Extension looks just fine. For your particular clam, A Croacia right?it looks ok, don't worry. From my understanding, when gaping, see in the first pic the siphon "hole"? That would be open extremely wide. My Croacia looks like that all the time and is just fine. As long as your parameters are good, alk and calc you should see growth monthly. Albeit, very slow but you will see it.
JR