Hi guys, thanks for the replies! Don -- yes, it (so far) is stationary, other than the opening/closing. I was especially taken aback by the fact that it can "see" so well. I can be several feet from the tank and make a relatively small movement, and the little guy will quickly snap shut. the shell halves (assuming at this point that it is in fact some type of bivalve), come to a point, which looks almost like it has a pair of interlocking teeth on the tips. this is what made me think it was something more lively than a bivalve initially. the whole "jaw" is about 1 cm across; not a terribly big creature.
I did notice when I fed the fish this evening, that the creature stayed closed. I was watching quickly to see how it would react when the scent of food came into the water. | 
