It depends on what you mean by "shell fish". There are several species of estuarine crustaceans and bivalves than can tolerate the lower salinities that one might encounter in these habitats. There are also many species of fresh water crustaceans, which include a number of shrimp species (and the common crayfish, to which may already be familiar). Probably one of the most impressive is the Malaysian giant freshwater prawn (Macrobranchium sp.) which is farmed commercially in many parts if the world for human consumption.

I suggest that you think about varying the DEGREE of saltiness as opposed to plus or minus salt. You probably already have thought about this, but consider that "not all salt water is the same. "For example, compare the middle of the Santa Barbara Channel to the Carpinteria mud flats. Do you think that the water is the same? Also think about temperature, levels of oxygen and potential fouling agents. You might be able to make more contacts and get more ideas by checking out the Marine Science Institute Website: MSI at UCSB