I could be
very wrong (so don't get discouraged until you hear from others or research a little more),but the Clam looks a little stressed to me. Overall, it looks great, but it looks like it might be beginning to gape a bit. Thus the lack of the filaments that you are searching for. Google image search "gaping reef clam" and you will see that the filaments are retracted due to the stretching.
Like I said, I could be wrong. But when I saw the original picture, my first thought was, "It looks like it's gaping". Just keep an eye on it and good luck!
