Clam? Barnacle?

[Zaccolwlell]

I'm not sure what this is, help please!

 

[Betta132]

I think it might be a jewel box clam, or some kind of oyster. What does it do?

 

[Zaccolwlell]

It just opens slightly and closes when disturbed. It's completely formed into the rock too

 

[pagojoe]

Oyster. It's probably a Lopha species, but there are several other genera with the irregular zigzag lip.