Clams are bivalves (filter feeding) animals that get their food by filtering small particles from the water. In addition, Tridacna clams also have a symbiotic relationship w/zooxanthellae (single-celled algae)...the algae lives in the mantle tissue, the clam spreads out its mantle tissue during the day so that the algae can photosynthesize and in turn provide food for the clam. Also, the zooxanthellae is what gives these giant clams their bright & interesting colors.