easy to scare them, Fiddler crabs are not especially shy crabs. They can easily be observed scavenging for food and interacting with one another. Males have only one claw that is useful for feeding, so they find and eat slower than females. Male fiddler crabs may spend up to twice as much time eating as females do.
In the wild, feeding and burrowing activity of Fiddler crabs is restricted to low tide period. Once the tide rises they enter their tunnels and plug the burrow entrance with mud.
Combat and Defense
According to the observations, Fiddler crabs always maintain and defend territories around the burrow area. They delimit territories by waving the male major claw chela and by standing in a statuesque pose known as “posing”. Although, males use their major claw as the main weapon in combat and defense of their territory. They can also use their legs to kick the opponent in an attempt to flip over.
The length of a territorial dispute depends upon the value of a burrow as a resource, cost to maintain it and the local population density. Food levels can affect time spent fighting as well. Duration of fights may be reduced by crabs building structures near their burrows that help territorial defense.
Fiddler Crabs Molting
Like all crabs, Fiddler crabs molt periodically throughout their lives. These molting periods begin close together when the crab is young and growing quickly. As the crab ages, these molting periods will begin to happen further and further apart. For example, adults molt every 2 – 3 months.
The molting process starts in 8 – 12 days when the upper layer of the cells starts separating from the cuticle. At that time you will notice that Fiddler crab start eating less and less the closer is the moment of molting. Another sign of molting is some slight color change (brown-muddy hue).
The process of molting does not take much time (5 – 15 minutes). Crabs are vulnerable and helpless during and after molting, so it is necessarily for them to go into hiding and not reappear until the process is complete and their new carapace has begun to harden.
According to the study, Fiddlers can also use a molt to get rid of harmful toxins, which concentrate in the chitin or regrow lost limbs.
Male fiddler crabs are capable of partial molting. Unlike most crabs, male fiddler crabs have the ability to quickly regrow their large claw without a complete molt, if it is damaged in a fight. These quickly regenerated claws are often not as strong, or as durable as their original claws. However, it has been observed that while this new claw is less adapted to fighting, it is lighter and more easily waved during mating rituals and more attractive to females.
Like all invertebrates, Fiddler crabs need adequate calcium supplement and trace minerals in the molting process. I highly recommend reading my article “How to Supplement Shrimp and Snails with Calcium”.
Note: Do not remove old exoskeleton from the tank. It contains lots of minerals and your crab will eat it later.
Fiddler Crabs Description
Fiddler crabs can be a variety of colors, ranging from violet to black, with the males usually being the most colorful. This genus of crabs gets their name from the asymmetrical claws seen on the male crabs. One claw is much larger than the other and is held up by the crab, similar to the way a fiddle would be held.
Adult Size of Fiddler Crabs
Fiddler crabs are relatively small when compared to other crabs species in the aquarium hobby, though they are a bit larger than Micro crabs. The average adult size for all Fiddler crab species rarely exceeds 5 cm (2 inches) across the leg span. Their carapace is usually about 3.5 cm (~1.5 inches) in diameter.
The females tend to be slightly smaller than the males. Scientists presume that is because the growth of females slows or ceases temporarily once they attain the size of sexual maturity.
Determining Gender of Fiddler Crabs
Determining the gender of your fiddler crabs is easier than with some crab species. Fiddler crabs are sexually dimorphic. The males have one claw that is drastically larger than the other, while female fiddler crabs have two small claws. The males are often more brightly colored than the females, though some females may be exceptionally colorful.
Fiddler Crabs Lifespan
The average lifespan of a pet Fiddler crab is 2 – 3 years. Some specimens have been reported to have lived longer. As with any pet, the quality of care plays a major role in life expectancy.
Feeding Your Fiddler Crabs
Fiddler crabs are omnivores feeders that require water for the feeding process. In their natural habitat, they survive on bits of organic matter, insects, the detritus of decomposing plants and benthic algae they find in the sand along the shoreline. In captivity, Fiddler crabs enjoy a varied diet and will readily eat just about anything they’re offered.
Foods Fiddler Crab Will Enjoy (examples with links to check the price on Amazon)
- Shrimp pellets
- Shrimp Granules
- Fish food (TetraMin® flakes, etc)
- Shrimp food (Hikari’s crustacean food like Hikari Shrimp Cuisine, Algae wafers, Brine shrimp, etc )
- Frozen blood worms
- Vegetables like spinach, peas, squash, leafy greens, etc.
The best foods are those that easily sink to the bottom of the tank, or can be sprinkled along the edge of their dry land, to stimulate their natural scavenging habit.
Ideally, you need to give them different types of food to get all necessary microelements. When you decide to give Fiddler crabs vegetables, you need to do it the safe way. Read my article “How to Blanch Сucumbers and Zucchini for Shrimp, Snails and Fish the Right Way”.
