Forgive my figure of speech. Clams are phytoplantonic eaters benefititng from unicellular green algae.
Thanks for asking before you flamed. At least do research as to the meaning of the post as this is how false judgements begin to pass around and they pass fast.
Deb,
Big opening is where water goes in, little is where it goes out. The hole should not be gapping. If it is, that is a sign of stress.