Just from my own experiences Ren I have bought small clams like this in (3) the past only to have them die within a one month time. When I finally go an answer on my questioning of why the thing that stuck out to me is that anything smaller than 3" would need some supplemental feeding. Took this to heart and bought a 3.5" T. corcea and have had it now since August of 08.
Do not know what supplemental feeding would consist of or how often, I just thought I would share my experience with you.