there is a big difference between shell fish and other fish. shellfish are hosts to a high number of bacteria, pathogens, and parasites that live inside the shells, between the flesh of the fish and the shell. microscopic organisms find this environment to be perfect, it provides food and shelter. the bacteria and parasite counts in shellfish are ALWAYS higher, sometimes much higher, than with other types of fish.
crab is a bit different for the simple reason that the crab legs are a completely closed system. the flesh/shell area of organisms like clams or shrimp are exposed directly to the ocean and thus bacteria and parasites can take up habitation there. crab legs are a closed system, so its not quite as easy for microbes to find their way in there.
as far as mercury goes, tests have shown that many wild caught alaskan salmon test extremely low or totally negative for mercury. also sardines also often test extremely low or negligible levels of mercury. mercola sells wild caught salmon on his site that test no mercury at all.