The group of crabs known commonly as fiddler crabs actually encompasses over 90 species and subspecies of the genus Uca. 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 two 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 musical instrument called a fiddle, is how fiddler crabs got their names.
Choosing a 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 their 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.
Housing Fiddler Crabs
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. The ideal amount of salt to add is controversial but you want to add enough salt to the water to attain a specific gravity of around 1.005 - 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 75-85 degrees Fahrenheit (24-29 degrees Celsius). Supplemental heat (a heating pad, water heater, or heat light) must be provided if the day and night temperatures do not fall within these ranges in your home. You don't want your fiddler crabs getting chilled.
In addition to the water and heat, you will also need to provide your crabs with dry land to climb on to. 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.
Feeding Fiddler Crabs
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 (sinking tablets, shrimp pellets, etc.), 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.
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 like 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.