Most of the rivers and water streams are contaminated with toxins like pesticide, lead, arsenic etc. To remove these toxins from water is a costly affair. Now biologists found a natural way to remove these toxins and clean the water. They used clams for cleaning river streams.
According to biologist clams are filter- feeders means when they draw water into their shells, they remove the food and suspended particles found in the water. That means they remove the toxins from water and release the fresh water. They draw in more food-rich water to continue feeding. This means that lots of water works its way through their shells.
Source.
http://www.sciencedaily.com/videos/2009/0110-clam_cleanup.htm