More than 90 species and subspecies of fiddler crabs can be found ranging from northern Australia to southern Asia, northern South America to southern North America and around the coastline of Africa. No matter where they're native to, fiddler crabs have a distinctive large front claw; males have a much larger claw than females. Fiddler crabs are a popular addition to a home aquarium, but for multiple reasons, they cannot survive in a fish bowl.
Most fish bowls are not large enough to provide a full square foot; your average fish bowl is about 1 to 2 gallons. Fiddler crabs are generally fairly small -- maxing out at about 2 inches wide, most averaging about 1.5 inches wide. Each needs about a square foot of space. You want to house one fiddler crab in a 7-gallon tank; up to four crabs will be fine in a 10-gallon tank as long as there are enough caves.
Even the largest fish bowl is not going to be large enough to create a proper semi-aquatic environment. Fiddler crabs are not solely aquatic; they need an aquarium that can provide both land and water. Generally, the tank should be 50 percent land and 50 percent water -- but if you only have one or two crabs in a 10 gallon tank, a minimum of 25 percent of the surface space will suffice as the land area.
Filtering Brackish Water
A fish bowl cannot efficiently maintain water salinity, and filters are not suitable for fish bowls. Pet stores often sell fiddler crabs as freshwater crustaceans -- but when housed in freshwater, they will only live about a month. Fiddler crabs need brackish water to survive a full life span, which is about two to three years. You will need to add aquarium salt to your tank, and you will need to monitor the water to ensure the specific gravity is about 1.005 to 1.015. A filter is also necessary for the water portion of the tank.
You can put just about any substrate at the bottom of a fish bowl, but you still can't create the proper land habitat with natural substrate. When setting up housing for fiddler crabs, consider that they need land and water. You can use gravel to set up the substrate for the water because it's easy to keep clean with a gravel vacuum, but the land should be sand or coconut fiber to create a more natural setting for burrowing behaviors.