Hi Sideways,
Better buy just one clam as a test. When I learned that Sam was eating clams, I bought one for Ollie. He hadn't the faintest idea what it was for;, even though I let the clam live there in the tank for about two weeks. (The pencil urchin knew it was food and climbed all over it, but was unable to open the clam.) So it seems to depend on the individual bimac - maybe you have a clam eater like Sam.
Good luck!
Nancy