It's funny that of the 5 or 6 notes I've gotten about fiddlers, all but one have been about so-called "fresh water" fiddlers sold in pet stores. You are correct: there is no such thing. According to the world's expert on fiddler crabs, Joslyn Crane (in "Fiddler Crabs of the World"), fiddler crabs are an _exclusively_marine_genus_. There are a few that can live in brackish water, but none can live in fresh water. Also, they like to be near, but not submerged in, water.
I suspect the pet store is selling some sort of mud crab and calling it a fiddler. Or, they have Uca Minax (the brackish-water fiddler) and it can tolerat pure fresh water for a while. However, most of the people who wrote me did so saying their crab had gotten 'sick' after a month and seemed to be sluggish or etc. This is likely what would happen if you kept a brackish-water fiddler submerged in fresh water for a long period of time.
I think fiddler crabs are really cute. I also think they would not make good pets (or else I'd have a few... goodness knows I can get as many as I want). They like to be kept in a tidal environment with lots of sediment, the sediment has to contain lots of organic material for them, and the water would have to be changed very regularly to prevent anoxia. None of these things make them good pets. I recommend against it (much as I like them).
It's unfortunate that fiddler crabs (real ones) are as cute as they are while also being as hard to keep as pets as they are. Even working during the summer at a field station with running sea water and a wet lab, I never tried keeping them as pets. Intertidal animals such as fiddlers require a tidal cycle and often have very strict requirements of how wet (as well as how dry) they can be. Oh, they can tolerate submergence (or dryness) for a while, but not for months. It makes them an interesting group to study (and easier to get to than deep-sea organisms of course), but it makes them almost impossible to keep as pets.
Steven V. Viscido
Dept. of Biological Sciences, Univ. of S.C., Columbia, SC 29208
http://tbone.biol.sc.edu/~steven/fiddler_crabs.html
Last modified: December 22, 2001