Doesn't look like a deresa to me but it's hard to tell from that pic. Can you get on when it is more open and with a little less blue to it? I am leaning towards T. crocea by the smooth shell and the paterning on the mantle. Deresas have more of a lined pattern on their mantle. If it is indeed a crocea you will want it up on the rocks as they are a rock dwelling clam and aren't found on the sand. They are also the most light demanding of all the Tridacna species and those T5's may not be enough to sustain in unless you get it close to the lights. Basically at the top of your rock work.