It is normal for Fiddler crabs to consume decaying matter. Therefore, Fiddler crabs should be supplied with Almond leaves, dead beech, or oak leaves regularly as they feed on these and require the detritus from the leaves. Read my article “Indian Almond Leaves and Alder Cones in a Shrimp Tank”.
Note: Always keep an eye on the nitrate levels in your tank and remove any uneaten food.
Fact: Some Fiddler species have modified mouthparts that allow them to feed on the sand. They possess specialized setae that scrape off the organic material so that the sand particles are discarded before the organic material is passed to the buccal cavity. Therefore, crabs that feed on sand consume fewer indigestible particles than crabs that feed on mud.
Keeping Fiddler Crabs
Fiddler crabs are not fully aquatic, they require an enclosure that offers both water and dry land. The best setup for their needs is a paludarium tank. A paludarium is an aquarium that has one half designated to dry land, which slopes gradually into a water-filled area and resembles a shoreline in appearance.
When done properly a Fiddler crab may also be housed in a fully aquatic tank that has a floating land area, with dry substrate where the crab can exit the water. A partially filled aquarium with large rocks above the water level can also be used. These kinds of setups need to be done carefully to ensure the crab has easy access to the dock, but can not escape the enclosure. Because of this, a paludarium is the best choice.
Note: Fiddler crabs are more than capable escape artists who use cables, tubes or other items that go in and out of the tank to climb out and escape.
Substrate for Fiddler Crabs
In their natural habitat fiddler crabs burrow into mud and sand along the shoreline. Therefore, the best substrate for fiddler crabs is one that caters to their burrowing habits and won’t easily collapse.
Sand is the most popular substrate choice. Ideally, Fiddler crabs need a substrate that is deep enough to allow them to burrow, while at the same time providing a gradual transition into the water portion of the tank to allow easy access to both sections.
Small gravel can be used as well, as long as you provide sandy burrowing areas.
Burrows and Hiding Places
Fiddler crabs, like all crabs, enjoy having a safe place to hide during their molting periods and reproduction. You should provide a few hiding places for your fiddler crabs. These hiding places can be store-bought tank decor, plants, or whatever you choose as long as they provide a hiding spot, your crab won’t care.
Note: Biologists studied burrow morphology and observed that generally all Fiddler crab’s burrows were nearly vertical and straight, mostly unbranched with an enlargement at the terminal end. In general, their length was about 10 – 20 cm (4 – 8 inches). Keep it in mind when you decide to provide them with some. In the wild, Burrows did not necessarily contain standing water but were usually only damp at low tides. Fiddler crabs return to them every 10 to 30 minutes during feeding to renew the respiratory water lost.
Plants and Fiddler Crabs
Unfortunately, there is a extremely high chance that Fiddler crabs will damage your live plants. They are not plant safe.
Most Fiddler crabs in the pet trade come from brackish swamps with low salinity levels, because of this some pet stores mislabel them as freshwater crabs. Fiddler crabs can live in water with a low salinity level, but they can not survive in freshwater for a long time. They are not freshwater crabs.
The preferred saline ratio for fiddler crabs is teaspoon of marine salt, for every gallon of water (~4.5 liters) of water. I would advise using Instant ocean marine salt (link to check the price on Amazon) for brackish setup. You will also need a Therm/ Hydrometer (link to check the price on Amazon).
Note: Chlorine is dangerous to Fiddler crabs, so it is important to dechlorinate any water before it is added to the tank. Fiddler crabs also need high levels of oxygen in their water, because they’ve evolved along beaches where the water is always changing and always moving.
When finished your tank should meet these requirements:
Salinity level between 1.01 and 1.08. Constant temps between 25 – 31C (~75 – 87F), with no major fluctuations, pH level of 7.2 – 8.2. Adequate aeration from an air pump, or filter outlet. Although Fiddler crabs can osmoregulate powerfully over a wide salinity range. They do not like the high level of salinity. In the wild, when salinities are extreme, they go underground and seal their burrows.
Tank Equipment (examples with links to check the price on Amazon)
Note: Sponge and Hang on Back Filters do not work well with lowered water levels.
Fiddler Crabs and Tankmates
Keeping Fiddler crabs with other creatures can be difficult due to their semi-terrestrial needs, but it can be done. Whenever you keep a community tank, it is important to ensure the needs of all included species are fully met. In the case of Fiddler crabs, any tankmates must be saltwater animals that can survive in the lower salinity levels of brackish water. For example, Neon Tetras, Guppies, Mollies, etc.
Fiddler crabs are small, so keep this in mind when choosing tank mates, larger predators may decide to snack on it. In contrast, smaller fish may be harassed and even killed by the crabs, so the options are limited.
