About Fiddler Crabs
The group of crabs known as fiddler crabs actually encompasses over 90 species and subspecies of the genus Uca that come from a variety of locations. Males and females are quite easily distinguished by their claws - the females have small claws but the males have one very distinctive large claw and the largest fiddler crabs only reach a mature size of about 2 inches across their body.
Fiddler crabs can be found on ocean beaches in the inter-tidal zone, retreating to muddy burrows as the tide goes out (so they are exposed to water that ranges from brackish to salt water), while other species are found in brackish water swamps. Fiddler crabs are scavengers, feeding on organic matter that they find on the surface of rocks and in the mud.
Choosing a Fiddler Crab
The fiddler crabs found in pet stores are most likely semi-terrestrial brackish crabs which means they need some salt in their water as well as access to air and dry land. Unfortunately, many pet stores keep fiddlers in a freshwater aquatic set up and then recommend the same to new owners. The crabs may do fine in a fresh water and fully aquatic set up for weeks but they will eventually become weak and die. If you are looking to purchase a fiddler crab, find a store that keeps theirs in brackish water or wait for a new shipment so the crab's time spent in fresh water is minimal and choose a crab that isn't missing any feet or claws.
Housing Fiddler Crabs
Because most pet fiddler crabs are from brackish waters, most experts recommend putting aquarium salt (never use table salt) in the water. The ideal amount of salt to add is controversial but it is probably best to get a hydrometer and add enough salt to the water to attain a specific gravity of around 1.005 - 1.010.
Hydrometers are not all that expensive and can be found at pet stores 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 (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.
Some provision should be made to provide the crabs with access to land/air. Owners that do not do this often find their crabs crawling up the filter intake and into or on top the filter in an attempt to find land. Providing a sloped bottom to the tank with part of the gravel or sand out of the water works well. 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. 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.
Fiddler Crab Molting
Signs of health in 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 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.