Unknown bivalves serve as mushroom attachment point

[Elysia]

I could use some help identifying some animals that make up the attachment site of some "Ultra" mushrooms that I recently purchased. There appears to be two bivalves, one on top of the other, that make up the bulk of what is suppose to be "rock." They are currently alive (they shut when disturbed) and would like to keep them that way, as they are quite large (a bit larger than the average human fist.) I do not know where the piece originated from.

In this image, near the bottom you can see a whitish mantle between the shell of one of the animals. Then, above that you can see the open shell of another (?) animal.

This image may show the gap formed by the two shells of the upper animal a bit better:

In addition to these bivalves, there is also a large barnacle in tow. Its diameter is the same as an adult human's thumb.

Does anyone have any ideas what these animals are, specifically? Thanks in advance.

 

[plateboy3293]

Free

 

[philter4]

It is an oyster. In the atlantic here we have thorny oysters and there are lots of live inverts attached to the oyster. While diving you notice the hitchhikers move as the oyster closes, this gives the oyster better camaflage, but I do not know if the hitchikers gain anything from being on it's back. they are filterfeeders and do no harm to your reef, so enjoy

 

[Elysia]

Thank you, philter4. I wondered when I saw the shell, but I wasn't certain how far south oysters lived (as I have seen them in MD before.) Any idea of what it is oysters eat -- phytoplankton? I do add preserved phyto to the tank, but not live. I'm not sure if I should be doing anything special to keep these inverts alive.