Basically, keeping fish, snails, and shrimp with Fiddler crabs can really be a game of chance, so it is not recommended unless you are prepared to potentially lose some fish, snail, shrimp or a crab.
Do not keep them with other crab species. For example, even Sesarmid crabs (like Red claw crabs – read the guide), wo are their neighbor in the wild) are more likely to win disputes against Fiddler crabs and harass them.
However, they can easily be kept in small groups of their own species.
It is recommended that you keep this group as a harem, one male will multiple females, as males will fight. They don’t require a ton of space and up to 4 crabs can easily be housed in a 10 gallon aquarium. For each additional crab after 4, add 3-5 gallons of space.
Fiddler Crabs and Mating Behavior
Fiddler crab mating is an interesting ordeal. First of all, these species show two types of mating behavior in sex-mixed colonies. In both cases, males initiate the pairing by waving their large claw to attract females.
- In surface mating.
A male courts a nearby female, usually the nearest one. The male leaves his burrow and approaches the female. If the female is receptive, the pair copulates at the entrance of the female’s burrow.
- In underground mating.
A burrow holding male induces a wandering female into his burrow. If a female is ready to mate she will choose one of the waving males. In case the mating is not successful, the female will emerge from the burrow in a few moments and continue wandering. If it is successful she will not emerge, but the male will come out after about 5 minutes and begins to gather up sand to block the entrance to his den. He barricades his den from the inside and seals him and the female inside, where they continue to mate up to a few days.
Each male consistently courted females within 1.5 m of his burrow. Now it is obvious why it is a bad idea to keep several males in one tank.
Breeding Fiddler Crabs
After underground mating, the male will leave the den and make, or fight for another one. The female stays underground for about two weeks (11-14 days), while she incubates her eggs. The eggs must be kept moist before they are released into the water.
After the incubation period, the female will enter the water and release her eggs, the larvae will hatch immediately. The larvae are called zoea and the closely resemble water fleas. They are free swimmers, they survive on rotifers and freshly hatched brine shrimp nauplii.
After three or so molting periods, into different stages of zoea, the baby crabs will molt into their final larva state. This stage is called megalopa and is similar to a crayfish in appearance. They are still free swimming at this stage, but tend to settle. After a final molt, the young crabs finally look like their parents and they start the typical fiddler crab lifestyle.
Important: Rearing of Fiddler crab larvae is only possible in marine water. However, in the wild, breeding periods are not coincident with the wettest times of the year, but with the months following these times. Furthermore, this coincided with the beginning of the growth of new cohorts. This breeding pattern may reflect adaptation to reduce mortality on the larvae or juveniles caused by a decrease in creek salinity.
Fiddler crabs are semi-terrestrial organisms and require land to survive and feed. This is extremely important to know if you are planning to keep them as pets. These crabs are easy to care for, which makes them a great choice for a beginner aquarist.
- The Ecology of Fiddler Crab Uca forcipata in Mangrove. Conference Paper. November 2013.
- The behavioural ecology of Fiddler crabs (Genus: Uca) that live in the mangrove forests of Darwin harbor by Madeleine Nobbs (the requirements for the degree of Doctor).
- Burrow morphology of three species of fiddler crab (Uca) along the coast of Pakistan. J. Zool., 142 (2): 114-126 July 2012.
- Comparison of mating behavior of the fiddler crab Uca lactea in relation to Density. January 2011.
- Spatial Ecology of Fiddler Crabs, Uca pugnax, in Southern New England Salt Marsh Landscapes: Potential Habitat Expansion in Relation to Salt Marsh Change. Article in Northeastern Naturalist. June 2013
- For whom the male waves: Four types of claw-waving display and their audiences in the fiddler crab, Uca lactea Article in Journal of Ethology. January 2010.
- On the reproductive behavior of the fiddler crab Uca thayeri, with comparisons to U. pugilator and U. vocans: Evidence for behavioral convergence. Article in Journal of Crustacean Biology. February 1987
- Taxonomic Revision Of The Wide-Front Fiddler Crabs Of The Uca Lactea Group (Crustacea: Decapoda: Brachyura: Ocypodidae) In The Indo-West Pacific. Article in June 2010
- Sand ornaments used to attract females are avoided by rival males in the fiddler crab Uca lacteal. Article in Ethology Ecology and Evolution. November 2016
- Courtship herding in the Wddler crab Uca elegans. Martin J. How · Jan M. Hemmi. Published: 10 October 2008. Springer-Verlag 2008
- Predation and Cannibalism by the Male Fiddler Crab Uca tetragonon. by Tsunenori Koga, Seiji Goshima, Minoru Murai and S. Poovachiranon.
- Effects of mud fiddler crabs (Uca pugnax) on the recruitment of halophyte seedlings in salt marsh dieback areas of Cape Cod (Massachusetts, USA). Article in Ecological Research. January 2011