Clam?

[Bectan01]

Hi all, I bought this piece yesterday for the zoas and this morning noticed the front section of rock opens and closes! Just slightly tho, first pic open second closed. Thoughts?

 

[Qube]

Most likely some sort of small clam. I have a few throughout the live rock in my nano. Also have one that is attached to an old cerith shell that a hermit has now commandeered.

 

[Armoredspider]

It is most likely an oyster. They sometimes come in on live rock and survive because they don't need extreme light/supplemental feeding when young. Plus it doesn't have a soft ring like part around the shell.