Asian fresh water clams
Freshwater clams often called mussels are no longer recommended as food. Pollutants build up in their bodies since they are filter feeders. You can cook them very well and kill any bacteria, but there is no way to get rid of chemical toxins. Salt water clams are said to taste better anyway. I checked with the texas parks and wildlife dept. They too don't seem too thrilled with people eating them. Taking clams from public waters is prohibited, but then the rules get confusing. Just take a look at their website and you will see what I mean.
Finally the chinese freshwater clam is known to be a host to trematodes, protozoans, and bacteria that will get into the liver. If I remember correctly, the cycle of the flat worms requires an intermediate host and this depends on the species of worm. It may be snail to fish to clam and if a human gets in the middle, or well, he is now a host. There are plenty of good clams in the market. Bon appetit!