Chemical
The fiddler crab, unlike the blue crabs you will find in a bay, need a clean, chemical free environment to survive. If a contaminant such as polychlorinated biphenyl, a chemical formerly used for electric insulation and coolants, gets into the water a fiddler crab will take this out of the seawater and concentrate it for food. They find this chemical most often in the sediments, and once they ingest it they pass it on to any animal that eats them, creating a chain of sick animals. As well, when there is an oil spill, years later the fiddler crabs of the nearby salt marshes have been observed to change their habits. They will not dig any more that a few centimeters into the sediments if they are near a place that was highly contaminated, and when they go through and eat from sediments that are or have been previously contaminated with the oil, they react to everything slower and move sluggishly. Finally, any heavy metals like mercury, zinc or copper are toxic to fiddler crabs and can cause slow development and deformities in young crabs.
Geological
Fiddler crabs in habit coasts from Mass. down to Florida. They use the soft sand of salt marshes to their advantage. They make burrows into the sand roughly 12 to 18 centimeters deep on average. These burrows are very useful to the fiddler crab; they provide protection when running from a predator, when the tide comes in they take a small piece of mud and plug the top of their burrow so the water doesn't flood the burrow because unlike most crabs fiddlers breath air, they provide a place to stay wet when the tides go out and the ground at the top of the burrow has no water above it, when the sun gets to be too much for them they go and cool of and wet their gills at the bottom of the burrow, and finally they use these burrows to procreate. After the male crabs do their displays with their large claws, a female that is attracted to him follows him back to his burrow "where the magic happens". When they burrow, the fiddler crab brings organic material up to the surface, which helps the overall health of the environment.
Physical
The fiddler crab is very tolerant of temperature differences as shown by how they habit almost anywhere on the east coast from Mass. and south. Their difference in color is also related to their area, which connects as well with the amount of sunlight in that area. Similarly, the fiddler crab is known to inhabit areas of many different salinities (.2 to 36.2 ppt), but they are much more common in high salinity areas. In fact in salt marshes with very high salinities you are likely to find hundreds of these little guys living together.
Random Facts
- When a male loses it's large claw in a fight, to a predator, etc. its small claw grows and becomes the big one and a small claw appears where the former big one was.
- Adults molt once or twice a year, during which time they remain very close to their burrows.
- Fiddler crab burrows can reach up to 2 feet deep.
Sources
http://www.dnr.sc.gov/cwcs/pdf/FiddlerCrab.pdf
http://www.whoi.edu/page.do?pid=12467&tid=282&cid=25746
http://www.sms.si.edu/irlspec/Uca_pugila.htm
http://www.chesapeakebay.net/fiddler_crab.htm
*All text sources I found were experiments, thesis papers and information that wasn't usable
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