Can anyone identify this?

[arking_mark]

Is that black thing a muscle or some sort of clam?

[sethsolomon]

maybe a keyhole limpet or a black sponge?

 

[arking_mark]

I don't think its a keyhole limped or black sponge.

 

It looks like it has a clam like mantle.

 

If it wasn't so hard, I'd pull the rock up for a closer look.

[elbowdeep88]

So a limpet will move around the tank right?

 

And a clam/mussel will close/retract if poked?

 

A sponge will do neither.