The group of crabs known commonly as fiddler crabs actually encompasses over 90 species and subspecies of the genus Uca. These crabs need brackish water (despite how many pet stores treat them) and are easy to keep as pets, with few health concerns and docile personalities.
Common Name: Fiddler Crab
Scientific Name: Uca minar
Adult Size: About 2 inches
Life Expectancy: Up to 3 years in captivity
Difficulty of Care: Easy
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Fiddler Crab Behavior and Temperament
Fiddler crabs are closely related to ghost crabs and can be found in the wild on ocean beaches in the inter-tidal zone.
They retreat to muddy burrows as the ocean tide goes out and they can also be found in brackish water swamps, which is where most pet fiddler crabs actually come from. Unlike land hermit crabs, fiddler crabs spend a lot of their time in water and do not require shells to live in.
The largest fiddler crabs only reach a mature size of about 2 inches across their body. Male and female crabs are quite easily distinguished by their claws since the females have small claws but the males have one very distinctive large claw. This large claw, held in such a way that it resembles a fiddle, is how fiddler crabs got their names.
Housing the Fiddler Crab
In captivity, fiddler crabs need brackish water and a "land" area in their aquariums. Providing a sloped beach from the water by using aquarium gravel or sand is ideal. Alternatively, use a partly filled fish tank and place large rocks into the water that the crabs can use to climb out of the water.
Don't house more than four crabs in one aquarium, because overcrowding is a big source of stress for crabs, and can lead to health problems. For one to four crabs, a 10-gallon tank is ideal.
One of the most important elements of a proper fiddler crab terrarium is the salt in the water. Most experts recommend putting aquarium salt (not table salt) in the water to mimic the brackish water in which your fiddler crab came from.
Add enough salt to the water to attain a specific gravity between 1.005 and 1.010. The specific gravity can be determined by using an inexpensive hydrometer. Hydrometers can be found at pet stores in the fish section and also at home beer brewing shops.
Alternatively, your package of aquarium salt may have instructions for producing brackish water conditions by adding a certain amount of salt to a certain amount of water. It is okay to vary the salt concentration/specific gravity slightly as fiddler crabs would naturally experience some variations of salinity in the wild.
Fiddler crabs do well at a range of temperatures between about 75 and 85 degrees Fahrenheit. Supplemental heat (a heating pad, water heater, or heat light) must be provided if temperatures do not fall within these ranges in your home. You don't want your fiddler crabs getting chilled.
Food and Water
In the wild, fiddler crabs are scavengers that eat bits of organic matter they find in the sand and mud. Thankfully, though, in captivity, they can be fed sinking crab food, fish food meant for scavengers (such as sinking tablets and shrimp pellets), and freeze-dried plankton and shrimp. These items should be readily available for you to purchase from any fish store. Offering food a few times a week should suffice.
They need calcium (which they can get from eating a shed exoskeleton), and the water's pH and temperature should be closely monitored.
Common Health Problems
Although you can house fiddler crabs together, since males are highly territorial, avoid putting more than one male in a tank. They may fight and injure each other.
Most of the health issues that fiddlers and other crabs experience in captivity have to do with poor water quality or an inadequate diet. You also want to take note of any foul odors coming from your crab's enclosure, which may indicate a fungus, or a problem with the water salinity.
Fiddler Crab Molting
Signs of a healthy fiddler crab includes growth and regular molting. Once a crab molts, its previous exoskeleton can be found in its entirety in the tank. This exoskeleton looks a bit like your crab would look if it were a ghost.
It is a good idea to leave this exoskeleton in the tank for at least a week or so in case your crab wants to ingest part of it. Exoskeletons serve as excellent sources of calcium, which a fiddler crab needs to produce a new one. Claws and legs may be lost during molts but they will regenerate over a couple of molting sessions.
Fiddler Crab Reproduction
Fiddler crabs may produce eggs in captivity but just because you see a female fiddler crab carrying eggs on its belly doesn't mean you'll have baby crabs. Successful rearing of young fiddler crabs in an aquarium is next to impossible due to the way their natural life cycle works.
In the wild, the larvae of fiddler crabs grow in deep ocean waters and return to shore at maturity. Since aquariums are not deep enough to mimic this, the crab life cycle is rarely successfully completed in captivity.
Purchasing Your Fiddler Crab
Since the fiddler crabs that are found in pet stores are most likely semi-terrestrial, brackish water crabs, they will need some salt in their water in addition to dry land.
Unfortunately, many pet stores keep fiddler crabs in a freshwater aquatic set up (and even refer to them as freshwater crabs) and then recommend the same to new owners. These crabs that are housed in freshwater may do fine for a few weeks but they will eventually become weak and die if they are not moved into brackish water.
Therefore, if you are looking to purchase a new fiddler crab, find a pet store that keeps its crabs in brackish water or wait for a new shipment to arrive. You want to keep any time spent in freshwater to a minimum.
Be sure to choose a crab that isn't missing any feet or claws as these can be indications of poor health. As fiddler crabs deteriorate in health, they begin to lose their limbs.