I have to disagree with Seth on this one...from the photos you posted, I'd ID the clam as T. crocea. The shell is very smooth (maximas tend to have at least a few scutes), and altho both T. maxima and T. crocea have asymmetrical valves (shell halves), T. maxima is longer (wider) than it is tall (e.g., is much more asymmetrical), and T. crocea has a symmetrical curve to its upper valve margin. Seth mentioned the irridipores (light sensing organs) around the periphery of the mantle, however, T. maxima has a definite line of them around the mantle edge as opposed to having them interspersed thruout the mantle. Finally, as Seth mentioned, the byssal opening of T. crocea is quite large, so turn your clam over if you want to check.
As for the gaping...T. crocea tends to be a very "gapy" clam as opposed to the other Tridacnids, and what I'm seeing in your clam wouldn't alarm me.
Finally, T. crocea has the finest gill structures of all the Tridacnids, and they can become clogged fairly easily due to "sandstorms" or detritus being disturbed. In nature, this clam is a rock-dweller, and is also the most light hungry of the genus, so keeping it in the rockwork is a better choice than on the substrate. If you think you may move the clam around occasionally, bury a clam halfshell or small flat piece of rock just beneath the substrate and let the clam attach to that. this makes it really simple to move the clam if needed. This will also prevent the clam from attaching to the tank bottom if you decide to keep it on the substrate.
HTH a bit...before the scorp bug bit me, I kept several Tridacnids in my reef (I did have to search the ol' memory banks tho!).
