Depends on how long it has been in your tank. What kind of clam it is, etc. Lighting has some to do with it, but not all. Granted it is a HUGE portion of it for Croceas and Maximas. I have read about 5 books in the last 3 weeks because I purchased 2 of every kind of clam. If the clam is a relatively new addition or you mess with it too much, it will gape. Also one sign that they like where they are at is the fact that if they are in your substrate, they will have 50 millions tons of sand attached to their byssal glands. But if they are gapeing, you can check for several of different things. What is in the sand beneath them, how much light are they getting, what kind of water current they are getting, blah blah blah....water conditions. I have two squamosas and two derasas that dont gape - ever and they are both fatter then anything. I have one crocea that doesnt gape and another that I added a week ago that you could probably fit a dime in his incurrent syphon. Hopefully that goes away.