About Fiddler Crabs
The group of crabs known 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, retreating to muddy burrows as the tide goes out, and in brackish water swamps (which is where most pet fiddler crabs 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.
Choosing a Fiddler Crab
Since the fiddler crabs that are found in pet stores are most likely semi-terrestrial, brackish crabs, they will need some salt in their water as well as access to air and 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 may do fine in a freshwater and fully aquatic set up for a few weeks but they will eventually become weak and die. If you are looking to purchase a fiddler crab, find a store that keeps their crabs in brackish water or wait for a new shipment to arrive so the crab's time spent in fresh water is minimal. Also, be sure to choose a crab that isn't missing any feet or claws as these can be indications of poor health.
Housing Fiddler Crabs
One of the most important elements of a fiddler crab set up 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. This can be done by using an inexpensive hydrometer. Hydrometers can be found at pet stores in the fish section and also home brewing shops. Alternatively, your package of aquarium salt may have instructions for producing brackish water conditions. It is okay to vary the salt concentration/specific gravity slightly as these crabs would naturally experience some variations in 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.
In addition to the water, you need to provide your crabs with dry land to climb on to. Providing a sloped beach from the water with aquarium gravel or sand is ideal. Alternatively, use a partly filled tank with large rocks on which the crabs can 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 from any fish store.
Fiddler Crab Molting
Signs of a healthy fiddler crabs include growth and regular molting. Once a crab molts, their previous exoskeleton can be found in it's entirety in the tank and will looking eerily like 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 since it serves as an excellent source of calcium (which they need to produce their new exoskeletons). Claws and legs may be lost during molts but they will regenerate over a couple of molts.
Fiddler Crab Reproduction
Fiddler crabs may produce eggs in captivity (the female carries these eggs on her belly) but successful rearing of young in an aquarium is next to impossible due to the way their natural life cycle works. The larvae of fiddler crabs grow in deep ocean waters and return to shore at maturity and since aquariums are not deep enough the crab life cycle cannot be completed.