Clam??

[offshore80]

I just noticed that my brain coral is sitting on a clam. He never really opens up much. But, he does open enough to filter water in and out. I noticed him the othe night when I was searching for my lobster with a flashlight and he dropped shut. He's about the size of a baseball. and doesn't have the typical clam shape.
All I have on it is PC's. It doesn't open up and expose any type of mantle or anything. Here is a picture. Thanks!

 

[spline9]

Looks like a Thorny Oyster to me. Check out images.google.com for more images. Some are "thorny" as their name implies but there are others that look just like yours.
I dont know much about them except that they look like a giant rock. If you find more info, please post about it. I'd like to know more.

 

[jimlssc24]

Yep looks like a Oyster. They are fun when they are small. They move every where

 

[offshore80]

Yeah, I think it's a oyster... Thanks for the quick ID.