I disagree with ireef. Just being open does not mean the clam is in a good spot. I have had a crocea open inside a bag on the way home from the lfs. I don't think it would have liked staying in there, though his mantle was extended. These clams are routinely out of water at reef tops, exposed to full daylight. Why would you not want to give the clam the best home possible outside of the top of a reef? i would move it up, crocea's live on rock in nature, so even though he may be open, he will feel more at home and get more light higher up. If you had 400w, then maybe it would be ok to keep it lower. The question is are you trying to make the clam as happy as he can be (read- as much light as possible), or do you want to have him where you want him to be?